Nitrogen limitation significantly reduces the competitive advantage of toxic Microcystis at high light conditions

Modifying luteolin's algicidal effect on Microcystis by virgin and diversely-aged polystyrene microplastics: Unveiling novel mechanisms through microalgal adaptive strategies

Luteolin has shown great potential in inhibiting Microcystis-dominated cyanobacterial blooms. However, widespread microplastics (MPs) in natural aquatic systems often serve as substrates for cyanobacterial growth, which could impact cyanobacterial resistance to external stresses and interfere with luteolin's algicidal effect. This study explored the influence of virgin and diversely-aged polystyrene microplastics (PS-MPs) on inhibitory effect of luteolin on Microcystis growth and its microcystins (MCs) production/release. Moreover, the underlying mechanisms were also revealed by jointly analyzing SEM image, antioxidant response, exopolymeric substances (EPSs) production, and functional gene expression. Results suggested that 0.5, 5, and 50 mg/L virgin and diversely-aged PS-MPs almost weakened growth inhibition and oxidative damage of two doses of luteolin against Microcystisby stimulating its EPSs production and inducing self-aggregation of Microcystis cells and/or hetero-aggregation between Microcystis cells and PS-MPs. Compared to virgin PS-MPs, photo-aged PS-MPs possessed rougher flaky surfaces, and hydrothermal-aged PS-MPs showed internal cracking. These characteristics led to greater stimulation of EPS production and exhibited more significant protective effects on Microcystis. Notably, PS-MPs also decreased MCs content in aqueous phase, likely because they adsorbed some MCs. Such toxigenic hetero-aggregates formed by MCs, MPs, and Microcystis cells would directly poison grazing organisms that consume them and create more pathways for MCs into food web, posing greater eco-risks. This is the first study to clarify the influence and mechanisms of virgin and diversely-aged MPs on allelopathic algicidal effects from the perspective of microalgal inherent adaptive strategies.

Changes in CO2 concentration drive a succession of toxic and non-toxic strains of Microcystis blooms

The dynamic changes between toxic and non-toxic strains of Microcystis blooms have always been a hot topic. Previous studies have found that low CO2 favors toxic strains, but how changing dissolved CO2 (CO2 [aq]) in water body influences the succession of toxic and non-toxic strains in Microcystis blooms remains uncertain. Here, we combined laboratory competition experiments, field observations, and a machine learning model to reveal the links between CO2 changes and the succession. Laboratory experiments showed that under low CO2 conditions (100-150 ppm), the toxic strains could make better use of CO2 (aq) and be dominant. The non-toxic strains demonstrated a growth advantage as CO2 concentration increased (400-1000 ppm). Field observations from June to November in Lake Taihu showed that the percentage of toxic strains increased as CO2 (aq) decreased. Machine learning highlighted links between the inorganic carbon concentration and the proportion of advantageous strains. Our findings provide new insights for cyanoHABs prediction and prevention.

Spatio-temporal variation of toxin-producing gene abundance in Microcystis aeruginosa from Poyang Lake

Microcystis aeruginosa (M. aeruginosa) causes massive blooms in eutrophic freshwater and releases microcystin. Poyang Lake is the largest freshwater lake in China and has kept a mid-nutrient level in recent years. However, there is little research on microcystin production in Poyang Lake. In this study, water and sediment samples from ten sampling sites in Poyang Lake were collected from May to December in 2020, and from January to April in 2021 respectively. Microcystis genes (mcyA, mcyB, 16 s rDNA) were quantified by real-time fluorescence quantitative PCR analysis, and then the spatial and temporal variation of mcy genes, physicochemical factors, and bacterial population structure in the lake was analyzed. The relationship between the abundance of mcy genes and physicochemical factors in water column was also revealed. Results indicated that the microcystin-producing genes mcyA and mcyB showed significant differences in spatial and temporal levels as well, which is closely related to the physicochemical factors especially the water temperature (p < 0.05) and the nitrogen content (p < 0.05). The abundance of mcy genes in the sediment in December affected the abundance of mcy genes in the water column in the next year, while the toxic Microcystis would accumulate in the sediment. In addition to the toxic Microcystis, we also found a large number of non-toxic Microcystis in the water column and sediment, and the ratio of toxic to non-toxic species can also affect the toxicity production of M. aeruginosa. Overall, the results showed that M. aeruginosa toxin-producing genes in Poyang Lake distributed spatially and temporally which related to the physicochemical factors of Poyang Lake.

Environmental concentrations of anionic surfactants in lake surface microlayers enhance the toxicity of Microcystis blooms: Insight from photosynthesis, interspecies competition, and MC production

Anionic surfactants represented by linear alkylbenzene sulfonate (LAS) exhibit vertical heterogeneity of concentrations in aquatic environments owing to their amphiphilic structure. Field investigations showed that the concentration of anionic surfactants (mainly LAS) in the water surface microlayer (SML) of Lake Taihu reached 580 μg/L, higher than that in the lower layer. Floating Microcystis blooms overlap in space with the high concentration of anionic surfactants in SML. However, few studies have focused on the effects of anionic surfactants (e.g., LAS) on the interspecies competition between toxic and nontoxic Microcystis. In this study, coculture and monoculture experiments were conducted with both toxic and nontoxic Microcystis species to explore how the environmental concentration of LAS regulates the dominance of toxic Microcystis and toxicity from the perspective of photosynthesis, species dominance, and MC production. The results showed that LAS concentrations above 0.267 or 0.431 mg/L (depending on light conditions) selectively promoted the photosynthetic competitive advantage of toxic Microcystis, leading to its higher population proportion in the community. Additionally, LAS concentrations above 0.5 mg/L induced the synthesis and release of microcystins (MCs). The results of chlorophyll fluorescence analysis, electron microscopy and transcriptome sequencing suggested that compared with nontoxic Microcystis, toxic Microcystis can better resist LAS stress by dissipating excess light, maintaining an intact membrane structure and maintaining cellular homeostasis. Transcriptome sequencing revealed that the photosynthetic damage of nontoxic Microcystis might be attributed to the impacts of LAS on the absorption and assimilation of nitrogen, which finally resulted in the degradation of phycobilisomes. This study can provide novel insight for establishing standards and safety management of wastewater discharge.

Effect of light-mediated variations of colony morphology on the buoyancy regulation of Microcystis colonies

Light is an important driver of algal growth and for the formation of surface blooms. Long-term buoyancy maintenance of Microcystis colonies is crucial for their aggregation at the water surface and the following algal bloom development. However, the effect of light-mediated variations of colony morphology on the buoyancy regulation of Microcystis colonies remains unclear. In this study, growth parameters, colony morphology and floatation/sinking performance of Microcystis colonies were determined to explore how variations in colony morphology influence the buoyancy of colonies under different light conditions. We quantified colony compactness through the cell volume to colony volume ratio (VR) and found different responses of colony size and VR under different light intensities. Microcystis colonies with higher VR could stay longer at the water surface under low light conditions, which was beneficial for the long-term growth and buoyancy maintenance. However, increased colony size and decreased compactness were observed at a later growth stage under relatively higher light intensity (i.e., >108 µmol photons m^-2 s^-1). Interestingly, we found a counterintuitive negative correlation between colony size and buoyancy of Microcystis under high light intensity. Additionally, we found that the influence of colony morphology on buoyancy was stronger at high light intensity. These results indicate that light could regulate the buoyancy via colonial morphology and that the role of colony morphology in buoyancy regulation needs to be accounted for in further studies under variable environmental conditions.

Phosphorus level impacts luteolin effect on Microcystis aeruginosa growth and microcystin-pollution risk - Novel perspective from correlation between exopolymers substances fractions and microcystin-production/release

Luteolin as a phytogenic algicide can inhibit the growth and microcystins (MCs) release of Microcystis, a dominant genus during cyanobacterial blooms, but how phosphorus (P) level impacts luteolin effect on its growth and MC-pollution risk is unclear. By employing Microcystis aeruginosa as test alga, this study addressed this concern and explored response mechanisms from novel insights of relationship between extracellular polysaccharide (ex-poly) and protein (ex-pro) contents and MC-production/release. At each P level (0.05-5 mg/L), rising luteolin dose more greatly inhibited Microcystis growth and MC-pollution risk, with growth inhibition ratio of around 10%-30%, 20%-50% and 40%-90% for 3, 6 and 12 mg/L luteolin, respectively, but almost increasingly enhanced cellular ability of MC-production/conservation and total and bound ex-poly/ex-pro production. Rising P level promoted Microcystis growth and intracellular/extracellular MCs content (IMC, EMC) in test system at each luteolin dose, thus higher P level weakened algicidal and MC-removal effects of luteolin, indicating that P-decrease was required for stronger application outcome of luteolin. Total and bound ex-poly/ex-pro amount were positively correlated with cellular MC-production/conservation ability, IMC and EMC, which constituted cooperative stress-defense of Microcystis at each P level. Besides, rising luteolin dose posed stronger algicidal effect by inactivating gene expression involving peroxidase synthesis (especially at P-limitation), photosynthesis and P acquisition, while rising P level alleviated algicidal and MC-pollution inhibition effects of luteolin by enhancing gene expression involving N acquisition and peroxidase synthesis. This study shed novel insights for P-dependent effect and mechanisms of luteolin on toxigenic Microcystis growth and MC-pollution control, which guided to mitigating toxigenic Microcystis-dominated cyanobacterial blooms in different P-level water areas.

Assessment of nitrogen interaction with temperature on the growth and toxin production of mat-forming toxin-producing Anagnostidinema carotinosum

AIMS

Global warming and eutrophication contribute to the severity of cyanobacteria blooms. However, it is unclear how these factors influence the growth and toxin production of Anagnostidinema carotinosum.

METHODS AND RESULTS

Based on morphological and molecular analysis, this is the first time A. carotinosum was identified in South Korea. The interactive effect of temperature (25, 30 or 34 °C) and nitrogen (2.5, 3.5 or 4.5 mg NO₃ -N l^-1 ) on A. carotinosum growth and toxin production were studied. Increasing nitrogen limitation reflects reduced growth and chlorophyll-a content at all temperatures. However, the growth was effective under nitrogen limitation when temperatures exceeded 25 °C. The maximum growth was found at 30 °C, followed by 34 °C under higher nitrate levels (3.5 and 4.5 mg l^-1 ). In addition, the cell microcystin and anatoxin-a quota increased significantly at 25 °C with increasing nitrate limitation, decreasing considerably at 30 °C in the same nitrate gradient.

CONCLUSION

These results suggested temperatures stimulate A. carotinosum growth at 30 and 34 °C and cellular toxin quota at 25 and 34 °C with increasing NO₃ -N levels.

SIGNIFICANCE AND IMPACT OF THE STUDY

These findings imply that limiting nitrogen input alone can effectively reduce biomass; however, controlling A. carotinosum and its toxins at higher temperatures under nitrate limitation is necessary for water quality.

Synergistic promoting effect of increasing aquatic ammonium and CO2 on Microcystis aeruginosa

Owing to climate change and intensive agricultural development, freshwater bodies have been affected by increases in both CO₂ levels and chemically-reduced forms of N. However, little is known about how these changes affect cyanobacterial growth and blooms. This study explored a range of light conditions (30, 80, 130, or 200 μmol photons/m²/s) wherein Microcystis aeruginosa, a widespread bloom-forming species, was exposed to different concentrations of CO₂ (400 parts per million (ppm) and 1000 ppm) in a medium containing NH₄⁺ or NO₃－. The interactive effects of N sources and CO₂ levels on the C/N metabolic balance and energy balance were examined to assess changes in the growth of M. aeruginosa. When the light intensity was 80 μmol photons/m²/s, elevated CO₂ could reduce intracellular reactive oxygen species (ROS) in NH₄⁺-grown M. aeruginosa. Meanwhile, cell density and chlorophyll a (Chl a) increased with increasing CO₂ levels, and the increase in Chl a was significantly greater in NH₄⁺-grown M. aeruginosa than in NO₃－-grown M. aeruginosa. Under light conditions of 200 μmol photons/m²/s, elevated CO₂ concentration caused NO₃－-grown M. aeruginosa to be affected by a large amount of ROS, and the growth of NO₃－-grown M. aeruginosa was finally suppressed. However, NH₄⁺-grown M. aeruginosa had a smaller amount of ROS and showed improved growth as CO₂ was elevated. This difference can be attributed to the faster metabolic pathways in the NH₄⁺ environment, which manifested in a lower accumulation of 2-oxoglutarate and fatty acids as CO₂ was elevated. These findings suggest that the simultaneous increase in ammonium and CO₂ in aquatic ecosystems confers cyanobacteria with greater advantages than the combination of nitrate and CO₂, which may aggravate cyanobacterial blooms.

Cyanobacterial Harmful Algal Blooms in Aquatic Ecosystems: A Comprehensive Outlook on Current and Emerging Mitigation and Control Approaches

An intensification of toxic cyanobacteria blooms has occurred over the last three decades, severely affecting coastal and lake water quality in many parts of the world. Extensive research is being conducted in an attempt to gain a better understanding of the driving forces that alter the ecological balance in water bodies and of the biological role of the secondary metabolites, toxins included, produced by the cyanobacteria. In the long-term, such knowledge may help to develop the needed procedures to restore the phytoplankton community to the pre-toxic blooms era. In the short-term, the mission of the scientific community is to develop novel approaches to mitigate the blooms and thereby restore the ability of affected communities to enjoy coastal and lake waters. Here, we critically review some of the recently proposed, currently leading, and potentially emerging mitigation approaches in-lake novel methodologies and applications relevant to drinking-water treatment.

Influence of Environmental Factors on Occurrence of Cyanobacteria and Abundance of Saxitoxin-Producing Cyanobacteria in a Subtropical Drinking Water Reservoir in Brazil

Blooms of cyanobacteria are frequent in Brazilian water reservoirs used for drinking water. The warning for the presence of potential toxin-producing cyanobacteria is typically based on time-consuming microscopy, rather than specific molecular detection of toxic genes in cyanobacteria. In this study, we developed a quantitative PCR assay for the detection of cyanobacteria producing the neurotoxin saxitoxin (STX). The assay targets the sxtA gene in the sxt gene cluster. Potential and dominant STX-producers in the Itupararanga reservoir were the genera Raphidiopsis, Aphanizomenon and Geitlerinema. Numbers of the sxtA gene varied from 6.76 × 103 to 7.33 × 105 cells mL−1 and correlated positively with SXT concentrations in the water. Concentrations of STX and the sxtA gene also correlated positively with TN:TP ratio and pH, but correlated negatively with inorganic nutrients and turbidity, confirming that regulation of the SXT production was impacted by environmental variables. In contrast, the occurrence of another cyanotoxin, microcystin, did not correlate with any environmental variables. The developed qPCR assay was found to be a rapid and robust approach for the specific quantification of potential STX-producing cyanobacteria and should be considered in future investigations on toxic cyanobacteria to provide an early warning of potential toxin episodes.

Ultraviolet-B radiation stress alters the competitive outcome of algae: Based on analyzing population dynamics and photosynthesis

The solar ultraviolet-B radiation (UVB) is increasingly affecting the aquatic ecosystems due to the long-term antropic damage to the stratospheric ozone. The distrupted interspecies competition is one of the primary causes driving the plankton community composition shifts under UVB stress. To reveal the competitive responses to enhanced UVB radiation, we grew two green algae Scenedesmus obliquus and Chlorella pyrenoidosa, and the unicellular cyanobacterium Microcystis aeruginosa in monocultures and in cocultures under differerent UVB intensities (0, 0.3 and 0.7 W m^-2), respectively. Results showed that elevated UVB radiation consistently decreased the population carrying capacies and the photosynthesis of the three species in monocultures. While cocultivated, C. pyrenoidosa was competively excluded by the presence of S. obliquus, and the competitive outcome was not affected by UVB exposure. By contrast, unicellular M. aeruginosa overwhelmingly suppressed the population growth of S. obliquus under no UVB, yet S. obliquus tended to be a better competitor under 0.3-0.7 W m^-2 UVB exposure. The species-specific photosynthesis sensitivity to UVB can partly explain the different tolerance of the algae to UVB and the change of competition outcome under elevated UVB. The present study elucidated the potential role of increased UVB radiation in determining the competitions between phytoplankton species, contributing to the understanding of phytoplankton community shifts under enhanced UVB stress.

Assessment of water quality in eutrophized water bodies through the application of indexes and toxicity

It is essential to have tools that can be used to diagnose water resources. For this reason, this study sets out on the one hand to assess water quality in three reservoirs in Spain (Salas, A Baxe and Conchas) with Cyano-Habs problems through the application of water quality indexes: The National Sanitation Foundation Water Quality Index (NFSWQI), the General Quality Index (GQI), the Trophic State Index (TSI), and the Trophic Contamination Index (ICOTRO). On the other hand, it seeks to learn how parameters such as nitrogen and phosphorus influence the presence of cyanotoxin, specifically Microcystis aeruginosa. To that end, samples from the three reservoirs are cultured and physical-chemical parameters and the toxicity of the water are measured. The results show that Salas reservoir has the worst water quality rating (GQI is bad and NFSWQI medium), while As Conchas and A Baxe obtain very good figures for NFSWQI. This contrasts with the data obtained via the TSI of moderately eutrophic conditions for all three reservoirs, and hypereutrophic levels for As Conchas and A Baxe downstream. On the other hand, the toxicity analysis shows levels of 1.12 ± 0.06 μg/l microcystin-LR (MC-LR) for As Conchas, 0.64 ± 0.04 μg/l MC-LR for Salas, and 1.24 ± 0.05 μg/l MC-LR for A Baxe, of which 20% corresponds to free MC-LR. This study finds that nitrogen is the parameter that most favors the production of MC-LR. We conclude that the eutrophication indexes are more reliable when studying the presence of cyanobacteria. Furthermore, nitrogen and phosphorous are the most significant parameters in this regard. They are taken into account in the quality indices (GQI, NFSWI), but they are not sufficiently representative. It is recommended as a future line of research that water quality indices be adapted or designed to incorporate eutrophication levels and even water toxicity.