Prediction of chlorophyll a and risk assessment of water blooms in Poyang Lake based on a machine learning method

Four different methods were used to identify the important factors influencing chlorophyll-a (Chl-a) content: correlation analysis (CC-NMI), principal component analysis (PCA), decision tree (DT), and random forest recursive feature elimination (RF-RFE). Considering the relationship between Chl-a and its active and passive factors, we established machine learning combination models based on multiple linear regression (MLR), multi-layer perceptron (MLP), and support vector regression (SVR) to predict Chl-a content for Poyang Lake, China. Then, the predictive effects of different combination models were compared and evaluated from multiple perspectives. Considering the actual needs for eutrophication prevention and control, the concept of risk probability was then introduced to assess the risk degree of risk associated with water blooms in Poyang Lake. The results indicated that the mean R2 for the Chl-a predictions using the MLR, MLP, and SVR models was 0.21, 0.61, and 0.75, respectively. Consequently, the SVR model demonstrated higher precision and more accurate predictions. Compared to other methods, integrating the SVR model with the RF-RFE method significantly improved the prediction accuracy, with the R2 increasing to 0.94. For Poyang Lake, 8.8% of random samples indicated a low risk level with a water bloom probability of 21.1%-36.5%; one sample indicated a medium risk level with a risk probability of 45.5%. The research results offer valuable insights for predicting eutrophication and conducting risk assessments for Poyang Lake. They also provide reliable scientific support for making decisions about eutrophication in lakes and reservoirs. Therefore, the results hold significant theoretical importance, practical value, and potential for widespread application.

Non-traditional abiotic drivers explain variability of chlorophyll-a in a shallow estuarine embayment

Understanding the factors influencing eutrophication, as represented by concentrations of chlorophyll-a (Chl-a), is needed to inform effective management and conservation strategies promoting ecological resilience. The objective of this study was to evaluate a unique combination of abiotic explanatory factors to describe Chl-a concentrations within the study estuary (North Biscayne Bay, Florida, USA). Multiple linear regression determined the strength and direction of influence of factors using data from 10 water quality monitoring stations. The analysis also considered time scales for evaluating cumulative effects of freshwater inflow and wind. Results show that dominant drivers of Chl-a were temperature, freshwater volume (whose cumulative effects were evaluated up to a 60-day time scale), and turbidity, which were statistically significant at 60, 60, and 70 % of the investigated stations, respectively. All drivers collectively accounted for 22 to 63 % of the variability of Chl-a measurements. Of the nine variables evaluated, nutrient concentrations (orthophosphate and ammonia) were not among the top three overall drivers. Despite nutrients historically being cited in the literature as the most significant factor, this study asserts that non-nutrient factors often govern Chl-a levels, necessitating a paradigm shift in management strategies to bolster estuarine resilience against climate change.

Exciton interactions of chlorophyll tetramer in water-soluble chlorophyll-binding protein BoWSCP

The exciton interaction of four chlorophyll a (Chl a) molecules in a symmetrical tetrameric complex of the water-soluble chlorophyll-binding protein BoWSCP was analyzed in the pH range of 3-11. Exciton splitting ΔE = 232 ± 2 cm-1 of the Qy band of Chl a into two subcomponents with relative intensities of 78.1 ± 0.7 % and 21.9 ± 0.7 % was determined by a joint decomposition of the absorption and circular dichroism spectra into Gaussian functions. The exciton coupling parameters were calculated based on the BoWSCP atomic structure in three approximations: the point dipole model, the distributed atomic monopoles, and direct ab initio calculations in the TDDFT/PCM approximation. The Coulomb interactions of monomers were calculated within the continuum model using three values of optical permittivity. The models based on the properties of free Chl a in solution suffer from significant errors both in estimating the absolute value of the exciton interaction and in the relative intensity of exciton transitions. Calculations within the TDDFT/PCM approximation reproduce the experimentally determined parameters of the exciton splitting and the relative intensities of the exciton bands. The following factors of pigment-protein and pigment-pigment interactions were examined: deviation of the macrocycle geometry from the planar conformation of free Chl; the formation of hydrogen bonds between the macrocycle and water molecules; the overlap of wave functions of monomers at close distances. The most significant factor is the geometrical deformation of the porphyrin macrocycle, which leads to an increase in the dipole moment of Chl monomer from 5.5 to 6.9 D and to a rotation of the dipole moment by 15° towards the cyclopentane ring. The contributions of resonant charge-transfer states to the wave functions of the Chl dimer were determined and the transition dipole moments of the symmetric and antisymmetric charge-transfer states were estimated.

Effect of salinity stress and surfactant treatment with zinc and boron on morpho-physiological and biochemical indices of fenugreek (Trigonella foenum-graecum)

Micronutrient application has a crucial role in mitigating salinity stress in crop plants. This study was carried out to investigate the effect of zinc (Zn) and boron (B) as foliar applications on fenugreek growth and physiology under salt stress (0 and 120 mM). After 35 days of salt treatments, three levels of zinc (0, 50, and 100 ppm) and two levels of boron (0 and 2 ppm) were applied as a foliar application. Salinity significantly reduced root length (72.7%) and shoot length (33.9%), plant height (36%), leaf area (37%), root fresh weight (48%) and shoot fresh weight (75%), root dry weight (80%) and shoot dry weight (67%), photosynthetic pigments (78%), number of branches (50%), and seeds per pod (56%). Fenugreek's growth and physiology were improved by foliar spray of zinc and boron, which increased the length of the shoot (6%) and root length (2%), fresh root weight (18%), and dry root weight (8%), and chlorophyll a (1%), chlorophyll b (25%), total soluble protein content (3%), shoot calcium (9%) and potassium (5%) contents by significantly decreasing sodium ion (11%) content. Moreover, 100 ppm of Zn and 2 ppm of B enhanced the growth and physiology of fenugreek by reducing the effect of salt stress. Overall, boron and zinc foliar spray is suggested for improvement in fenugreek growth under salinity stress.

Impact of wildfire ash on bacterioplankton abundance and community composition in a coastal embayment (Ría de Vigo, NW Spain)

Wildfire ash can have an impact on coastal prokaryotic plankton. To understand the extent to which community composition and abundance of coastal prokaryotes are affected by ash, two ash addition experiments were performed. Ash from a massive wildfire that took place in the Ría de Vigo watershed in October 2017 was added to natural surface water samples collected in the middle sector of the ría during the summer of 2019 and winter of 2020, and incubated for 72 h, under natural water temperature and irradiance conditions. Plankton responses were assessed through chlorophyll a and bacterial abundance measurements. Prokaryotic DNA was analyzed using 16S rRNA gene partial sequencing. In summer, when nutrient concentrations were low in the ría, the addition of ash led to an increase in phytoplankton and bacterial abundance, increasing the proportions of Alteromonadales, Flavobacteriales, and the potentially pathogenic Vibrio, among other taxa. After the winter runoff events, nutrient concentrations in the Ría de Vigo were high, and only minor changes in bacterial abundance were detected. Our findings suggest that the compounds associated with wildfire ash can alter the composition of bacterioplanktonic communities, which is relevant information for the management of coastal ecosystems in fire-prone areas.

Chlorophyll a and novel synthetic derivatives alleviate atopic dermatitis by suppressing Th2 cell differentiation via IL-4 receptor modulation

Atopic dermatitis (AD) treatment has largely relied on non-specific broad immunosuppressants despite their long-term toxicities until the approval of dupilumab, which blocks IL-4 signaling to target Th2 cell responses. Here, we report the discovery of compound 4aa, a novel compound derived from the structure of chlorophyll a, and the efficacy of chlorophyll a to alleviate AD symptoms by oral administration in human AD patients. 4aa downregulated GATA3 and IL-4 in differentiating Th2 cells by potently blocking IL-4 receptor dimerization. In the murine model, oral administration of 4aa reduced the clinical severity of symptoms and scratching behavior by 76% and 72%, respectively. Notably, the elevated serum levels of Th2 cytokines reduced to levels similar to those in the normal group after oral administration of 4aa. Additionally, the toxicological studies showed favorable safety profiles and good tolerance. In conclusion, 4aa may be applied for novel therapeutic developments for patients with AD.

Environmental forcing of phytoplankton carbon-to-diversity ratio and carbon-to-chlorophyll ratio: A case study in Jiaozhou Bay, the Yellow Sea

The relationships between phytoplankton carbon (C) biomass and diversity (i.e., C-to-H' ratio) and chlorophyll a (i.e., C-to-Chl a ratio) are good indicators of marine ecosystem functioning and stability. Here we conducted four cruises spanning 2 years in Jiaozhou Bay to explore the dynamics of C-to-H' and C-to-Chl a ratios. The results showed that the phytoplankton C biomass and diversity were dominated by diatoms, followed by dinoflagellates. The average C-to-H' ratio ranged from 84.10 to 912.17, with high values occurring in the northern region of the bay. In contrast, the average C-to-Chl a ratio ranged between 15.55 and 89.47, and high values primarily appeared in the northern or northeastern part of the bay. In addition, the redundancy analysis showed that temperature and phosphate (DIP) were significantly correlated with both ratios in most cases, indicating that temperature and DIP may be key factors affecting the dynamics of C-to-H' and C-to-Chl a ratios.

The unprecedented 2022 extreme summer heatwaves increased harmful cyanobacteria blooms

Heatwaves are increasing and expected to intensify in coming decades with global warming. However, direct evidence and knowledge of the mechanisms of the effects of heatwaves on harmful cyanobacteria blooms are limited and unclear. In 2022, we measured chlorophyll-a (Chla) at 20-s intervals based on a novel ground-based proximal sensing system (GBPSs) in the shallow eutrophic Lake Taihu and combined in situ Chla measurements with meteorological data to explore the impacts of heatwaves on cyanobacterial blooms and the potential relevant mechanisms. We found that three unprecedented summer heatwaves (July 4-15, July 22-August 16, and August 18-23) lasting a total of 44 days were observed with average maximum air temperatures (MATs) of 38.1 ± 1.9 °C, 38.7 ± 1.9 °C, and 40.2 ± 2.1 °C, respectively, and that these heatwaves were characterized by high air temperature, strong PAR, low wind speed and rainfall. The daily Chla significantly increased with increasing MAT and photosynthetically active radiation (PAR) and decreasing wind speed, revealing a clear promotion effect on harmful cyanobacteria blooms from the heatwaves. Moreover, the combined effects of high temperature, strong PAR and low wind, enhanced the stability of the water column, the light availability and the phosphorus release from the sediment which ultimately boosted cyanobacteria blooms. The projected increase in heatwave occurrence under future climate change underscores the urgency of reducing nutrient input to eutrophic lakes to combat cyanobacteria growth and of improving early warning systems to ensure secure water management.

Long-term study of desert dust deposition effects on phytoplankton biomass in the Persian Gulf using Google Earth Engine

Persian Gulf is surrounded by the most active dust source regions of the world. Every year, millions of tons of dust are deposited in this area causing significant adverse effects. This research aimed to fill the gap in the field of dust impact quantification regarding different and important regions in the Persian Gulf. We evaluated aerosol optical thickness (AOT) as dust, chlorophyll a (Chl a) as phytoplankton biomass, sea surface temperature (SST), and particulate organic carbon (POC) using Moderate Resolution Imaging Spectroradiometer (MODIS) data to quantify and analyze the impacts of dust. According to dust frequency maps over 21 years, 4 critical regions were selected including the Northern, Central, and Southern regions of the Persian Gulf and Strait of Hormuz. Slightly lower Chl a values were noted in the southern region and Strait of Hormuz compared to the Northern and Central regions. The Mann-Kendall (MK) test and Sen's slope estimator, which can determine trend parameters, were used to analyze 4 selected regions using 8 days, monthly, and yearly averaged data. The results of the MK test showed a significant positive trend for SST and a significant negative trend for POC. AOT and Chl a trends varied based on their locations. The cross-correlation test with a time lag showed maximum correlations between Chl a and dust (AOT) with a delay of 6-7 months. Also, to quantify the impacts of dust on phytoplankton biomass, R2 and analysis of variance (ANOVA) regression were used for data with a 6-7-month time lag. The results showed that the contribution of dust in the amount of Chl a variation was about 10 %-20 %. This study showed that the simultaneous use of remote sensing and statistical methods could provide necessary and timely warnings (regarding management) to prevent algal blooms and aquatic loss in this important and strategic area.

Partitioning Recovery of Natural Pigments from Spirulina platensis

In the last few decades, the recovery of high-added value compounds with high food potential using microalgae as feedstock has been one of the main challenges for both research and industry. This study provides a simple, affordable, and cost-saving approach for the selective recovery of C-phycocyanin (C-PC), chlorophyll a and carotenoids from Spirulina platensis biomass by using biocompatible and industrially approved solvents (such as CaCl2 and ethanol). The concentration and yield of each pigment in the liquid extract have been spectrophotometrically detected, whereas the decolorized protein-rich biomass has been analyzed by CIELab parameters. The most concentrated (566.4 μg/mL) and food-grade C-PC extract (purity index 0.7) was obtained by applying a biomass/solvent ratio (1:10) for the first round of extraction (20 min), followed by a second round at 1:5 using CaCl2 1.5% (w/v) aqueous solution. Additionally, the same trial enabled the production of the brightest decolorized protein-rich biomass (L* = 46.2), characterized by a yellow-orange tonality (h° values = 81.3°).

Relationship between cell volume and particulate organic matter for different size phytoplankton

Phytoplankton biomass is an intuitive indicator to evaluate the stability of marine ecosystems. How to obtain phytoplankton biomass quickly and accurately is the focus of marine research. In this work, we selected 12 phytoplankton species to explore the relationship between cell volume and cellular compounds. Results indicated that the volume of 12 species spanned 6 orders of magnitude from 0.11 μm3 (Synechococcus sp. XM-5) to 1.26 × 105 μm3 (Ditulum brightwellii). Besides, a strong negative linear correlation between cell abundance and volume was observed. For per-cell of phytoplankton, the log-log relationship between cell volume and particulate organic carbon (POC), particulate organic nitrogen (PON), chlorophyll a (Chl-a) results reflected a positive correlation. On the contrary, the density of cellular compounds (POC, PON and Chl-a) presented a negative correlation in cell volume. In addition, for ratio of particulate organic carbon and particulate organic nitrogen (C/N), the average value was 5.46, diatoms had the lowest values, 4.72. Moreover, the ratio of carbon and chlorophyll a (C/Chl-a) varied from 15.09 to 79.75, indicating that the C/Chl-a values affected by the different categories. Meanwhile, diatoms usually have a lower C/Chl-a value for their special structure. Finally, we excluded the Synechococcus group of small individuals and found that the slope of their corresponding carbon-volume relationship was not very different, indicating that the smaller individuals had no effect on the assessment of carbon-volume relationship. The physiological variation, based on the cell volume, provides us with a new perspective to simplify the relationship between carbon and volume, and further simplify the methods and analysis of experiments and model simulations of ecosystem biological rate and flux.

Trophic status and lake depth play important roles in determining the nutrient-chlorophyll a relationship: Evidence from thousands of lakes globally

A fundamental problem in lake eutrophication management is that the nutrient-chlorophyll a (Chl a) relationship shows high variability due to diverse influences of for example lake depth, lake trophic status, and latitude. To accommodate the variability induced by spatial heterogeneity, a reliable and general insight into the nutrient-Chl a relationship may be achieved by applying probabilistic methods to analyze data compiled across a broad spatial scale. Here, the roles of two critical factors determining the nutrient-Chl a relationship, lake depth and trophic status, were explored by applying Bayesian networks (BNs) and a Bayesian hierarchical linear regression model (BHM) to a compiled global dataset from 2849 lakes and 25083 observations. We categorized the lakes into three groups (shallow, transitional, and deep) according to mean and maximum depth relative to mixing depth. We found that despite a stronger effect of total phosphorus (TP) and total nitrogen (TN) on Chl a when combined, TP played a dominant role in determining Chl a, regardless of lake depth. However, when the lake was hypereutrophic and/or TP was >40 μg/L, TN had a greater impact on Chl a, especially in shallow lakes. The response curve of Chl a to TP and TN varied with lake depth, with deep lakes having the lowest yield Chl a per unit of nutrient, followed by transitional lakes, while shallow lakes had the highest ratio. Moreover, we found a decrease of TN/TP with increasing Chl a concentrations and lake depth (represented as mixing depth/mean depth). Our established BHM may help estimating lake type and/or lake-specific acceptable TN and TP concentrations that comply with target Chl a concentrations with higher certainty than can be obtained when bulking all lake types.

Spatio-temporal structures of satellite-derived water quality indicators along the Korean South Coast

The structure of 9-year time series data for Sea Surface Temperature (SST), Chlorophyll a (Chl-a) and Total Suspended Solids (TSS), derived from the Visible Infrared Imaging Radiometer Suite (VIIRS), was examined in this study. Authors found that there exists strong seasonality among the three variables with spatial heterogeneity along the Korean South Coast (KSC). In specific, SST was in phase with Chl-a, but out of phase with TSS by six months. A strong inversed spectral power with six-month phase-lag was found between Chl-a and TSS. This could be attributed to different dynamics and environmental settings. For example, Chl-a concentration seemed to have strong positive correlation with SST indicating typical seasonality of marine biogeochemical processes such as primary production; while a strong negative correlation between TSS and SST might have been influenced by changes in physical oceanographic processes, such as stratification and monsoonal wind-driven vertical mixing. In addition, the strong east-west heterogeneity of Chl-a suggests that the marine coastal environments are predominantly governed by distinct local hydrological conditions and human activities associated with land cover and land use, while the east-west spatial pattern revealed in TSS timeseries was associated with the gradient of tidal forcings and topographical changes keeping tidally induced resuspension low eastward.

Depth profile of reservoir water quality in the Southwest of Ethiopia

The objective of this research was to investigate the variation of water quality parameters at different depths of the Gilgel Gibe I reservoir in Oromia Jimma zone during wet seasons. Five stations within the reservoir were selected, and water quality parameters were determined at four different depths (surface, 5 m, 10 m, and 15 m). Water quality parameters were analyzed on-site using a HACH, HQ40d portable multi-meter, and turbidity was measured using Wag tech turbidity meter and in the laboratory using the standard method. Comparison of water quality parameters among depths were conducted using one-way ANOVA and Tukey's pairwise comparisons with 5% significance level. The probable contributing source of the investigated physicochemical water quality parameters at different depth was identified using Principal components analysis. The results show that depth wise except for total suspended solids (TSS), total dissolved solids (TDS), total phosphorus (TP) and soluble reactive phosphorus (SRP) the rest showed statistically significant difference at p < 0.05 level. Negative correlations were found between depth and dissolved oxygen (DO), water temperature, pH, nitrate (N0₃^-) and chlorophyll a (Chl.a) while positive correlations were found between electrical conductivity (EC), biochemical oxygen demand (BOD₅), turbidity and N0₃^-. The study revealed that the release of nutrients associated with increased concentration of BOD5 at the bottom depth caused low concentration of dissolved oxygen due to oxygen consumption. This was further aggravated through the decomposition of organic matter, indicating organic pollution resulting from runoff from the catchment. The presence of dense masses of blue-green algae in the pelagic zone of Gilgel Gibe I reservoir suggested the presence of ample nutrients for its blooming and significant reduction of water quality, indicating possible eutrophic conditions. Therefore, catchment management is required to protect aquatic life and the reservoir function as a whole from reservoir water quality degradation.

Sediments are important in regulating the algae-derived off-flavor (β-cyclocitral) in eutrophic lakes

In recent years, due to global warming and water eutrophication, cyanobacterial blooms have occurred frequently worldwide, resulting in a series of water quality problems, among which the odor problem in lakes is one of the focuses of attention. In the late stage of the bloom, a large amount of algae accumulated on the surface sediment, which will be a great hidden danger to cause odor pollution in lakes. β-Cyclocitral is one of the typical algae-derived odor compounds that cause odor in lakes. In this study, an annual survey of 13 eutrophic lakes in the Taihu Lake basin was investigated to assess the effects of abiotic and biotic factors on β-cyclocitral in water. Our results showed that high concentrations of β-cyclocitral in the pore water (pore-β-cyclocitral) were detected in the sediment and far exceeded that in the water column, with an average of about 100.37 times. Structural equation modeling indicated that algal biomass and pore-β-cyclocitral can directly regulate the concentrations of β-cyclocitral in the water column, and total phosphorus (TP) and temperature (Temp) promoted the algal biomass which further enhanced the production of β-cyclocitral both in the water column and pore water. It was worth noting that when Chla ≥30 μg/L, the effects of algae on pore-β-cyclocitral were significantly enhanced, and pore-β-cyclocitral played a major role in the regulation of β-cyclocitral concentrations in water column. Overall, our study facilitated a comprehensive and systematic understanding of the effects of algae on odorants and the dynamic regulatory processes in complex aquatic ecosystems, and revealed a long-neglected process, that was, the important contribution of sediments to β-cyclocitral in the water column in eutrophic lakes, which would conduce to a more accurate understanding of the evolution of off flavors in lakes and also useful for the management of odors in lakes in the future.

Use of a Bayesian network as a decision support tool for watershed management: a case study in a highly managed river-dominated estuary

Decision making in water resource management has many dimensions including water supply, flood protection, and meeting ecological needs, therefore, is complex, full of uncertainties, and often contentious due to competing needs and distrust among stakeholders. It benefits from robust tools for supporting the decision-making process and for communicating with stakeholders. This paper presents a Bayesian network (BN) modeling framework for analyzing various management interventions regulating freshwater discharges to an estuary. This BN was constructed using empirical data from 98 months of monitoring the Caloosahatchee River Estuary in south Florida during the period 2008-2021 as a case study to illustrate the potential advantages of the BN approach. Results from three different management scenarios and their implications on down-estuary conditions as they affected eastern oysters (Crassostrea virginica) and seagrass (Halodule wrightii) are presented and discussed. Finally, the directions for future applications of the BN modeling framework to support management in similar systems are offered.

Prediction and sensitivity analysis of chlorophyll a based on a support vector machine regression algorithm

Outbreaks of planktonic algae seriously affect the water quality of rivers and are difficult to control. Based on the analysis of the temporal and spatial variation characteristics of environmental factors, this study uses a support vector machine regression (SVR) algorithm to establish a chlorophyll a (Chl-a) prediction model and conduct Chl-a sensitivity analysis. In 2018, the average Chl-a content was 126.25 ug/L. The maximum total nitrogen (TN) content was 16.68 mg/L and high year-round. The average NH₄⁺-N and total phosphorous (TP) contents were only 0.78 and 0.18 mg/L. The content of NH₄⁺-N was higher in spring and increased significantly along the water flow, while TP decreased slightly along the water flow. We used a radial basis function kernel SVR model and tenfold cross-validation method to optimize parameters. The penalty parameter c was 1.4142, the kernel function parameter g was 1, and the training and verification errors were only 0.032 and 0.067, respectively, indicating a good model fit. Based on a sensitivity analysis of the SVR prediction model, the maximum sensitivity coefficients of Chl-a to TP and WT were 0.571 and 0.394, respectively, and the contributions were 33% and 22%, respectively. The next highest sensitivity coefficients were those of DO (0.28, 16%) and pH (0.243, 14%). The sensitivity coefficients of TN and NH₄⁺-N were the lowest. According to the current water environment pollution conditions, TP is the limiting factor of Chl-a in the Qingshui River, and it is also the main prevention and control factor of phytoplankton outbreak.

Effects of benthic fish and light regimes on water quality and the growth of Vallisneria natans with two sediment types

In shal low eutrophic lakes, submersed macrophytes are essential for maintaining a clear water state and they are significantly affected by benthic fish disturbance, light availability, and sediment types. We conducted a mesocosm experiment with benthic fish (Misgurnus anguillicaudatus), two light regimes, and submerged macrophyte (Vallisneria natans) growing in two sediment types to investigate the ecological effects of benthic fish and light regimes on water quality and the growth of submersed macrophyte. Our findings indicated that the benthic fish increased the concentrations of total nitrogen, total phosphorus, and total dissolved phosphorus in the overlying water. The effects of benthic fish on ammonia-nitrogen (NH4⁺-N) and chlorophyll a (Chl-a) contents were related to light regimes. Fish disturbance indirectly promoted the growth of macrophytes growing in sand by increasing NH4⁺-N content in overlying water. However, the increasing Chl-a content stimulated by fish disturbance and high light regime reduced the growth of submersed macrophytes growing in clay due to shading. Macrophytes with different sediments had different strategies coping with light. Plants growing in sand responded to low light mainly by adjusting the leaf and root biomass allocation, whereas plants growing in clay responded to low light by physiologically adjusting the soluble carbohydrate content. The findings of this study might help restore lake vegetation to some degree, and using nutrient-poor sediment might be an appropriate method to avoid the detrimental effects of fish-mediated disturbances on the growth of submerged macrophytes.

Energy dissipation efficiency in the CP43 assembly intermediate complex of photosystem II

Photosystem II in oxygenic organisms is a large membrane bound rapidly turning over pigment protein complex. During its biogenesis, multiple assembly intermediates are formed, including the CP43-preassembly complex (pCP43). To understand the energy transfer dynamics in pCP43, we first engineered a His-tagged version of the CP43 in a CP47-less strain of the cyanobacterium Synechocystis 6803. Isolated pCP43 from this engineered strain was subjected to advanced spectroscopic analysis to evaluate its excitation energy dissipation characteristics. These included measurements of steady-state absorption and fluorescence emission spectra for which correlation was tested with Stepanov relation. Comparison of fluorescence excitation and absorptance spectra determined that efficiency of energy transfer from β-carotene to chlorophyll a is 39 %. Time-resolved fluorescence images of pCP43-bound Chl a were recorded on streak camera, and fluorescence decay dynamics were evaluated with global fitting. These demonstrated that the decay kinetics strongly depends on temperature and buffer used to disperse the protein sample and fluorescence decay lifetime was estimated in 3.2-5.7 ns time range, depending on conditions. The pCP43 complex was also investigated with femtosecond and nanosecond time-resolved absorption spectroscopy upon excitation of Chl a and β-carotene to reveal pathways of singlet excitation relaxation/decay, Chl a triplet dynamics and Chl a → β-carotene triplet state sensitization process. The latter demonstrated that Chl a triplet in the pCP43 complex is not efficiently quenched by carotenoids. Finally, detailed kinetic analysis of the rise of the population of β-carotene triplets determined that the time constant of the carotenoid triplet sensitization is 40 ns.

Phytoplankton pigment dynamics in marine lake fluctuating between stratified and holomictic euxinic conditions

Biomass dynamics in the marine lake are strongly dependent on seasonal variability in vertical stratification, indicating rapid adaptation of phytoplankton to short-term changes in the water column. A small marine lake (Rogoznica Lake, Croatia), which fluctuates between stably stratified and holomictic euxinic conditions, was used as a model to study the phytoplankton responses to environmental perturbations, in particular the anoxic stress, caused by periodic holomixia. The epilimnion showed significant temporal and vertical variability with a chlorophyll a subsurface maximum with the highest biomass near the chemocline. Fucoxanthin-containing biomass (diatoms) dominated in the epilimnion in colder seasons and was first to recover after holomictic euxinic events. The shift towards the smaller groups prevailed during highly stratified water column conditions in warmer seasons. Results for the hypolimnion were more enigmatic, with high concentrations of alloxanthin, zeaxanthin, and violaxanthin indicating the presence of a viable small-size mixotrophic community under extreme conditions.

Effect of specific irradiance on productivity and pigment and protein production of Porphyridium purpureum (Rhodophyta) semi-continuous culture

Porphyridium purpureum is a promising microalga species due to the content of various valuable compounds. In this study, specific irradiance parameter, representing the amount of light energy per unit of microalgae biomass, was introduced. The growth characteristics and pigments and protein accumulation of P. purpureum culture were investigated under semi-continuous mode. Varying dilution rate and surface irradiance resulted in a specific irradiance of 0.2-6.7 W g^-1. Using mathematical modeling, we determined the patterns of changes in biomass, pigments, protein content and productivity of P. purpureum culture depending on specific irradiance. The content of target compounds was maximized under the lowest level of specific irradiance (0.2-1.2 W g^-1), while the highest productivity of this components was reached under 1.2-1.7 W g^-1. Overall, lower irradiance levels were favorable for P. purpureum cultivation based on the energy consumption and production characteristics of this species.

Biosynthesis of 2-methylisoborneol is regulated by chromatic acclimation of Pseudanabaena

Cyanobacteria can sense different light color by adjusting the components of photosynthetic pigments including chlorophyll a (Chl a), phycoerythrin (PE), and phycocyanin (PC), etc. Filamentous cyanobacteria are the main producer of 2-methylisoborneol (MIB) and many can increase their PE levels so that they are more competitive in subsurface layer where green light is more abundant, and have caused extensive odor problems in drinking water reservoirs. Here, we identified the potential correlation between MIB biosynthesis and ambient light color induced chromatic acclimation (CA) of a MIB-producing Pseudanabaena strain. The results suggest Pseudanabaena regulates the pigment proportion through Type III CA (CA3), by increasing PE abundance and decreasing PC in green light. The biosynthesis of MIB and Chl a share the common precursor, and are positively correlated with statistical significance regardless of light color (R²=0.68; p<0.001). Besides, the PE abundance is also positively correlated with Chl a in green light (R²=0.57; p=0.019) since PE is the antenna that can only transfer the energy to PC and Chl a. In addition, significantly higher MIB production was observed in green light since more Chl a was synthesized.

Spatially distributed water quality responses to freshwater discharge in a tropical estuary, Hilo Bay, Hawai'i

Hilo Bay, Hawai'i, is an estuary of great importance to its neighboring coastal community, but it is threatened by impaired water quality indicated by excessive turbidity and chlorophyll a associated with river discharges of sediments and nutrients. The Wailuku River in the western half of the bay is the primary source of freshwater discharge, hypothesized here to form a surface water-dominant half of the bay with different water quality traits than the groundwater-dominant, eastern half of the bay where the spring-fed Wailoa River discharges. The water quality of both halves of the bay over different flow conditions of the Wailuku River is examined in this study using spatially distributed water quality sampling which collects hundreds of samples in either half of the bay at a distance of about every 40 m. The dense sample shows significant differences between the two halves of the bay, with greater salinity dilution and turbidity in the surface water-dominant area. Both salinity and turbidity have a predictable relation to discharge, with salinity decreasing and turbidity increasing in higher flow conditions. Chlorophyll a, however, has a more complex relation to discharge, as chlorophyll a concentrations are greatest in high-flow conditions, but this may be because the water quality samples were collected in different seasons. Furthermore, significantly greater chlorophyll a concentrations in the groundwater-dominant half of the bay in low-flow conditions show that discharge may be spuriously correlated to chlorophyll a, and further studies of the effects of surface water discharge on chlorophyll a concentrations are warranted.

Evaluation of the effect of UV-B radiation on growth, photosynthetic pigment, and antioxidant enzymes of some cyanobacteria

The current study is focused on the effects of artificial UV-B radiation on growth, proteins, and pigments, as well as the activities of several enzymatic and non-enzymatic antioxidant enzymes in some cyanobacterial strains. Cultures were maintained at 25 °C ± 1 °C under a white fluorescent tube of intensity 30-40 μE m -2s^-1 with a 14:10 light and dark cycle in the laboratory and analyzed at an interval of 25, 32, 39, 46, and 53 days. The test cultures were exposed to UV-B stress for 24 h at the same intervals. We found that exposure to UV-B showed increased production of phycocyanin and carotenoids in four strains, namely, Scytonema javanicum, Nostoc muscorum, Aphanothece naegeli, and Synechococcus elongates. We also look into the effects of UV-B radiation on the proline content, non-protein thiols, radical scavenging activity, ascorbic acid, and tocopherol, total flavonoid content (TFC), total phenolic content (TPC) on these strains. Variation in the non-enzymatic antioxidants and expression levels of enzymatic enzymes and reducing power activity as compared to the non-irradiated control was found. Our study showed that cyanobacteria impart prominent antioxidant and radical scavenging properties which facilitate the defence mechanism against UV-B induced cell damage.

Early warning of algal blooms based on the optimization support vector machine regression in a typical tributary bay of the Three Gorges Reservoir, China

Algal blooms caused by climate change and human activities have received considerable attention in recent years. Since chlorophyll a (Chl-a) can be used as an indicator of phytoplankton biomass, it has been selected as a direct indicator for monitoring and early warning of algal blooms. With the development of artificial intelligence, data-driven approaches with small sample data and high accuracy prediction have been gradually applied to water quality prediction. This study aimed at using environment factors (water quality and meteorological data) to assist the prediction of Chl-a concentration based on the optimization support vector machine (SVM) model. The most relevant environment factors were extracted from the commonly used environment factors according to the method of cosine similarity. The traditional particle swarm optimization (PSO) algorithm was adopted to optimize the ANN and SVM models, respectively. Then, the better prediction model PSO-SVM can be obtained according to the results of three scientific evaluation indicators. The latest optimization algorithm of grey wolf optimizer (GWO) was also proposed to optimize the SVM to realize high-accuracy Chl-a concentration predication. The GWO-SVM model achieved higher accuracy than the other models both in training and validation processes. Therefore, the dimension of the input vector could be reduced with using the cosine similarity method, and the prediction of Chl-a concentration in high accuracy and the early warning of algal blooms in the study area of this paper could also achieved.

Water quality analysis based on phytoplankton and metal indices: a case study in the Sauce Grande River Basin (Argentina)

The increasing landscape alterations due to anthropogenic activities is of global concern since it affects aquatic ecosystems, often resulting in compromise of the ecological integrity and the water quality. In this sense, the evaluation, monitoring, and prediction of the aquatic ecosystem quality becomes an important research subject. This study presents the first integrated water quality assessment of the Sauce Grande River Basin, in Argentina, based on the spatial distribution of the phytoplankton community, the physicochemical parameters, and the metal concentrations (Cd, Cu, Cr, Fe, Mn, Ni, Pb, and Zn) found in the particulate fraction. According to the trophic indices and the phytoplankton abundance, composition, and diversity, the water quality showed significant deterioration in the lower basin after the Sauce Grande lake. The trophic state index indicated that water was oligotrophic in over 75% of the sampling sites, increasing downstream, where two sites were characterized as mesotrophic, and one described as hypertrophic. The phytoplankton community was dominated by diatoms in zones with low anthropogenic impact and conductivity, whereas high densities of Euglenophyta, Chlorophyta, and Cyanobacteria were found in the middle-lower basin, associated with higher organic matter and eutrophication. The conductivity, turbidity, and most metal concentrations also increased towards the downstream area, even exceeding recommended levels for the metals Cu, Cr, Mn, and Pb in the middle and lower reaches of the basin (Cu: 3.5 µg L^-1; Cr: 2.4 µg L^-1; Pb: 1.2 µg L^-1; Mn 170 µg L^-1). This study generates a database for the water quality of the Sauce Grande River Basin and sets an example of how the water quality varies along a basin that crosses different topographic environments, land covers, and anthropogenic influences.

A ground-based remote sensing system for high-frequency and real-time monitoring of phytoplankton blooms

The worldwide expansion of phytoplankton blooms has severely threatened water quality, food webs, habitat stability and human health. Due to the rapidity of phytoplankton migration and reproduction, high-frequency information on phytoplankton bloom dynamics is crucial for their forecasting, treatment, and management. While several approaches involving satellites, in situ observations and automated underwater monitoring stations have been widely used in the past several decades, they cannot fully provide high-frequency and continuous observations of phytoplankton blooms at low cost and with high accuracy. Thus, we propose a novel ground-based remote sensing system (GRSS) that can monitor real-time chlorophyll a concentrations (Chla) in inland waters with a high frequency. The GRSS mainly consists of three platforms: the spectral measurement platform, the data-processing platform, and the remote access control, display and storage platform. The GRSS is capable of obtaining a remote sensing irradiance ratio (R(λ)) of 400-1000 nm at a high frequency of 20 s. Eight different Chla retrieval algorithms were calibrated and validated using a dataset of 481 pairs of GRSS R(λ) and in situ Chla measurements collected from four inland waters. The results showed that random forest regression achieved the best performance in deriving Chla (R² = 0.95, root mean square error = 13.40 μg/L, and mean relative error = 25.7%). The GRSS successfully captured two typical phytoplankton bloom events in August 2021 with rapid changes in Chla from 20 μg/L to 325 μg/L at the minute level, highlighting the critical role that this GRSS can play in the high-frequency monitoring of phytoplankton blooms. Although the algorithm embedded into the GRSS may be limited by the size of the training dataset, the high-frequency, continuous and real-time data acquisition capabilities of the GRSS can effectively compensate for the limitations of traditional observations. The initial application demonstrated that the GRSS can capture rapid changes of phytoplankton blooms in a short time and thus will play a critical role in phytoplankton bloom management. From a broader perspective, this approach can be extended to other carriers, such as aircraft, ships and unmanned aerial vehicles, to achieve the networked monitoring of phytoplankton blooms.

Elucidating phytoplankton limiting factors in lakes and reservoirs of the Chinese Eastern Plains ecoregion

Knowledge of phytoplankton limiting factors is essential for cost-efficient lake eutrophication management. Herein, we propose a statistical framework to explore site-specific phytoplankton limiting factors and their dependence on water depth (WD) in 54 lakes in the Chinese Eastern Plains ecoregion. First, the maximal chlorophyll a (Chla) response to total N (TN) or P (TP), representing a region-specific "standard" model where phytoplankton were primarily N- or P-limited, was quantified using a 95% quantile regression. Second, site-specific limiting factors were identified using analogical residual analysis. N- or P-limitation was inferred if Fraction_TN (i.e. fraction of Chla observed and predicted by the "standard" model for a given TN) > 0.95 or Fraction_TP >0.95; if both Fraction_TN and Fraction_TP <0.95 in a specific environmental condition (e.g. high non-algal turbidity), light limitation was suggested. As a result, 5%, 7%, 4%, 36%, 16%, 2%, and 30% of the sampling sites were limited by N, P, N+P, light availability, rapid flushing, abundant macrophytes, and unmeasured factors, respectively. Bloom control suggestions in the short run are proposed considering these actual limiting factors. Furthermore, the maximal Fraction_TN or Fraction_TP response to WD was explored, reflecting the effect of WD on Fraction_TN (or Fraction_TP) without significant confounders. The results indicated that phytoplankton in the studied freshwaters would be potentially light-limited, N-limited, N+P-co-limited, or P-limited depending on WD (<1.8, 1.8-2.1, 2.1-5.2, or >5.2 m, respectively), because N will gradually become surplus with increasing WD, while at very shallow depths, strong sediment re-suspension induces light limitation. This finding implies that long-term nutrient management strategies in the studied freshwaters that have WDs of 0-2.1, 2.1-5.2, and >5.2 m can entail control of N, N+P, and P, respectively. This study provides essential information for formulating context-dependent bloom control for lakes in our study area and serves as a valuable reference for developing a cost-efficient eutrophication management framework for other regions.

Pervasive changes in algal indicators since pre-industrial times: A paleolimnological study of changes in primary production and diatom assemblages from ~200 Canadian lakes

Anthropogenic stressors affect lakes around the world, ranging in scale from catchment-specific pollutants to the global impacts of climate change. Canada has a large number and diversity of lakes, yet it is not well understood how, where, and when human impacts have affected these lakes at a national scale. The NSERC Canadian Lake Pulse Network sought to create the first nationwide database of Canadian lake health, undertaking a multi-year survey of 664 lakes spanning 12 ecozones across Canada. A key objective of the network is to determine where, by how much, and why have Canadian lakes changed during the Anthropocene. To address this objective, we compared sedimentary chlorophyll a and diatoms from modern and pre-industrial sediment intervals of ~200 lakes. The lakes spanned a range of sizes, ecozones, and degrees of within-catchment land use change. We inferred the quantity of chlorophyll a, its isomers and main diagenetic products using visible reflectance spectroscopy. We found widespread increases in primary production since pre-industrial times. Primary production increased, on average, across all ecozones, human impact classes, and stratification classes. Likewise, an increase in planktonic diatom taxa over time was detected in the majority of sampled lakes, likely due to recent climate warming. However, regional factors (ecozones) explained the most variation in modern diatom species assemblages as well as their temporal turnover. Furthermore, lakes with high human impact (i.e., higher weighted proportions of human land use in the catchment) exhibited greater taxonomic turnover than lakes with a low human impact class. The greatest diatom turnover was found in the agriculture-rich Prairies and the lowest in the sparsely populated Boreal Shield and Taiga Cordillera ecozones. Overall, our study highlights that drivers operating at different geographic scales (i.e., climatic and land-use changes) have led to significant alterations in algal indicators since pre-industrial times across the country.

Toxicity of TiO2 nanoparticles to the marine microalga Chaetoceros muelleri Lemmermann, 1898 under long-term exposure

Titanium dioxide nanoparticles (TiO₂NPs) have been extensively used in industry, raising many concerns about their release into the aquatic environments. In marine ecosystems, microalgae are major primary producers; among them, Chaetoceros muelleri is an important microalga in the aquaculture industry as live feed. The impacts of TiO₂NPs on the growth, photosynthetic pigments, protein and lipid contents, and the interaction of TiO₂NPs with the cell wall of C. muelleri were investigated in the present study. Algal cells were exposed to concentrations of 5, 10, 50, 100, 200, and 400 mg/L TiO₂NPs for 10 days. There was a significant difference in the growth between the control and TiO₂NPs treatments on each day. The half-maximal inhibitory concentration (IC₅₀) of TiO₂NPs on algal cells was found to be 10.08 and 5.01 mg/L on the 3rd and 10th days, respectively. The contents of chlorophyll a and c reduced significantly in the TiO₂NPs-treated microalgae. TiO₂NPs also reduced the protein and lipid contents in the treated microalgae, up to 13.02% and 47.6% respectively, at the highest concentration. The interaction of TiO₂NPs with the C. muelleri cells was obvious based on Fourier transform infrared spectroscopy, microscopic images, EDS, and Mapping analyses. Toxic effects of the released TiO₂NPs can damage the stocks of C. muelleri as an important live feed in mariculture.

Single toxicity of arsenic and combined trace metal exposure to a microalga of ecological and commercial interest: Diacronema lutheri

Eco-toxicological assays with species of economic interest such as Diacronema lutheri are essential for industries that produce aquaculture feed, natural food additives and also in drug developing industries. Our study involved the exposure of a single and combined toxicity of arsenic (As V) to D. lutheri for the entire algal growth phase and highlighted that a combined exposure of As V with other essential (Copper, Cu; Nickel, Ni) and non-essential (Cadmium, Cd; Lead, Pb) trace metals reduced significantly the cell number, chlorophyll a content, and also significantly increased the de-epoxidation ratio (DR) as a stress response when compared to the single toxicity of As V. Arsenic, as one of the ubiquitous trace metal and an active industrial effluent is reported to have an increased bio-concentration factor when in mixture with other trace metals in this study. In the combined exposure, the concentration of total As bio-accumulated by D. lutheri was higher than in the single exposure. Hence, polluted areas with the prevalence of multiple contaminants along with the highly toxic trace metals like As can impose a greater risk to the exposed organisms that may get further bio-magnified in the food chain. Our study highlights the consequences and the response of D. lutheri in terms of contamination from single and multiple trace metals in order to obtain a safer biomass production for the growing need of natural derivatives.

Seamount effect on phytoplankton biomass and community above a deep seamount in the tropical western Pacific

It is generally believed that the enhancement of phytoplankton appears only in shallow and intermediate depth seamounts, while the phenomenon has also been observed in some deep seamounts by satellites recently. To figure out what effect do deep seamounts have on phytoplankton and the relevant mechanisms, the phytoplankton biomass and community on the Kocebu Seamount (depth: 1198 m) were studied. The results showed that the average Chl a concentration of the seamount was 0.09 mg·m^-3, and the Chl a maximum layer was mainly located at 150 m, and picophytoplankton such as prochlorophytes and cyanobacteria were the dominant groups. High Chl a patches (>0.2 mg·m^-3) were mainly distributed within 20 km of the peak, and both nitrate and orthophosphate were obviously uplifted at the peak. The physical data indicated the uplifted of nutrients could be caused by the internal tides, which generated by the interaction of topography and tide. This is the first time that the promotion of phytoplankton was observed in situ on a deep seamount, and this study expounded relevant mechanisms and suggested that the ecological functions of deep seamounts may have been previously neglected.

Cyanobacterial pigment concentrations in inland waters: Novel semi-analytical algorithms for multi- and hyperspectral remote sensing data

Cyanobacteria are notorious for producing harmful algal blooms that present an ever-increasing serious threat to aquatic ecosystems worldwide, impacting the quality of drinking water and disrupting the recreational use of many water bodies. Remote sensing techniques for the detection and quantification of cyanobacterial blooms are required to monitor their initiation and spatiotemporal variability. In this study, we developed a novel semi-analytical approach to estimate the concentration of cyanobacteria-specific pigment phycocyanin (PC) and common phytoplankton pigment chlorophyll a (Chl a) from hyperspectral remote sensing data. The PC algorithm was derived from absorbance-concentration relationship, and the Chl a algorithm was devised based on a conceptual three-band structure model. The developed algorithms were applied to satellite imageries obtained by the Hyperspectral Imager for the Coastal Ocean (HICO™) sensor and tested in Lake Kinneret (Israel) during strong cyanobacterium Microcystis sp. bloom and out-of-bloom times. The sensitivity of the algorithms to errors was evaluated. The Chl a and PC concentrations were estimated with a mean absolute percentage difference (MAPD) of 16% and 28%, respectively. Sensitivity analysis shows that the influences of backscattering and other water constituents do not affect the estimation accuracy of PC (~2% MAPD). The reliable PC/Chl a ratios can be obtained at PC concentrations above 10 mg m^-3. The computed PC/Chl a ratio depicts the contribution of cyanobacteria to the total phytoplankton biomass and permits investigating the role of ambient factors in the formation of a complex planktonic community. The novel algorithms have extensive practical applicability and should be suitable for the quantification of PC and Chl a in aquatic ecosystems using hyperspectral remote sensing data as well as data from future multispectral remote sensing satellites, if the respective bands are featured in the sensor.

Coastal planktonic community unaffected by Boreal hydropower complex in Québec, Canada

Comprehensive studies of the impact of hydropower on coastal environments are rare. This study examines the impact of commissioning the hydropower plants of the Romaine complex on the freshwater discharge of the Rivière Romaine near its estuary and on the Chenal de Mingan ecosystem in the summers of 2015, 2017 and 2019. Continuous temperature, salinity and chlorophyll a data were obtained from two instrumented buoys, and nutrients as well as the phytoplankton and zooplankton communities were sampled five times a year at 11 stations. The results demonstrate the major influence of offshore waters on temperature and salinity in the study area, and the decreasing influence of the Rivière Romaine with distance from its mouth. Nutrient concentrations in the estuary did not covary with river discharge or with nutrient concentrations in the river. Importantly, impoundment of the reservoirs of the complex had no measurable effect on nutrient stoichiometry in the Chenal de Mingan. Overall, the chlorophyll a concentrations ranged from 0.1 to 7.6 µg L^-1 in the channel, the community was dominated by diatoms, and phytoplankton growth was either nitrate limited or under predation pressure. The zooplankton community has been composed of the same groups of species and has been dominated by cyclopoids and calanoids since 2015. Our study underlines the importance of including regional meteorological trends in the analysis to avoid biased conclusions on the impact of hydropower projects. The study concluded that modulation of the Rivière Romaine discharge and related changes in water quality did not lead to measurable change in plankton production in the Chenal de Mingan.

Exposure-based ecotoxicity assessment of Co3O4 nanoparticles in marine microalgae

The exposure-effect study was conducted to evaluate the effect of Co₃O₄ nanoparticles on Tetraselmis suecica. The growth suppressing effect has been observed during the interaction between nanoparticles and microalgae as indicated by 72 h EC₅₀ (effective concentration of a chemical at which 50% of its effect is observed) value (45.13±3.95 mg/L) of Co₃O₄ nanoparticles for Tetraselmis suecica. Decline in chlorophyll a content also indicated the compromised photosynthetic ability and physiological state of microalgae. Further biochemical investigation such as increase in extracellular LDH (lactate dehydrogenase) level, ROS (reactive oxygen species), and levels of membrane lipid peroxidation in treated samples signifies the compromised cellular health and membrane disintegration caused by nanoparticles. Parallel to this, the cell entrapment, membrane damage, and attachment of nanoparticles on cell surface were also visualized by SEM-EDX (scanning electron microscope-energy dispersive X-ray) microscopy. The overall results of this study clearly indicated that Co₃O₄ nanoparticles might have toxic effects on growth of marine microalgae and other aquatic life forms as well. Hence, release of Co₃O₄ nanoparticles in aquatic ecosystem and resulting ecotoxic effect should be broadly addressed.

Bayesian change point quantile regression approach to enhance the understanding of shifting phytoplankton-dimethyl sulfide relationships in aquatic ecosystems

Dimethyl sulfide (DMS) serves as an anti-greenhouse gas, plays multiple roles in aquatic ecosystems, and contributes to the global sulfur cycle. The chlorophyll a (CHL, an indicator of phytoplankton biomass)-DMS relationship is critical for estimating DMS emissions from aquatic ecosystems. Importantly, recent research has identified that the CHL-DMS relationship has a breakpoint, where the relationship is positive below a CHL threshold and negative at higher CHL concentrations. Conventionally, mean regression methods are employed to characterize the CHL-DMS relationship. However, these approaches focus on the response of mean conditions and cannot illustrate responses of other parts of the DMS distribution, which could be important in order to obtain a complete view of the CHL-DMS relationship. In this study, for the first time, we proposed a novel Bayesian change point quantile regression (BCPQR) model that integrates and inherits advantages of Bayesian change point models and Bayesian quantile regression models. Our objective was to examine whether or not the BCPQR approach could enhance the understanding of shifting CHL-DMS relationships in aquatic ecosystems. We fitted BCPQR models at five regression quantiles for freshwater lakes and for seas. We found that BCPQR models could provide a relatively complete view on the CHL-DMS relationship. In particular, it quantified the upper boundary of the relationship, representing the limiting effect of CHL on DMS. Based on the results of paired parameter comparisons, we revealed the inequality of regression slopes in BCPQR models for seas, indicating that applying the mean regression method to develop the CHL-DMS relationship in seas might not be appropriate. We also confirmed relationship differences between lakes and seas at multiple regression quantiles. Further, by introducing the concept of DMS emission potential, we found that pH was not likely a key factor leading to the change of the CHL-DMS relationship in lakes. These findings cannot be revealed using piecewise linear regression. We thereby concluded that the BCPQR model does indeed enhance the understanding of shifting CHL-DMS relationships in aquatic ecosystems and is expected to benefit efforts aimed at estimating DMS emissions. Considering that shifting (threshold) relationships are not rare and that the BCPQR model can easily be adapted to different systems, the BCPQR approach is expected to have great potential for generalization in other environmental and ecological studies.

Meteorological drivers and ENSO influence on phytoplankton biomass dynamics in a shallow subtropical lake

Meteorological features influence the dynamics of aquatic ecosystems and consequently their biotas. This study aimed to identify the meteorological drivers of phytoplankton biomass (chlorophyll a), sampled seasonally over a period of 12 years (2001-2013) in Lake Mangueira, a large shallow subtropical lake in southern Brazil. The lake is 90 km long and 3-10 km wide with a mean depth of 3 m and is oligo-mesotrophic and highly affected by wind action. In general, non-parametric multiplicative regression analysis identified wind direction, radiation, and the Oceanic Niño Index as the main drivers of variation in chlorophyll a. Notably, ENSO periods caused changes in physical, chemical, and meteorological parameters, including conductivity, total suspended solids, total and dissolved nitrogen, alkalinity, soluble reactive silica, wind speed, and precipitation. Phytoplankton biomass showed significant differences between ENSO periods and the periods without events, occurring in the highest values during La Niña years. This study showed that meteorological variables can significantly influence productivity patterns, indicating the importance of including them in limnological studies.

Machine learning for anomaly detection in cyanobacterial fluorescence signals

Many drinking water utilities drawing from waters susceptible to harmful algal blooms (HABs) are implementing monitoring tools that can alert them to the onset of blooms. Some have invested in fluorescence-based online monitoring probes to measure phycocyanin, a pigment found in cyanobacteria, but it is not clear how to best use the data generated. Previous studies have focused on correlating phycocyanin fluorescence and cyanobacteria cell counts. However, not all utilities collect cell count data, making this method impossible to apply in some cases. Instead, this paper proposes a novel approach to determine when a utility needs to respond to a HAB based on machine learning by identifying anomalies in phycocyanin fluorescence data without the need for corresponding cell counts or biovolume. Four widespread and open source algorithms are evaluated on data collected at four buoys in Lake Erie from 2014 to 2019: local outlier factor (LOF), One-Class Support Vector Machine (SVM), elliptic envelope, and Isolation Forest (iForest). When trained on standardized historical data from 2014 to 2018 and tested on labelled 2019 data collected at each buoy, the One-Class SVM and elliptic envelope models both achieve a maximum average F1 score of 0.86 among the four datasets. Therefore, One-Class SVM and elliptic envelope are promising algorithms for detecting potential HABs using fluorescence data only.

Differential sensitivity to oxygen among the bacteriochlorophylls g in the type-I reaction centers of Heliobacterium modesticaldum

The type-I, homodimeric photosynthetic reaction center (RC) of Heliobacteria (HbRC) is the only known RC in which bacteriochlorophyll g (BChl g) is found. It is also simpler than other RCs, having the smallest number of protein subunits and bound chromophores of any type-I RC. In the presence of oxygen, BChl g isomerizes to 8¹-hydroxychlorophyll a_F (Chl a_F). This naturally occurring process provides a way of altering the chlorophylls and studying the effect of these changes on energy and electron transfer. Transient absorbance difference spectroscopy reveals that triplet-state formation occurs in the antenna chlorophylls of HbRCs but does not provide site-specific information. Here, we report on an extended optically detected magnetic resonance (ODMR) study of the antenna triplet states in HbRCs with differing levels of conversion of BChl g to Chl a_F. The data reveal pools of BChl g molecules with different triplet zero-field splitting parameters and different susceptibilities to chemical oxidation. By relating the detailed spectroscopic characteristics derived from the ODMR data to the recently solved crystallographic structure, we have tentatively identified BChl g molecules in which the probability of triplet formation is high and sites at which BChl g conversion is more likely, providing useful information about the fate of the excitation in the complex.

Influence of microphytobenthos on the sedimentary organic matter composition in two contrasting estuarine microhabitats

The influence of microphytobenthic community on the composition of sedimentary organic matter was evaluated in two Bahía Blanca estuary microhabitats contrasting in vegetation and hydrodynamic conditions; namely, S1 located in a Sarcocornia perennis saltmarsh with macroscopic microbial mats and S2 in a mudflat without macroscopic microbial mats or vegetation. Moisture, organic matter, chlorophyll a, phaeopigments, carbohydrates (CH), proteins (PRT), and abundance and structure of microphytobenthic community were evaluated in surface sediments. Higher moisture was observed at S2 and was related to the proximity of this site to the subtidal zone and the effect of the environmental variables temperature and rain. No significant differences were found in organic matter content between sites; however, at S1, a higher concentration was registered during winter and early spring associated to the period of higher microphytobenthic biomass. Chlorophyll a and phaeopigments were higher at S1, attributed to the higher microphytobenthos abundance at this site. Differences in microphytobenthos were observed not only in quantity but also in community structure since at S1 filamentous cyanobacteria dominated the community, whereas at S2, higher abundance of centric diatoms and the absence of cyanobacteria were observed during most of the study. S1 showed higher concentration of proteins and carbohydrates which could be attributed to the higher production of fresh organic matter by microphytobenthos. The total protein and carbohydrate concentrations allowed us to classify both microhabitats into the meso-oligotrophic category, contrasting with the classification made by other authors using water column proxies.

Picophytoplankton distribution along Khatanga Bay-shelf-continental slope environment gradients in the western Laptev Sea

The spatial variations of photosynthetic picoplankton abundance and biomass and the picoplankton's contribution to chlorophyll a concentration along the transect from Khatanga Bay to the continental slope in the western part of the Laptev Sea were studied in September 2017. Picoeukaryotes dominated in the picophytoplankton communities. Picophytoplankton in Khatanga Bay showed more variability than those over the Laptev shelf and continental slope: abundance and biomass were the highest in the southern part of the bay and markedly decreased with increasing salinity in its northern part. Picocyanobacteria were found over the shelf and slope at temperatures of +2.4 to -1.6°С and salinity from 22 to 34. Picophytoplankton contribution to total chlorophyll a on the shelf was higher than in Khatanga Bay. The study of picophytoplankton of Khatanga Bay and in the western Laptev Sea can serve as a baseline for future assessment of the Laptev Sea ecosystem response to interannual and climate changes.

Linking water environmental factors and the local watershed landscape to the chlorophyll a concentration in reservoir bays

The frequency of harmful algal blooms caused by eutrophication is increasing globally, posing serious threats to human health and economic development. Reservoir bays, affected by water environment and local watershed landscape, are more prone to eutrophication and algal blooms. The chlorophyll a (Chl a) concentration is an important indicator for the degree of eutrophication and algal bloom. Exploring the complex relationships between water environment and landscape background, and Chl a concentration in the reservoir bays are crucial for ensuring high-quality drinking water from reservoirs. In this study, we monitored Chl a concentrations of 66 bays in Danjiangkou Reservoir and the related water quality parameters (e.g., water temperature, turbidity, nutrients) in waterbodies of these reservoir bays in the storage and discharge periods from 2015 to 2018. Partial least squares-structural equation modeling (PLS-SEM) was used to quantify the relationship between water environmental factors and watershed landscapes, and Chl a concentrations in reservoir bays. The results showed that mean Chl a concentration was higher in storage period than that in discharge period. Two optimal PLS-SEMs explained 66.8% and 53.6% of Chl a concentration variation in the storage and discharge periods, respectively. The net effect of water chemistry on Chl a concentration was more pronounced during the discharge period (total effect = 0.61, 37% of the total effect on Chl a), while the net effect of land-use composition on Chl a concentration was more significant during the storage period (total effect = 0.57, 30% of the total effect on Chl a). The landscape pattern had significant indirect effects on Chl a concentration, especially during the discharge period (indirect effect = -0.31, 19% of the total effect on Chl a). Our results provide valuable information for managers to make rational decisions, thereby contributing to the prevention of eutrophication and algal blooms in reservoir bays.

Facilitating harmful algae removal in fresh water via joint effects of multi-species algicidal bacteria

Harmful algae blooms posing serious threats to the ecological environment occur frequently across the world. Multi-species algicidal bacteria were enriched by utilizing immobilized carriers in a pilot scale experiment, which significantly promoted the effect of algal control in the reactors. Under the optimal condition, the algicidal ratio and chlorophyll a degradation rate reached 87.69% and 47.00 μg/(L·d), respectively. The growth of Cyanophyta, diatom, Dinoflagellate and Cryptophyta was inhibited significantly by the joint action of algicidal bacteria and light shading of fillers, accounting for 53.74% and 36.47%, respectively. The results of 16S rRNA high-throughput sequencing suggested algicidal bacteria (10.17%) belonging to 13 genera were enriched. Among the algicidal process, Bacillus and Pseudomonas played crucial roles. Fluorescence spectroscopy and UV₂₅₄ were adopted to assess the release of dissolved organic matter (DOM) and the precursors of disinfection by-products (DBPs). Two efficient algicidal strains (C1, C4) were isolated which showed high homology with Enterobacter asburiae JCM6051(T) and Pseudomonas simiae oli(T), respectively. This study provided new insights into the in-situ bioremediation of eutrophication in fresh water.

Development of Spirulina sea-weed raw extract/polyamidoamine hydrogel system as novel platform in photodynamic therapy: Photostability and photoactivity of chlorophyll a

The aim of this paper is to present and characterize Polyamidoamine-based hydrogels (PAA) as scaffolds to host photoactive Chlorophyll a (Chl a) from Spirulina (Arthrospira platensis) sea-weed Extract (SE), for potential applications in Photodynamic Therapy (PDT). The pigment extracted from SE was blended inside PAA without further purification, according to Green Chemistry principles. A comprehensive investigation of this hybrid platform, PAA/SE-based, was thus performed in our laboratory and, by means of Visible absorption and emission spectroscopies, the Chl a features, stability and photoactivity were studied. The obtained results evidenced the presence of two main Chl a forms, monomeric and dimeric, interacting with hydrogel polyamidoamines network. To better understand the nature of this interaction, the spectroscopic investigation of this system was performed both before and after the solidification of the hydrogel, that occurred at least in 24 h. Then, focusing the attention on solid scaffold, the ¹Chl a^⁎ fluorescence lifetime and FTIR-ATR analyses of PAA/SE were carried out, confirming the findings. The swelling and Point Zero Charge (PZC) measurements of solid PAA and PAA/SE were additionally performed to investigate the hydrogel behavior in water. Chl a molecules blended in PAA were (photo) stable and photoactive, and this latter feature was demonstrated showing that the pigment induced, when swelled in water and under irradiation, the formation of singlet oxygen (¹O₂), measured by direct and indirect methods.

Photosynthesis of the Cyanidioschyzon merolae cells in blue, red, and white light

Photosynthesis and respiration rates, pigment contents, CO2 compensation point, and carbonic anhydrase activity in Cyanidioschizon merolae cultivated in blue, red, and white light were measured. At the same light quality as during the growth, the photosynthesis of cells in blue light was significantly lowered, while under red light only slightly decreased as compared with white control. In white light, the quality of light during growth had no effect on the rate of photosynthesis at low O2 and high CO2 concentration, whereas their atmospheric level caused only slight decrease. Blue light reduced markedly photosynthesis rate of cells grown in white and red light, whereas the effect of red light was not so great. Only cells grown in the blue light showed increased respiration rate following the period of both the darkness and illumination. Cells grown in red light had the greatest amount of chlorophyll a, zeaxanthin, and β-carotene, while those in blue light had more phycocyanin. The dependence on O2 concentration of the CO2 compensation point and the rate of photosynthesis indicate that this alga possessed photorespiration. Differences in the rate of photosynthesis at different light qualities are discussed in relation to the content of pigments and transferred light energy together with the possible influence of related processes. Our data showed that blue and red light regulate photosynthesis in C. merolae for adjusting its metabolism to unfavorable for photosynthesis light conditions.

Small-scale distribution of microbes and biogeochemistry in the Great Barrier Reef

Microbial communities distribute heterogeneously at small-scales (mm-cm) due to physical, chemical and biological processes. To understand microbial processes and functions it is necessary to appreciate microbes and matter at small scales, however, few studies have determined microbial, viral, and biogeochemical distribution over space and time at these scales. In this study, the small-scale spatial and temporal distribution of microbes (bacteria and chlorophyll a), viruses, dissolved inorganic nutrients and dissolved organic carbon were determined at five locations (spatial) along the Great Barrier Reef (Australia), and over 4 consecutive days (temporal) at a coastal location. Our results show that: (1) the parameters show high small-scale heterogeneity; (2) none of the parameters measured explained the bacterial abundance distributions at these scales spatially or temporally; (3) chemical (ammonium, nitrate/nitrite, phosphate, dissolved organic carbon, and total dissolved nitrogen) and biological (chl a, and bacterial and viral abundances) measurements did not reveal significant relationships at the small scale; and (4) statistically significant differences were found between sites/days for all parameter measured but without a clear pattern.

Improved predictability of peak periphyton in rivers using site-specific accrual periods and long-term water quality datasets

Prevention of excessive periphyton standing crop (quantified as chlorophyll a) is among primary objectives for river management. Defensible instream nutrient criteria to achieve periphyton chlorophyll a targets at the site scale require robust predictive models. Such models have proved elusive because peak chlorophyll a depends on multiple factors in addition to nutrients. A key predictor may be accrual period, which depends on river flow variability and the flow magnitudes (effective flows, EF) at which periphyton biomass removal is initiated. In this study we used a seven-year dataset from 44 gravel-bed river sites in the Manawatū-Whanganui region, New Zealand, to explore the relative importance of accrual period, nutrients, and other variables in explaining peak chlorophyll a, using a regression approach. We also assessed the effect of combining data from multiple years. Previous empirical studies have used a universal flow metric (3 × median flow) to define accrual period (Da3). We calculated site-specific EF, which varied from 2 × to 15 × median flow. Accrual period based on EF (DaEF) outperformed Da3 in models. However, in the study region, more variance in chlorophyll a was explained by conductivity (EC) and dissolved inorganic nitrogen (DIN) than by DaEF. The best models derived from multi-year datasets included EC, DIN and DaEF as predictors and accounted for up to 82% of the variance in peak chlorophyll a. Models from annual data were weaker and more variable in strength and predictors. The models indicated that EC and DaEF should be considered when setting DIN criteria for periphyton outcomes in the study region. The principles we used in developing the models may have broad relevance to the management of periphyton in other regions.

The magnitude and drivers of harmful algal blooms in China's lakes and reservoirs: A national-scale characterization

Harmful algal blooms (HABs) can have dire repercussions on aquatic wildlife and human health, and may negatively affect recreational uses, aesthetics, taste, and odor in drinking water. The factors that influence the occurrence and magnitude of harmful algal blooms and toxin production remain poorly understood and can vary in space and time. It is within this context that we use machine learning (ML) and two 14-year (2005-2018) data sets on water quality and meteorological conditions of China's lakes and reservoirs to shed light on the magnitude and associated drivers of HAB events. General regression neural network (GRNN) models are developed to predict chlorophyll a concentrations for each lake and reservoir during two study periods (2005-2010 and 2011-2018). The developed models with an acceptable model fit are then analyzed by two indices to determine the areal HAB magnitudes and associated drivers. Our national assessment suggests that HAB magnitudes for China's lakes and reservoirs displayed a decreasing trend from 2006 (1363.3 km²) to 2013 (665.2 km²), and a slightly increasing trend from 2013 to 2018 (775.4 km²). Among the 142 studied lakes and reservoirs, most severe HABs were found in Lakes Taihu, Dianchi and Chaohu with their contribution to the total HAB magnitude varying from 89.2% (2013) to 62.6% (2018). HABs in Lakes Taihu and Chaohu were strongly associated with both total phosphorus and nitrogen concentrations, while our results were inconclusive with respect to the predominant environmental factors shaping the eutrophication phenomena in Lake Dianchi. The present study provides evidence that effective HAB mitigation may require both nitrogen and phosphorus reductions and longer recovery times; especially in view of the current climate-change projections. ML represents a robust strategy to elucidate water quality patterns in lakes, where the available information is sufficient to train the constructed algorithms. Our mapping of HAB magnitudes and associated environmental/meteorological drivers can help managers to delineate hot-spots at a national scale, and comprehensively design the best management practices for mitigating the eutrophication severity in China's lakes and reservoirs.

A simplified approach to detect a significant carbon dioxide reduction by phytoplankton in lakes and rivers on a regional and global scale

Carbon dioxide (CO₂) uptake by phytoplankton can significantly reduce the partial pressure of CO₂ (pCO₂) in lakes and rivers, and thereby CO₂ emissions. Presently, it is not known in which inland waters on Earth a significant pCO₂ reduction by phytoplankton is likely. Since detailed, comparable carbon budgets are currently not available for most inland waters, we modified a proxy to assess the pCO₂ reduction by phytoplankton, originally developed for boreal lakes, for application on a global scale. Using data from 61 rivers and 125 lakes distributed over five continents, we show that a significant pCO₂ reduction by phytoplankton is widespread across the temperate and sub-/tropical region, but absent in the cold regions on Earth. More specifically, we found that a significant pCO₂ reduction by phytoplankton might occur in 24% of the lakes in the temperate region, and 39% of the lakes in the sub-/tropical region. We also showed that such a reduction might occur in 21% of the rivers in the temperate region, and 5% of the rivers in the sub-/tropical region. Our results indicate that CO₂ uptake by phytoplankton is a relevant flux in regional and global carbon budgets. This highlights the need for more accurate approaches to quantify CO₂ uptake by primary producers in inland waters, particularly in the temperate and sub-/tropical region.

Decreasing underwater ultraviolet radiation exposure strongly driven by increasing ultraviolet attenuation in lakes in eastern and southwest China

Underwater light attenuation plays an important role in modulating aquatic ecosystems and is considered a sentinel of climate change and human activity. However, knowledge of the long-term exposure of underwater ultraviolet radiation (UVR) in aquatic ecosystem is still very limited. We carried out extensive UVR measurements in different seasons in five lakes at different altitudes, collected long-term Secchi disk depth (SDD) data, developed the models between UVR diffuse attenuation coefficient (K_d) and SDD, and further assessed the long-term underwater UVR exposure. Observation results from five lakes including 259 samples showed large spatial variabilities of K_d(313) (UVB) from 0.83 to 5.91 m^-1 and K_d(340) (UVA) from 0.51 to 4.67 m^-1. Chromophoric dissolved organic matter (CDOM) absorption coefficients were significantly correlated with K_d(313) and K_d(340). Thus, the effects of climate change and human activity on CDOM abundance, source and composition may significantly alter UVR attenuation in aquatic environments. The long-term underwater UVR exposure, which was estimated from significant positive correlations between 1/SDD and K_d(313) and K_d(340), and incident UVR, significantly decreased in Lake Fuxianhu, Lake Erhai, and Lake Qiandaohu. The regime shift from clear water state to turbid state in Lake Erhai around 2001-2003 dramatically decreased underwater UVR exposure. In conclusion, increasing UVR attenuation played a more important role in determining underwater UVR exposure than decreasing incident UVR with the relative contributions of 89.9% and 87.7% in Lake Fuxianhu, 98.0% and 97.7% in Lake Erhai, 94.4% and 92.5% in Lake Qiandaohu for UVB and UVA exposure, respectively. This is the first study to elucidate the long-term trend of underwater UVR exposure considering both increasing UVR attenuation and decreasing incident UVR.