Competition for nutrients and light: testing advances in resource competition with a natural phytoplankton community

Environmental modulators of algae-bacteria interactions at scale

Interactions between photosynthetic and heterotrophic microbes play a key role in global primary production. Understanding phototroph-heterotroph interactions remains challenging because these microbes reside in chemically complex environments. Here, we leverage a massively parallel droplet microfluidic platform that enables us to interrogate interactions between photosynthetic algae and heterotrophic bacteria in >100,000 communities across ∼525 environmental conditions with varying pH, carbon availability, and phosphorus availability. By developing a statistical framework to dissect interactions in this complex dataset, we reveal that the dependence of algae-bacteria interactions on nutrient availability is strongly modulated by pH and buffering capacity. Furthermore, we show that the chemical identity of the available organic carbon source controls how pH, buffering capacity, and nutrient availability modulate algae-bacteria interactions. Our study reveals the previously underappreciated role of pH in modulating phototroph-heterotroph interactions and provides a framework for thinking about interactions between phototrophs and heterotrophs in more natural contexts.

From disruption to adaptation: Response of phytoplankton communities in representative impounded lakes to China's South-to-North Water Diversion Project

Impounded lakes are often interconnected in large-scale water diversion projects to form a coordinated system for water allocation and regulation. The alternating runoff and transferred water can significantly impact local ecosystems, which are initially reflected in the sensitive phytoplankton. Nonetheless, limited information is available on the temporal dynamics and assembly patterns of phytoplankton community in impounded lakes responding to continuous and periodic water diversion. Herein, a long-term monitoring from 2013 to 2020 were conducted to systematically investigate the response of phytoplankton community, including its characteristics, stability, and the ecological processes governing community assembly, in representative impounded lakes to the South-to-North Water Diversion Project (SNWDP) in China. In the initial stage of the SNWDP, the phytoplankton diversity indices experienced a decrease during both non-water diversion periods (8.5 %∼21.2 %) and water diversion periods (5.6 %∼12.2 %), implying a disruption in the aquatic ecosystem. But the regular delivery of high-quality water from the Yangtze River gradually increased phytoplankton diversity and mediated ecological assembly processes shifting from stochastic to deterministic. Meanwhile, reduced nutrients restricted the growth of phytoplankton, pushing species to interact more closely to maintain the functionality and stability of the co-occurrence network. The partial least squares path model revealed that ecological process (path coefficient = 0.525, p < 0.01) and interspecies interactions in networks (path coefficient = -0.806, p < 0.01) jointly influenced the keystone and dominant species, ultimately resulting in an improvement in stability (path coefficient = 0.878, p < 0.01). Overall, the phytoplankton communities experienced an evolutionary process from short-term disruption to long-term adaptation, demonstrating resilience and adaptability in response to the challenges posed by the SNWDP. This study revealed the response and adaptation mechanism of phytoplankton communities in impounded lakes to water diversion projects, which is helpful for maintaining the lake ecological health and formulating rational water management strategies.

Image-derived indicators of phytoplankton community responses to Pseudo-nitzschia blooms

Phytoplankton populations in the natural environment interact with each other. Despite rising global concern with Pseudo-nitzschia blooms, which can produce the potent neurotoxin domoic acid, we still do not fully understand how other phytoplankton genera respond to the presence of Pseudo-nitzschia. Here, we used a 4-year high-resolution imaging dataset for 9 commonly found phytoplankton genera in Narragansett Bay, alongside environmental data, to identify potential interactions between phytoplankton genera and their response to elevated Pseudo-nitzschia abundance. Our results indicate that Pseudo-nitzschia tends to bloom either concurrently with or right after other phytoplankton genera. Such bloom periods coincide with higher water temperatures and lower salinity. Pseudo-nitzschia image abundance tends to increase the most from March-May and peaks during May-Jun, whereas the image-derived biovolume and width of Pseudo-nitzschia chains increase the most during Jan-Feb. For most phytoplankton genera, their relationship with Pseudo-nitzschia abundance is noticeably different from their relationship with Pseudo-nitzschia image features. Despite the complexity in the phytoplankton community, our analysis suggests several ecological indicators that may be used to determine the risk of harmful algal blooms.

Explaining the mechanisms behind niche dimensionality and light-driving species diversity based on functional traits

Two prevalent ecological mechanisms, niche dimensionality and light asymmetry, may well explain species loss with fertilization gradients in grassland communities. Although there is still controversy surrounding the two competitive mechanisms that maintain species coexistence, few studies have examined the patterns of change in dissimilarity in species composition (β-diversity) and the relative explanatory contributions of plant functional traits to α- and β-diversity when multiple resources are added. To clarify this knowledge gap, we conducted a 6-year experiment of resource addition in an alpine meadow on the Qinghai-Tibet Plateau to assess how species richness and spatial β-diversity are affected by increasing numbers of added resources (NAR) and light limitation. Our results found that both NAR and light limitation led to decreased species richness, suggesting that niche dimensionality and light asymmetry may contribute equally to species loss, rather than either alone. Moreover, NAR is the primary factor responsible for the increase in β-diversity, which exhibits a negative relationship with species richness. Furthermore, the increase in height is the most likely explanation for β-diversity, while the increase in SLA is the most likely explanation for species richness, thereby indicating the changes in species richness and composition can be effectively explained by the response of certain morphological functional traits with the addition of multiple resources. Future research should focus on the complex interactions of different ecological mechanisms that contribute to the maintenance of biodiversity in grassland ecosystems all over the world.

Ignited competition: Impact of bioactive extracellular compounds on organelle functions and photosynthetic systems in harmful algal blooms

Prevalent interactions among marine phytoplankton triggered by long-range climatic stressors are well-known environmental disturbers of community structure. Dynamic response of phytoplankton physiology is likely to come from interspecies interactions rather than direct climatic effect on single species. However, studies on enigmatic interactions among interspecies, which are induced by bioactive extracellular compounds (BECs), especially between related harmful algae sharing similar shellfish toxins, are scarce. Here, we investigated how BECs provoke the interactions between two notorious algae, Alexandrium minutum and Gymnodinium catenatum, which have similar paralytic shellfish toxin (PST) profiles. Using techniques including electron microscopy and transcriptome analysis, marked disruptions in G. catenatum intracellular microenvironment were observed under BECs pressure, encompassing thylakoid membrane deformations, pyrenoid matrix shrinkage and starch sheaths disappearance. In addition, the upregulation of gene clusters responsible for photosystem-I Lhca1/4 and Rubisco were determined, leading to weaken photon captures and CO2 assimilation. The redistribution of lipids and proteins occurred at the subcellular level based on in situ focal plane array FTIR imaging approved the damages. Our findings illuminated an intense but underestimated interspecies interaction triggered by BECs, which is responsible for dysregulating photosynthesis and organelle function in inferior algae and may potentially account for fitness alteration in phytoplankton community.

Nitrogen enrichment changed the biogeochemical role of sesarmid crabs by shifting their diets in tropical mangrove ecosystems

Sesarmid crabs modulate nutrient dynamics of tropical mangroves through their leaf-eating habit. How N enrichment may alter this regulatory role, and the implications for mangrove nutrient dynamics, remain unclear. Using a mesocosm experiment, we tested how N enrichment could change the microphytobenthos (MPB) communities, thus modifying the crabs' diet and their role in nutrient dynamics. The factorial experiment combined with field investigation revealed a significant increase in the relative abundance of cyanobacteria. Stable isotope analysis suggested that the main carbon source of crabs shifted from leaf litter to cyanobacteria in mesocosms under both high (20×) and low (2×) N enrichment treatments. The significantly lower total cellulase activity of crabs in the mesocosms might explain the decreased carbon assimilation from leaf litter. The changes in the MPB and the microbiome with N enrichment in the presence of crabs may drive significantly higher carbon processing rate in tropical mangroves.

Differential Expression of Stress Adaptation Genes in a Diatom Ulnaria acus under Different Culture Conditions

Diatoms are a group of unicellular eukaryotes that are essential primary producers in aquatic ecosystems. The dynamic nature of their habitat necessitates a quick and specific response to various stresses. However, the molecular mechanisms of their physiological adaptations are still underexplored. In this work, we study the response of the cosmopolitan freshwater diatom Ulnaria acus (Bacillariophyceae, Fragilariophycidae, Licmophorales, Ulnariaceae, Ulnaria) in relation to a range of stress factors, namely silica deficiency, prolonged cultivation, and interaction with an algicidal bacterium. Fluorescent staining and light microscopy were used to determine the physiological state of cells under these stresses. To explore molecular reactions, we studied the genes involved in the stress response-type III metacaspase (MC), metacaspase-like proteases (MCP), death-specific protein (DSP), delta-1-pyrroline-5-carboxylate dehydrogenase (ALDH12), and glutathione synthetase (GSHS). We have described the structure of these genes, analyzed the predicted amino acid sequences, and measured their expression dynamics in vitro using qRT-PCR. We demonstrated that the expression of UaMC1, UaMC3, and UaDSP increased during the first five days of silicon starvation. On the seventh day, it was replaced with the expression of UaMC2, UaGSHS, and UaALDH. After 45 days of culture, cells stopped growing, and the expression of UaMC1, UaMC2, UaGSHS, and UaDSP increased. Exposure to an algicidal bacterial filtrate induced a higher expression of UaMC1 and UaGSHS. Thus, we can conclude that these proteins are involved in diatoms' adaptions to environmental changes. Further, these data show that the molecular adaptation mechanisms in diatoms depend on the nature and exposure duration of a stress factor.

Designing More Informative Multiple-Driver Experiments

For decades, multiple-driver/stressor research has examined interactions among drivers that will undergo large changes in the future: temperature, pH, nutrients, oxygen, pathogens, and more. However, the most commonly used experimental designs-present-versus-future and ANOVA-fail to contribute to general understanding or predictive power. Linking experimental design to process-based mathematical models would help us predict how ecosystems will behave in novel environmental conditions. We review a range of experimental designs and assess the best experimental path toward a predictive ecology. Full factorial response surface, fractional factorial, quadratic response surface, custom, space-filling, and especially optimal and sequential/adaptive designs can help us achieve more valuable scientific goals. Experiments using these designs are challenging to perform with long-lived organisms or at the community and ecosystem levels. But they remain our most promising path toward linking experiments and theory in multiple-driver research and making accurate, useful predictions.

Nutrition of Escherichia coli within the intestinal microbiome

In this chapter, we update our 2004 review of "The Life of Commensal Escherichia coli in the Mammalian Intestine" (https://doi.org/10.1128/ecosalplus.8.3.1.2), with a change of title that reflects the current focus on "Nutrition of E. coli within the Intestinal Microbiome." The earlier part of the previous two decades saw incremental improvements in understanding the carbon and energy sources that E. coli and Salmonella use to support intestinal colonization. Along with these investigations of electron donors came a better understanding of the electron acceptors that support the respiration of these facultative anaerobes in the gastrointestinal tract. Hundreds of recent papers add to what was known about the nutrition of commensal and pathogenic enteric bacteria. The fact that each biotype or pathotype grows on a different subset of the available nutrients suggested a mechanism for succession of commensal colonizers and invasion by enteric pathogens. Competition for nutrients in the intestine has also come to be recognized as one basis for colonization resistance, in which colonized strain(s) prevent colonization by a challenger. In the past decade, detailed investigations of fiber- and mucin-degrading anaerobes added greatly to our understanding of how complex polysaccharides support the hundreds of intestinal microbiome species. It is now clear that facultative anaerobes, which usually cannot degrade complex polysaccharides, live in symbiosis with the anaerobic degraders. This concept led to the "restaurant hypothesis," which emphasizes that facultative bacteria, such as E. coli, colonize the intestine as members of mixed biofilms and obtain the sugars they need for growth locally through cross-feeding from polysaccharide-degrading anaerobes. Each restaurant represents an intestinal niche. Competition for those niches determines whether or not invaders are able to overcome colonization resistance and become established. Topics centered on the nutritional basis of intestinal colonization and gastrointestinal health are explored here in detail.

Mangrove interaction with saltmarsh varies at different life stages

Mangroves and saltmarshes are two of the most relevant coastal habitats for humans. These ecosystems offer several services like coastal protection, climate mitigation, and nursery habitats for many artisanal and commercially exploited fish, crabs, and shellfish. They mostly dominate different latitudinal ranges but in several places around the world they co-occur and interact. Here, we summarize the current scientific knowledge on mangrove-saltmarsh ecological interactions and propose a conceptual model. We screened 1410 articles from 1945 to 2022 and selected 29 experiments that assessed mangrove-saltmarsh ecological interactions. Both positive and negative interactions are observed but there is variation along different mangrove life stages. Higher retention and establishment of mangrove propagules are found inside saltmarshes than on bare flats, i.e. facilitation, and these effects are higher at grass than at succulent saltmarsh species. Mangrove seedlings, saplings, or trees mostly compete with saltmarshes, negatively affecting mangrove growth. We propose a model with different outcomes considering the interaction between different mangrove's life stages and saltmarsh forms and discussed these interactions in the light of anthropogenic threats and climate change.

Food web structure for high carbon retention in marine plankton communities

Total annual net primary productions in marine and terrestrial ecosystems are similar. However, a large portion of the newly produced marine phytoplankton biomass is converted to carbon dioxide because of predation. Which food web structure retains high carbon biomass in the plankton community in the global ocean? In 6954 individual samples or locations containing phytoplankton, unicellular protozooplankton, and multicellular metazooplankton in the global ocean, phytoplankton-dominated bottom-heavy pyramids held higher carbon biomass than protozooplankton-dominated middle-heavy diamonds or metazooplankton-dominated top-heavy inverted pyramids. Bottom-heavy pyramids predominated, but the high predation impact by protozooplankton on phytoplankton or the vertical migration of metazooplankton temporarily changed bottom-heavy pyramids to middle-heavy diamonds or top-heavy inverted pyramids but returned to bottom-heavy pyramids shortly. This finding has profound implications for carbon retention by plankton communities in the global ocean.

Colonial morphology weakens the response of different inorganic carbon uptake systems to CO2 levels in Microcystis population

Rising atmospheric CO2 concentration negatively impacts aquatic ecosystems and may induce evolutionary changes in the CO2-concentrating mechanism (CCM) of cyanobacteria. As the most notorious freshwater cyanobacteria, Microcystis strains have high phenotypic plasticity to form colonies and blooms in lakes and reservoirs worldwide. However, phenotypic plasticity of Microcystis responses to elevated CO2 is still a major open question. Here, we studied how Microcystis strains with two genotype of inorganic carbon uptake systems, bicA and sbtA, and different colonial morphology response to 200 ppm, 400 ppm, and 800 ppm CO2 levels. The results revealed that sbtA genotypes showed significantly higher specific growth rates, Chl a concentration, and photosynthetic efficiency at 200 ppm CO2, whereas higher specific growth rates, Chl a concentration, and photosynthetic efficiency were found in bicA genotype at 800 ppm CO2. The highest values of specific growth rates, Chl a concentration, Fv/Fm, and maximal net photosynthesis (Pm) were observed in unicellular morphology, followed by small colony and large colonial morphology at all CO2 levels. The values of K0.5 (DIC), K0.5 (CO2), and K0.5 (HCO3-) in the large colonials increased with rising CO2 levels, but these values significantly decreased in the unicellular and small colonials. ANOSIM analysis indicated that colonial morphology reduced significantly inter-group differences between bicA and sbtA genotypes at all CO2 treatments. These results suggest that colonial morphology of Microcystis can weakens the response of different inorganic carbon uptake systems to CO2 levels. Moreover, phenotypic and genotypic plasticity is likely to broaden strongly the fitness of Microcystis from rising atmospheric CO2.

Periodic temporal environmental variations induce coexistence in resource competition models

Natural ecosystems, in particular on the microbial scale, are inhabited by a large number of species. The population size of each species is affected by interactions of individuals with each other and by spatial and temporal changes in environmental conditions, such as resource abundance. Here, we use a generic population dynamics model to study how, and under what conditions, a periodic temporal environmental variation can alter an ecosystem's composition and biodiversity. We demonstrate that using timescale separation allows one to qualitatively predict the long-term population dynamics of interacting species in varying environments. We show that the notion of Tilman's R* rule, a well-known principle that applies for constant environments, can be extended to periodically varying environments if the timescale of environmental changes (e.g., seasonal variations) is much faster than the timescale of population growth (doubling time in bacteria). When these timescales are similar, our analysis shows that a varying environment deters the system from reaching a steady state, and stable coexistence between multiple species becomes possible. Our results posit that biodiversity can in part be attributed to natural environmental variations.

Characterization of patterns and variability in the dynamics of outdoor aquatic mesocosms: exploring the capabilities and challenges in data supporting aquatic system models

Aquatic mesocosms are complex test systems used within regulatory risk assessment of plant protection products. These model ecosystems allow researchers to capture interactions of multiple species under realistic environmental conditions. They enable assessment of direct and indirect effects of stressors at all trophic levels (i.e., from primary producers to secondary consumers) and impacts on ecosystem functions. Due to the limited ability to test the multitude of potential exposure scenarios, cross-linking aquatic mesocosm studies with virtual mesocosms, i.e., aquatic system models (ASMs), can serve to meet the demand for more environmental realism and ecological relevance in risk assessment. In this study, full control data sets from seven aquatic mesocosm studies conducted at a single test facility under GLP were analysed graphically and using descriptive statistics. Thereby, not only a comprehensive data base but also an insight into the species present, their dynamics over time, and variability in unchallenged mesocosms was observed. While consistency in dynamics could be discerned for physical and chemical parameters, variability was evident for several biological endpoints. This variability points to amplification of small differences over time as well as to stochastic processes. The outline of existing gaps and uncertainties in data leads to the estimation of what can be expected to be captured and predicted by ASMs.

Investigation of a nutrient-plankton model with stochastic fluctuation and impulsive control

In this paper, we investigate a stochastic nutrient-plankton model with impulsive control of the nutrient concentration and zooplankton population. Analytically, we find that the population size is nonnegative for a sufficiently long time. We derive some sufficient conditions for the existence of stable periodic oscillations, which indicate that the plankton populations will behave periodically. The numerical results show that the plankton system experiences a transition from extinction to the coexistence of species due to the emergence of impulsive control. Additionally, we observe that the nutrient pulse has a stronger relationship with phytoplankton growth than the zooplankton pulse. Although the frequency of impulsive control and appropriate environmental fluctuations can promote the coexistence of plankton populations, an excessive intensity of noise can result in the collapse of the entire ecosystem. Our findings may provide some insights into the relationships among nutrients, phytoplankton and zooplankton in a stochastic environment.

Assessing the algal population dynamics using multiple machine learning approaches: Application to Macao reservoirs

The quality of reservoir water is important to the health and wellbeing of human and animals. Eutrophication is one of the most serious problems threatening the safety of reservoir water resource. Machine learning (ML) approaches are effective tools to understand and evaluate various environmental processes of concern, such as eutrophication. However, limited studies have compared the performances of different ML models to reveal algal dynamics using time-series data of redundant variables. In this study, the water quality data from two reservoirs in Macao were analyzed by adopting various ML approaches, including stepwise multiple linear regression (LR), principal component (PC)-LR, PC-artificial neuron network (ANN) and genetic algorithm (GA)-ANN-connective weight (CW) models. The influence of water quality parameters on algal growth and proliferation in two reservoirs was systematically investigated. The GA-ANN-CW model demonstrated the best performance in reducing the size of data and interpreting the algal population dynamics data, which displayed higher R-squared, lower mean absolute percentage error and lower root mean squared error values. Moreover, the variable contribution based on ML approaches suggest that water quality parameters, such as silica, phosphorus, nitrogen, and suspended solid have a direct impact on algal metabolisms in two reservoirs' water systems. This study can expand our capacity in adopting ML models in predicting algal population dynamics based on time-series data of redundant variables.

Niche differentiation in the light spectrum promotes coexistence of phytoplankton species: a spatial modelling approach

The paradox of the plankton highlights the apparent contradiction between Gause's law of competitive exclusion and the observed diversity of phytoplankton. It is well known that phytoplankton dynamics depend heavily on light availability. Here we treat light as a continuum of resources rather than a single resource by considering the visible light spectrum. We propose a spatially explicit reaction-diffusion-advection model to explore under what circumstance coexistence is possible from mathematical and biological perspectives. Furthermore, we provide biological context as to when coexistence is expected based on the degree of niche differentiation within the light spectrum and overall turbidity of the water.

Colony formation of Phaeocystis globosa: A case study of evolutionary strategy for competitive adaptation

Some algae possess a multi-morphic life cycle, either in the form of free-living solitary cells or colonies which constantly occur in algal blooms. Though colony formation seems to consume extra energy and materials, many algae tend to outbreak in form of colonies. Here, we hypothesized that colony formation is a selected evolutionary strategy to improve population competitiveness and environmental adaptation. To test the hypothesis, different sizes of colonies and solitary cells in a natural bloom of Phaeocystis globosa were investigated. The large colony showed a relatively low oxidant stress level, a nutrient trap effect, and high nutrient use efficiency. The colonial nitrogen and phosphorus concentrations were about 5-10 times higher than solitary cell phycosphere and cellular nutrient allocation decreased with the enlargement of the colonial diameter following the economies of scale law. These features provide the colony with monopolistic competence and could function as an evolutionary strategy for competitive adaptation.

Effect of Interactions between Phosphorus and Light Intensity on Metabolite Compositions in Tea Cultivar Longjing43

Light intensity influences energy production by increasing photosynthetic carbon, while phosphorus plays an important role in forming the complex nucleic acid structure for the regulation of protein synthesis. These two factors contribute to gene expression, metabolism, and plant growth regulation. In particular, shading is an effective agronomic practice and is widely used to improve the quality of green tea. Genotypic differences between tea cultivars have been observed as a metabolic response to phosphorus deficiency. However, little is known about how the phosphorus supply mediates the effect of shading on metabolites and how plant cultivar gene expression affects green tea quality. We elucidated the responses of the green tea cultivar Longjing43 under three light intensity levels and two levels of phosphorus supply based on a metabolomic analysis by GC×GC-TOF/MS (Two-dimensional Gas Chromatography coupled to Time-of-Flight Mass Spectrometry) and UPLC-Q-TOF/MS (Ultra-Performance Liquid Chromatography-Quadrupole-Time of Flight Mass Spectrometry), a targeted analysis by HPLC (High Performance Liquid Chromatography), and a gene expression analysis by qRT-PCR. In young shoots, the phosphorus concentration increased in line with the phosphate supply, and elevated light intensities were positively correlated with catechins, especially with epigallocatechin of Longjing43. Moreover, when the phosphorus concentration was sufficient, total amino acids in young shoots were enhanced by moderate shading which did not occur under phosphorus deprivation. By metabolomic analysis, phenylalanine, tyrosine, and tryptophan biosynthesis (PTT) were enriched due to light and phosphorus effects. Under shaded conditions, SPX2 (Pi transport, stress, sensing, and signaling), SWEET3 (bidirectional sugar transporter), AAP (amino acid permeases), and GSTb (glutathione S-transferase b) shared the same analogous correlations with primary and secondary metabolite pathways. Taken together, phosphorus status is a crucial factor when shading is applied to increase green tea quality.

Critical role of parasite-mediated energy pathway on community response to nutrient enrichment

Parasites form an integral part of food webs, however, they are often ignored in classic food web theory or limited to the investigation of trophic transmission pathways. Specifically, direct consumption of parasites by nonhost predators is rarely considered, while it can contribute substantially to energy flow in food webs. In aquatic systems, chytrids constitute a major group of fungal parasites whose free-living infective stages (zoospores) form a highly nutritional food source to zooplankton. Thereby, the consumption of zoospores can create an energy pathway from otherwise inedible phytoplankton to zooplankton ("mycoloop"). This parasite-mediated energy pathway might be of special importance during phytoplankton blooms dominated by inedible or toxic primary producers like cyanobacteria, which are on the rise with eutrophication and global warming. We theoretically investigated community dynamics and energy transfer in a food web consisting of an edible nonhost and an inedible host phytoplankton species, a parasitic fungus, and a zooplankton species grazing on edible phytoplankton and fungi. Food web dynamics were investigated along a nutrient gradient contrasting nonadaptive zooplankton species representative for filter feeders like cladocerans and zooplankton with the ability to actively adapt their feeding preferences like many copepod species. Overall, the importance of the mycoloop for zooplankton increases with nutrient availability. This increase is smooth for nonadaptive consumers. For adaptive consumers, we observe an abrupt shift from an almost exclusive preference for edible phytoplankton at low nutrient levels to a strong preference for parasitic fungi at high nutrient levels. The model predicts that parasitic fungi could contribute up to 50% of the zooplankton diet in nutrient-rich environments, which agrees with empirical observations on zooplankton gut content from eutrophic systems during blooms of inedible diatoms or cyanobacteria. Our findings highlight the role of parasite-mediated energy pathways for predictions of energy flow and community composition under current and future environmental change.

Unravelling nutrient fate and CO2 concentrations in the reservoirs of the Seine Basin using a modelling approach

Reservoirs are active reactors for the biogeochemical cycling of carbon (C) and nutrients (nitrogen: N, phosphorus: P, and silica: Si), however, our in-depth understanding of C and nutrient cycling in reservoirs is still limited by the fact that it involves a variety of closely linked and coupled biogeochemical and hydrological processes. In this study, the updated process-based Barman model was applied to three reservoirs of the Seine Basin during 2019-2020, considering the variations of carbon dioxide (CO₂) concentrations and key water quality variables. The model simulations captured well the observed seasonal variations of water quality variables, although discrepancies remained for some variables. According to the model, we found that: (1) the three reservoirs are autotrophic ecosystems and showed high removal efficiency of dissolved inorganic carbon and nutrients during 2019-2020; (2) phytoplankton assimilation, benthic denitrification, precipitation and dissolution of calcium carbonate, and gas exchange at the water-air interface are the dominant processes for water quality variations in reservoirs; (3) based on scenarios results, trophic state and mean water depth of reservoir would impact the biogeochemical processes and the retention efficiency of nitrate and dissolved silicate. Finally, we expect that the successful application of Barman model in the reservoirs of the Seine Basin could provide a useful tool for simulating reservoir water quality changes and thus evaluating the impacts of reservoirs on downstream water quality.

Adverse role of colonial morphology and favorable function of microcystins for Microcystis to compete with Scenedesmus

In eutrophic freshwaters, Microcystis usually becomes dominant in phytoplankton communities due to the synergistic effects of its special eco-physiological traits and environmental factors. Colonial morphology can protect Microcystis from zooplankton grazing, which indirectly favors Microcystis to outcompete other phytoplankton, although the colonial form is not conducive to the absorption of nutrients. Moreover, unicellular Microcystis usually has competitive advantages over other phytoplankton due to its efficient absorption capacity for nutrients and releasing microcystins. However, the consequence of direct competition between toxic colonial Microcystis and green algae without external grazing pressure still remained unknown. In this study, the competition between toxic colonial Microcystis aeruginosa and a common green alga Scenedesmus obliquus was explored. Results showed that: (1) colonial M. aeruginosa had a higher requirement for key macro-nutrient phosphorus than S. obliquus, and thus its population declined and was replaced by S. obliquus eventually; (2) microcystins released by colonial M. aeruginosa inhibited the photosynthetic activity and growth of S. obliquus at early stage of the competition; (3) the photosynthetic potential of colonial M. aeruginosa was stimulated in response to the competitive stress from S. obliquus, although the population of colonial M. aeruginosa declined eventually; (4) microcystin production of colonial M. aeruginosa was enhanced by phosphorus limitation due to S. obliquus competition and was positively related to photosynthetic potential of colonial M. aeruginosa. These results indicated that, in the absence of complex natural environment, colonial Microcystis cannot outcompete Scenedesmus in a pure competition, although microcystins can play a favorable role in the competition, which clarified the opposite role of colonies and microcystins in the competition of colonial Microcystis against other phytoplankton.

Ultrastructure of setae of a planktonic diatom, Chaetoceros coarctatus

Silica frustules of most planktonic diatoms have many shallow holes in which the length (L) is smaller than the width (W). The present study focuses on a silica ultrastructure of setae of a planktonic diatom having deep (L/W > 1) holes. Here, we characterized microscopically patterned nanoholes on the silica walls of thick, robust, and hollow setae of a colony of Chaetoceros coarctatus. Basically, tetragonal poroid arrangements with and without a costa pattern are observed on the inner and outer surfaces, respectively, for three kinds of curving hollow setae attached to the anterior, intercalary, and posterior parts of the colony. The seta structures including specific poroid arrangements and continuity of deep nanoholes depend on the location. The deep nanoholes ∼90 nm wide are elongated from 150 to 1500 nm (L/W ∼17) with an increase in the wall thickness of the polygonal tubes of the setae. The inside poroid array, with a period of 190 nm in the extension direction of setae, is lined by parallel plates of the costae. However, the poroid arrangement on the outer surface is disordered, with several holes obstructed with increasing wall thickness of the posterior terminal setae. According to the movement of a colony in a fluid microchannel, the thick curving terminal setae is suggested to involve attitude control and mechanical protection. Using an optical simulation, the patterned deep through-holes on the intercalary setae were suggested to contribute anti-reflection of blue light in the wavelength range of 400 to 500 nm for the promotion of photosynthesis in seawater.

Picocyanobacterial Synechococcus in marine ecosystem: Insights from genetic diversity, global distribution, and potential function

Marine Synechococcus, a main group of picocyanobacteria, has been ubiquitously observed across the global oceans. Synechococcus exhibits high phylogenetical and phenotypical diversity, and horizontal gene transfer makes its genetic evolution much more intricate. With the development of measurement technologies and analysis methods, the genomic information and niche partition of each Synechococcus lineage tend to be precisely described, but the global analysis is still lacking. Therefore, it is necessary to summarize existing studies and integrate published data to gain a comprehensive understanding of Synechococcus on genetic variation, niche division, and potential functions. In this review, the maximum likelihood trees are constructed based on existing sequence data, including both phylogenetic and pigmentary gene markers. The global distribution characteristics of abundance, lineages, and pigment types are concluded through pooled analysis of more than 700 samples obtained from approximately 50 scientific research cruises. The potential functions of Synechococcus are explored in element cycles and biological interactions. Future work on Synechococcus is suggested to focus on not only elucidating the nature of Synechococcus biodiversity but also demonstrating its interactions with the ecosystem by combining bioinformatics and macroscopic isotope-labeled environmental parameters.

Seaweed Fertilizer Prepared by EM-Fermentation Increases Abundance of Beneficial Soil Microbiome in Paddy (Oryza sativa L.) during Vegetative Stage

Excessive use of chemical fertilizer could potentially decrease soil productivity by decreasing soil microbiome diversity. In this study, we evaluated the effects of fermented seaweed fertilizer in the soil microbial community of paddy plants (Oryza sativa L.). The paddy seedlings were divided into five groups, control (C0), chemical fertilizer (CF), seaweed fertilizer (SF), chemical and seaweed fertilizer combination 50:50 (CFSF1), and chemical and fertilizer combination 75:25 (CFSF2). The CFSF1 combination showed to be the most effective in inducing plant height (83.99 ± 3.70 cm) and number of tillers (24.20 ± 4.08). After 8 weeks after transplantation, the isolated DNA from each soil treatment were subjected to 16S rRNA (v3–v4 region) next-generation sequencing. The beneficial Acidobacteriota was most abundant in CFSF1. At genus level, the nitrifying bacteria MND1 was seen to be abundant in CFSF1 and also present in other SF treatments. The genus Chujaibacter is highly abundant in CF, which potentially plays a role in denitrification resulting in soil degradation. In addition, the CFSF1-treated soils show significantly higher diversity of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA). The current results could potentially contribute to the utilization of SF as a bioremediator and promoting green agriculture practice by reducing the amount of CF usage.

Functional Responses of Phytoplankton Assemblages to Watershed Land Use and Environmental Gradients

Watershed land-use changes have been identified as major threats to lake fauna, subsequently affecting ecosystem functioning. In this study, the functional-based approach was used to examine the effects of land use and environmental changes on phytoplankton communities in four selected lakes in Northeast China. We also identified the sensitive functional traits as indicators of environmental stressors. The integration of RLQ analysis with the fourth-corner approach significantly identified five of 18 functional trait categories, including flagella, filamentous, unicellular, mixotrophic, and chlorophyll c, as potential indicators to changes in watershed land-use intensity and environmental gradients. Significant relationships between traits and land use and water quality highlighted the consequential indirect impact of extensive agricultural and urban development on phytoplankton via allochthonous nutrient inputs and various contaminants. In addition, the functional richness of phytoplankton assemblages generally increased along with surface area and forests, but decreased along with intensive agricultural and urban land use, implying that functional homogenization may cause a reduction in ecosystem productivity and reliability to land-use intensity. Given the superior performance of the functional-based approach, our findings also highlighted the importance of the application of both the biological traits and functional diversity index in monitoring programs for lake ecosystems.

Biodiversity promotes ecosystem functioning despite environmental change

Three decades of research have demonstrated that biodiversity can promote the functioning of ecosystems. Yet, it is unclear whether the positive effects of biodiversity on ecosystem functioning will persist under various types of global environmental change drivers. We conducted a meta‐analysis of 46 factorial experiments manipulating both species richness and the environment to test how global change drivers (i.e. warming, drought, nutrient addition or CO₂ enrichment) modulated the effect of biodiversity on multiple ecosystem functions across three taxonomic groups (microbes, phytoplankton and plants). We found that biodiversity increased ecosystem functioning in both ambient and manipulated environments, but often not to the same degree. In particular, biodiversity effects on ecosystem functioning were larger in stressful environments induced by global change drivers, indicating that high‐diversity communities were more resistant to environmental change. Using a subset of studies, we also found that the positive effects of biodiversity were mainly driven by interspecific complementarity and that these effects increased over time in both ambient and manipulated environments. Our findings support biodiversity conservation as a key strategy for sustainable ecosystem management in the face of global environmental change.

Successful microbial colonization of space in a more dispersed manner

Many organisms live in habitats with limited nutrients or space, competition for these resources is ubiquitous. Although spatial factors related to the population's manner of colonizing space influences its success in spatial competition, what these factors are and to what extent they influence the outcome remains underexplored. Here, we applied a simulated competitive model to explore the spatial factors affecting outcomes of competition for space. By quantifying spatial factors, we show that colonizing space in a more dispersed manner contributes to microbial competitive success. We also find that the competitive edge deriving from a more dispersed manner in colonization can compensate for the disadvantage arising from either a lower growth rate or lower initial abundance. These findings shed light on the role of space colonization manners on maintaining biodiversity within ecosystems and provide novel insights critical for understanding how competition for space drives evolutionary innovation.

High forest stand density exacerbates growth decline of conifers driven by warming but not broad-leaved trees in temperate mixed forest in northeast Asia

Increasing temperature over recent decades is expected to positively impact tree growth in humid regions. However, high stand density could increase the negative effects of warming-induced drought through inter-tree competition. How neighborhood competition impacts tree growth responding to climate change remains unclear. Here, we utilized the Changbai Mountain region in northeastern Asia as our study area. We quantified individual tree growth using tree-ring samples collected from three dominant tree species growing in three forest stand density levels. We estimated the effects of climate warming and forest stand density on growth processes and tested for a species-specific response to climate. Our results demonstrated that overall 25% of Korean pine, but only ~3% of Mongolian oak and ~ 4% of Manchurian ash experienced growth reduction. Increased forest density can also exacerbate growth reduction. We identified a climate turning point in 1984, where warming rapidly increased, and defined two groups, "enhance group" (EG) and "decline group" (DG), according to the individual tree growth trend after 1984. For the EG, climate warming increased temperature sensitivity, but the temperature sensitivity declined with increasing stand density for the whole study period. For the DG, tree growth sensitivity shifted from temperature to precipitation after 1984, driven by increased competition pressure under climate warming. Our study concludes that growth decline from warming-induced drought might be amplified by high forest stand density, was especially pronounced in conifer trees.

Impact of light quality on freshwater phytoplankton community in outdoor mesocosms

In shallow lakes, wind wave turbulence alters underwater spectral composition, but the influence of this phenomenon on phytoplankton community structure is poorly understood. We used 100L mesocosms to investigate the influence of light quality on a natural phytoplankton community collected from Taihu Lake in China. The communities in mesocosms were exposed to sunlight filtered for white, blue, green, and red light, while wave-making pumps simulated wind wave turbulence similar to Taihu Lake. Over the course of experiment, each filtered light reduced the total phytoplankton abundance compared to white light. The mean abundance of phytoplankton in controls was 1.72, 1.78, and 7.89 times of that in the red, blue, and green light treatments. Red, blue, and green light significantly promoted the growth of cyanobacteria, green algae, and diatoms, respectively, and induced successional change of the phytoplankton species under the tested conditions. The proportion of Microcystis to total phytoplankton abundance in controls and red light shifted from 87.09% at the beginning to 37.95% and 56.30% at the end of the experiment, respectively, and maintained its dominance, whereas Microcystis lost its dominance and was replaced by Scenedesmus (53.78%) and Synedra (53.18%) in the blue and green light, respectively. Given the process of how these phytoplankton compete in designated spectrum, exploring these influences could help provide new insights into the dominance formation of toxic cyanobacteria.

Underwater light climate and wavelength dependence of microalgae photosynthetic parameters in a temperate sea

Studying how natural phytoplankton adjust their photosynthetic properties to the quantity and quality of underwater light (i.e. light climate) is essential to understand primary production. A wavelength-dependent photoacclimation strategy was assessed using a multi-color pulse-amplitude-modulation chlorophyll fluorometer for phytoplankton samples collected in the spring at 19 locations across the English Channel. The functional absorption cross section of photosystem II, photosynthetic electron transport (PET_λ) parameters and non-photochemical quenching were analyzed using an original approach with a sequence of three statistical analyses. Linear mixed-effects models using wavelength as a longitudinal variable were first applied to distinguish the fixed effect of the population from the random effect of individuals. Population and individual trends of wavelength-dependent PET_λ parameters were consistent with photosynthesis and photoacclimation theories. The natural phytoplankton communities studied were in a photoprotective state for blue wavelengths (440 and 480 nm), but not for other wavelengths (green (540 nm), amber (590 nm) and light red (625 nm)). Population-detrended PET_λ values were then used in multivariate analyses (partial triadic analysis and redundancy analysis) to study ecological implications of PET_λ dynamics among water masses. Two wavelength ratios based on the microalgae saturation parameter E_k (in relative and absolute units), related to the hydrodynamic regime and underwater light climate, clearly confirmed the physiological state of microalgae. They also illustrate more accurately that natural phytoplankton communities can implement photoacclimation processes that are influenced by in situ light quality during the daylight cycle in temporarily and weakly stratified water. Ecological implications and consequences of PET_λ are discussed in the context of turbulent coastal ecosystems.

Transformations from specialists to generalists cause bacterial communities are more stable than micro-eukaryotic communities under anthropogenic activity disturbance

Different microbial components have different responses to environmental disturbances. Here, we found that the planktonic bacterial and micro-eukaryotic communities had different responses to anthropogenic activity disturbance in a subtropical river, because they had different survival strategies (generalist and specialist). We used nutrients (nitrogen and phosphorus) as indicators of anthropogenic activities. We found that river stretch 1 showed low nutrient concentrations from October 2018 to September 2019. However, a nutrient disturbance was observed in river stretch 2. The nutrient concentrations increased largely in December and January but recovered to low values in June. Bacterial communities had higher resilience under this disturbance than micro-eukaryotic communities in river stretch 2. The bacterial community composition were quite different between the two river stretches in December and January but were similar in June and July. However, the differences of micro-eukaryotic community composition between the two river stretches were always high during the study period. The bacterial communities in river stretch 2 contained more generalists and nutrient tolerant specialists. The bacterial nutrient tolerant specialists rapidly decreased in the low nutrient months and were replaced by the generalists. Bacteria which were involved in this shifts accounted for 29.3% of the total abundance. However, the micro-eukaryotic communities in river stretch 2 contained more moderate generalists. These moderate generalists were insensitive to the variation of nutrients and only 19.56% of the micro-eukaryotes had significant responses to the disturbance. The survival strategies caused bacterial communities had higher adaptability than eukaryotes to environmental fluctuation.

Priority effects in microbiome assembly

Advances in next-generation sequencing have enabled the widespread measurement of microbiome composition across systems and over the course of microbiome assembly. Despite substantial progress in understanding the deterministic drivers of community composition, the role of historical contingency remains poorly understood. The establishment of new species in a community can depend on the order and/or timing of their arrival, a phenomenon known as a priority effect. Here, we review the mechanisms of priority effects and evidence for their importance in microbial communities inhabiting a range of environments, including the mammalian gut, the plant phyllosphere and rhizosphere, soil, freshwaters and oceans. We describe approaches for the direct testing and prediction of priority effects in complex microbial communities and illustrate these with re-analysis of publicly available plant and animal microbiome datasets. Finally, we discuss the shared principles that emerge across study systems, focusing on eco-evolutionary dynamics and the importance of scale. Overall, we argue that predicting when and how current community state impacts the success of newly arriving microbial taxa is crucial for the management of microbiomes to sustain ecological function and host health. We conclude by discussing outstanding conceptual and practical challenges that are faced when measuring priority effects in microbiomes.

Assessment of water quality under various environmental features using a site-specific weighting water quality index

In a multiregional river system, environmental features such as natural conditions and anthropogenic activities vary among regions, resulting in spatiotemporal variations in water quality. Therefore, a robust water quality assessment method (e.g., water quality index [WQI]) that considers various environmental features is essential for water resources management. This study developed a min/max autocorrelation factor analysis (MAFA) based WQI framework (MAFAWQI). The statistical procedure reduces the bias of expert opinions. The MAFAWQI characterizes impaired water quality variables as indicators and assesses appropriate weighting values of indicators at each sampling site to reflect site-specific environmental features. The MAFAWQI was successful for assessing water quality in the middle and down streams of Han River in central China with site-specific pollution features such as nitrogen and phosphorus pollution related to multiple-source in tributaries, impacts of tributaries on the main stream, and phosphorus pollution related to nonpoint-source in agricultural regions. The MAFAWQI exhibited a balanced rating of water quality compared to the strict assessment method using a single indicator and the lenient assessment method using stationary weighting values of indicators. The MAFAWQI scores indicated that the water quality in tributaries and during the spring were significantly worse than those in and during the other regions and seasons in the middle and down streams of Han River, respectively. The framework and application of the MAFAWQI may provide a new perspective for developing WQIs.

Cell size influences inorganic carbon acquisition in artificially selected phytoplankton

Cell size influences the rate at which phytoplankton assimilate dissolved inorganic carbon (DIC), but it is unclear whether volume-specific carbon uptake should be greater in smaller or larger cells. On the one hand, Fick's Law predicts smaller cells to have a superior diffusive CO₂ supply. On the other, larger cells may have greater scope to invest metabolic energy to upregulate active transport per unit area through CO₂ -concentrating mechanisms (CCMs). Previous studies have focused on among-species comparisons, which complicates disentangling the role of cell size from other covarying traits. In this study, we investigated the DIC assimilation of the green alga Dunaliella tertiolecta after using artificial selection to evolve a 9.3-fold difference in cell volume. We compared CO₂ affinity, external carbonic anhydrase (CA_ext ), isotopic signatures (δ¹³ C) and growth among size-selected lineages. Evolving cells to larger sizes led to an upregulation of CCMs that improved the DIC uptake of this species, with higher CO₂ affinity, higher CA_ext and higher δ¹³ C. Larger cells also achieved faster growth and higher maximum biovolume densities. We showed that evolutionary shifts in cell size can alter the efficiency of DIC uptake systems to influence the fitness of a phytoplankton species.

The role of changes in environmental quality in multitrait plastic responses to environmental and social change in the model microalga Chlamydomonas reinhardtii

Intraspecific variation plays a key role in species' responses to environmental change; however, little is known about the role of changes in environmental quality (the population growth rate an environment supports) on intraspecific trait variation. Here, we hypothesize that intraspecific trait variation will be higher in ameliorated environments than in degraded ones. We first measure the range of multitrait phenotypes over a range of environmental qualities for three strains and two evolutionary histories of Chlamydomonas reinhardtii in laboratory conditions. We then explore how environmental quality and trait variation affect the predictability of lineage frequencies when lineage pairs are grown in indirect co-culture. Our results show that environmental quality has the potential to affect intraspecific variability both in terms of the variation in expressed trait values, and in terms of the genotype composition of rapidly growing populations. We found low phenotypic variability in degraded or same-quality environments and high phenotypic variability in ameliorated conditions. This variation can affect population composition, as monoculture growth rate is a less reliable predictor of lineage frequencies in ameliorated environments. Our study highlights that understanding whether populations experience environmental change as an increase or a decrease in quality relative to their recent history affects the changes in trait variation during plastic responses, including growth responses to the presence of conspecifics. This points toward a fundamental role for changes in overall environmental quality in driving phenotypic variation within closely related populations, with implications for microevolution.

Resource Colimitation Drives Competition Between Phytoplankton and Bacteria in the Southern Ocean

Across the Southern Ocean, phytoplankton growth is governed by iron and light, while bacterial growth is regulated by iron and labile dissolved organic carbon (LDOC). We use a mechanistic model to examine how competition for iron between phytoplankton and bacteria responds to changes in iron, light, and LDOC. Consistent with experimental evidence, increasing iron and light encourages phytoplankton dominance, while increasing LDOC and decreasing light favors bacterial dominance. Under elevated LDOC, bacteria can outcompete phytoplankton for iron, most easily under lower iron. Simulations reveal that bacteria are major iron consumers and suggest that luxury storage plays a key role in competitive iron uptake. Under seasonal conditions typical of the Southern Ocean, sources of LDOC besides phytoplankton exudation modulate the strength of competitive interactions. Continued investigations on the competitive fitness of bacteria in driving changes in primary production in iron-limited systems will be invaluable in refining these results.

Meta-analysis of multiple driver effects on marine phytoplankton highlights modulating role of pCO2

Responses of marine primary production to a changing climate are determined by a concert of multiple environmental changes, for example in temperature, light, pCO₂ , nutrients, and grazing. To make robust projections of future global marine primary production, it is crucial to understand multiple driver effects on phytoplankton. This meta-analysis quantifies individual and interactive effects of dual driver combinations on marine phytoplankton growth rates. Almost 50% of the single-species laboratory studies were excluded because central data and metadata (growth rates, carbonate system, experimental treatments) were insufficiently reported. The remaining data (42 studies) allowed for the analysis of interactions of pCO₂ with temperature, light, and nutrients, respectively. Growth rates mostly respond non-additively, whereby the interaction with increased pCO₂ profusely dampens growth-enhancing effects of high temperature and high light. Multiple and single driver effects on coccolithophores differ from other phytoplankton groups, especially in their high sensitivity to increasing pCO₂ . Polar species decrease their growth rate in response to high pCO₂ , while temperate and tropical species benefit under these conditions. Based on the observed interactions and projected changes, we anticipate primary productivity to: (a) first increase but eventually decrease in the Arctic Ocean once nutrient limitation outweighs the benefits of higher light availability; (b) decrease in the tropics and mid-latitudes due to intensifying nutrient limitation, possibly amplified by elevated pCO₂ ; and (c) increase in the Southern Ocean in view of higher nutrient availability and synergistic interaction with increasing pCO₂ . Growth-enhancing effect of high light and warming to coccolithophores, mainly Emiliania huxleyi, might increase their relative abundance as long as not offset by acidification. Dinoflagellates are expected to increase their relative abundance due to their positive growth response to increasing pCO₂ and light levels. Our analysis reveals gaps in the knowledge on multiple driver responses and provides recommendations for future work on phytoplankton.

Vibrational modes of water predict spectral niches for photosynthesis in lakes and oceans

Stretching and bending vibrations of water molecules absorb photons of specific wavelengths, a phenomenon that constrains light energy available for aquatic photosynthesis. Previous work suggested that these absorption properties of water create a series of spectral niches but the theory was still too simplified to enable prediction of the spectral niches in real aquatic ecosystems. Here, we show with a state-of-the-art radiative transfer model that the vibrational modes of the water molecule delineate five spectral niches, in the violet, blue, green, orange and red parts of the spectrum. These five niches are effectively captured by chlorophylls and phycobilin pigments of cyanobacteria and their eukaryotic descendants. Global distributions of the spectral niches are predicted by satellite remote sensing and validated with observed large-scale distribution patterns of cyanobacterial pigment types. Our findings provide an elegant explanation for the biogeographical distributions of photosynthetic pigments across the lakes and oceans of our planet.

Competitive traits of coral symbionts may alter the structure and function of the microbiome

In the face of global warming and unprecedented coral bleaching, a new avenue of research is focused on relatively rare algal symbionts and their ability to confer thermal tolerance to their host by association. Yet, thermal tolerance is just one of many physiological attributes inherent to the diversity of symbiodinians, a result of millions of years of competition and niche partitioning. Here, we revealed that competition among cocultured symbiodinians alters nutrient assimilation and compound production with species-specific responses. For Cladocopium goreaui, a species ubiquitous within stable coral associations, temperature stress increased sensitivity to competition eliciting a shift toward investment in cell replication, i.e., putative niche exploitation. Meanwhile, competition led Durusdinium trenchii, a thermally tolerant “background” symbiodinian, to divert resources from immediate growth to storage. As such, competition may be driving the dominance of C. goreaui outside of temperature stress, the destabilization of symbioses under thermal stress, the repopulation of coral tissues by D. trenchii following bleaching, and ultimately undermine the efficacy of symbiont turnover as an adaptive mechanism.

Photoacoustic and fluorescence lifetime imaging of diatoms

Photoacoustic and fluorescent methods are used intensely in biology and medicine. These approaches can also be used to investigate unicellular diatom algae that are extremely important for Earth's ecology. They are enveloped within silica frustules (exoskeletons), which can be used in drug delivery systems. Here, we report for the first time the successful application of photoacoustic (PA) and fluorescent visualization of diatoms. Chlorophyll a and c and fucoxanthin were found likely to be responsible for the photoacoustic effect in diatoms. The PA signal was obtained from gel drops containing diatoms and was found to increase with the diatom concentration. The fluorescence lifetime of the diatom chromophores ranged from 0.5 to 2 ns. The dynamic light scattering, absorbance, and SEM characterization techniques were also applied. The results were considered in combination to elucidate the nature of the photoacoustic signal. Possible biotechnological applications are proposed for the remote photoacoustic monitoring of algae.

Characteristics of microbial eukaryotic community recovery in eutrophic water by using ecological floating beds

Ecological floating beds can rapidly remove nutrients (nitrogen and phosphorus) from eutrophic water, but we still know little about whether this process can simultaneously recover microbial eukaryotic communities. To fill this gap, planktonic microbial eukaryotic communities were investigated using 18S rRNA high-throughput gene sequencing during nutrient removal by floating beds of Canna indica L. We found that nutrient concentrations were high in both the control and treatment groups during period 1 (days 0-5) but rapidly decreased in the treatment group during period 2 (days 6-9) and period 3 (days 10-18). However, the microbial eukaryotic species richness and community compositions were similar between the control and treatment groups during periods 1 and 2 but showed small differences during period 3. The microbial eukaryotic co-occurrence networks between the control and treatment groups also showed similar degree centrality and interconnected eukaryotic members. We found that some abundant fungi species significantly responded to nutrient variations, but a large number of abundant ciliates were insensitive to nutrient removal. Our findings suggest that ecological floating beds can rapidly remove nutrients in eutrophic waters but that it is difficult to quickly and simultaneously improve microbial eukaryotic communities. This result reveals the critical influence of nutrient pollution on aquatic ecosystems and therefore on long-term and comprehensive aquatic habitat restoration, as aquatic macrophyte recoveries should be conducted after nutrient controls have been implemented.

Light, but Not Nutrients, Drives Seasonal Congruence of Taxonomic and Functional Diversity of Phytoplankton in a Eutrophic Highland Lake in China

Information on temporal dynamics of phytoplankton communities and their responses to environmental factors can provide insights into mechanisms driving succession of phytoplankton communities that is useful in programs to manage and or remediate undesirable assemblages. Populations of phytoplankton can be controlled by bottom-up factors such as nutrients and temperature or top-down such as predation by zooplankton. Traditionally, taxonomic diversity based on morphologies has been the measure used for analysis of responses to environmental factors. Recently, according to functional groupings, including functional groups (FG), morpho-FG (MFG), and morphology-based FG (MBFG), functional diversity has been used to represent functional aspects of phytoplankton communities. However, to what extent these taxonomic and functional groupings are congruent at seasonal time-scales and the main environmental factors, which drive succession, have remained less studied. Here, we analyzed absolute and relative proportions of a phytoplankton community during a 3-year period in Lake Erhai, a eutrophic highland lake in China. Alpha diversity and beta diversity, as measured by Shannon-Wiener and Bray-Curtis indices of taxonomic grouping and three functional groupings (FG, MFG, and MBFG) were applied to investigate environmental factors determining diversity. Significant, positive relationships were observed between taxonomic diversity and functional diversity that were strongly linked through seasons. In order to exclude the influence of dominant species' tolerance to extreme environments, the dominant species were excluded one by one, and the results showed that residual communities still exhibited similar patterns of succession. This synchronous temporal pattern was not principally driven by the dominant genera (Microcystis, Psephonema, and Mougeotia). Instead, the entire phytoplankton community assemblages were important in the pattern. Most diversity indices of taxonomic and functional groupings were significantly correlated with solar irradiance, but not nutrient concentrations. Because the lake is eutrophic and there were already sufficient nutrients available, additional nutrients had little effect on seasonal taxonomic and functional diversity of phytoplankton in Lake Erhai.

Changes in water color shift competition between phytoplankton species with contrasting light-harvesting strategies

The color of many lakes and seas is changing, which is likely to affect the species composition of freshwater and marine phytoplankton communities. For example, cyanobacteria with phycobilisomes as light-harvesting antennae can effectively utilize green or orange-red light. However, recent studies show that they use blue light much less efficiently than phytoplankton species with chlorophyll-based light-harvesting complexes, even though both phytoplankton groups may absorb blue light to a similar extent. Can we advance ecological theory to predict how these differences in light-harvesting strategy affect competition between phytoplankton species? Here, we develop a new resource competition model in which the absorption and utilization efficiency of different colors of light are varied independently. The model was parameterized using monoculture experiments with a freshwater cyanobacterium and green alga, as representatives of phytoplankton with phycobilisome-based vs. chlorophyll-based light-harvesting antennae. The parameterized model was subsequently tested in a series of competition experiments. In agreement with the model predictions, the green alga won the competition in blue light whereas the cyanobacterium won in red light, irrespective of the initial relative abundances of the species. These results are in line with observed changes in phytoplankton community structure in response to lake brownification. Similarly, in marine waters, the model predicts dominance of Prochlorococcus with chlorophyll-based light-harvesting complexes in blue light but dominance of Synechococcus with phycobilisomes in green light, with a broad range of coexistence in between. These predictions agree well with the known biogeographical distributions of these two highly abundant marine taxa. Our results offer a novel trait-based approach to understand and predict competition between phytoplankton species with different photosynthetic pigments and light-harvesting strategies.

Perspective: Advancing the research agenda for improving understanding of cyanobacteria in a future of global change

Harmful cyanobacterial blooms (=cyanoHABs) are an increasing feature of many waterbodies throughout the world. Many bloom-forming species produce toxins, making them of particular concern for drinking water supplies, recreation and fisheries in waterbodies along the freshwater to marine continuum. Global changes resulting from human impacts, such as climate change, over-enrichment and hydrological alterations of waterways, are major drivers of cyanoHAB proliferation and persistence. This review advocates that to better predict and manage cyanoHABs in a changing world, researchers need to leverage studies undertaken to date, but adopt a more complex and definitive suite of experiments, observations, and models which can effectively capture the temporal scales of processes driven by eutrophication and a changing climate. Better integration of laboratory culture and field experiments, as well as whole system and multiple-system studies are needed to improve confidence in models predicting impacts of climate change and anthropogenic over-enrichment and hydrological modifications. Recent studies examining adaptation of species and strains to long-term perturbations, e.g. temperature and carbon dioxide (CO₂) levels, as well as incorporating multi-species and multi-stressor approaches emphasize the limitations of approaches focused on single stressors and individual species. There are also emerging species of concern, such as toxic benthic cyanobacteria, for which the effects of global change are less well understood, and require more detailed study. This review provides approaches and examples of studies tackling the challenging issue of understanding how global changes will affect cyanoHABs, and identifies critical information needs for effective prediction and management.

Stable coexistence of equivalent nutrient competitors through niche differentiation in the light spectrum

Niche-based theories and the neutral theory of biodiversity differ in their predictions of how the species composition of natural communities will respond to changes in nutrient availability. This is an issue of major environmental relevance, as many ecosystems have experienced changes in nitrogen (N) and phosphorus (P) due to anthropogenic manipulation of nutrient loading. To understand how changes in N and P limitation may impact community structure, we conducted laboratory competition experiments using a multispecies phytoplankton community sampled from the North Sea. Results showed that picocyanobacteria (Cyanobium sp.) won the competition under N limitation, while picocyanobacteria and nonmotile nanophytoplankton (Nannochloropsis sp.) coexisted at equal abundances under P limitation. Additional experiments using isolated monocultures confirmed that Cyanobium sp. depleted N to lower levels than Nannochloropsis sp., but that both species had nearly identical P requirements, suggesting a potential for neutral coexistence under P-limited conditions. Pairwise competition experiments with the two isolates seemed to support the consistency of these results, but P limitation resulted in stable species coexistence irrespective of the initial conditions rather than the random drift of species abundances predicted by neutral theory. Comparison of the light absorption spectra indicates that coexistence of the two species was stabilized through differential use of the underwater light spectrum. Our results provide an interesting experimental example of modern coexistence theory, where species were equal competitors in one niche dimension but their competitive traits differed in other niche dimensions, thus enabling stable species coexistence on a single limiting nutrient through niche differentiation in the light spectrum.