Spatiotemporal dynamics of cell abundance, colony size and intracellular toxin concentrations of pelagic and benthic Microcystis in Lake Caohai, China

Dynamics of Microcystis surface scum formation under different wind conditions: the role of hydrodynamic processes at the air-water interface

Due to climate change, Microcystis blooms occur at increasing frequencies in aquatic ecosystems worldwide. Wind-generated turbulence is a crucial environmental stressor that can vertically disperse the Microcystis surface scum, reducing its light availability. Yet, the interactions of Microcystis scum with the wind-generated hydrodynamic processes, particularly those at the air-water interface, remain poorly understood. Here, we explore the response of Microcystis (including colony size and migration dynamics) to varying magnitudes and durations of intermittent wind disturbances in a mesocosm system. The flow velocities, size of Microcystis colonies, and the areal coverage of the water surface by scum were measured through video observations. Our results demonstrate that low wind speeds increase colony size by providing a stable condition where Microcystis forms a scum layer and aggregates into large colonies at the air-water interface. In contrast, wind disturbances disperse scum and generate turbulence, resulting in smaller colonies with higher magnitudes of wind disturbance. We observed that surface scum can form rapidly following a long period (6 h) of high-magnitude (4.5 m s-1) wind disturbance. Furthermore, our results indicate reduced water surface tension caused by the presence of Microcystis, which can decrease surface flow velocity and counteract wind-driven mixing. The reduced surface tension may also drive lateral convection at the water surface. These findings suggest that Microcystis reduces surface tension, likely by releasing surface-active materials, as an adaptive response to various wind conditions. This could result in an increased rate of surface scum re-formation under wind conditions and potentially facilitate the lateral expansion of scum patches during weak wind periods. This study reveals new insights into how Microcystis copes with different wind conditions and highlights the importance of the air-water interface for Microcystis scum dynamics.

Selection of photosynthetic traits by turbulent mixing governs formation of cyanobacterial blooms in shallow eutrophic lakes

Prediction of the complex cyanobacteria-environment interactions is vital for understanding harmful bloom formation. Most previous studies on these interactions considered specific properties of cyanobacterial cells as representative for the entire population (e.g. growth rate, mortality, and photosynthetic capacity (Pmax)), and assumed that they remained spatiotemporally unchanged. Although, at the population level, the alteration of such traits can be driven by intraspecific competition, little is known about how traits and their plasticity change in response to environmental conditions and affect the bloom formation. Here we test the hypothesis that intraspecific variations in Pmax of cyanobacteria (Microcystis spp.) play an important role in its population dynamics. We coupled a one-dimensional hydrodynamic model with a trait-based phytoplankton model to simulate the effects of physical drivers (turbulence and turbidity) on the Pmax of Microcystis populations for a range of dynamic conditions typical for shallow eutrophic lakes. Our results revealed that turbulence acts as a directional selective driver for changes in Pmax. Depending on the intensity of daily-periodic turbulence, representing wind-driven mixing, a shift in population-averaged phenotypes occurred toward either low Pmax, allowing the population to capture additional light in the upper layers, or high Pmax, enhancing the efficiency of light utilization. Moreover, we observed that a high intraspecific diversity in Pmax accelerated the formation of surface scum by up to more than four times compared to a lower diversity. This study offers insights into mechanisms by which cyanobacteria populations respond to turbulence and underscores the significance of intraspecific variations in cyanobacterial bloom formation.

HIGHLIGHTS

Differences in survivability and toxic potential among Microcystis colonies of different sizes in sediment

Microcystis colonies have the ability to persist for extended periods in sediment and function as a "seed bank" for the succeeding summer bloom in water column. The colonial morphology and toxin production ability of Microcystis are important for their population maintenance and life history. However, it is unclear about the influence of the colony morphology and toxic potential of Microcystis colonies on their benthic process. To address this question, we classified field Microcystis samples into three groups based on their size (< 150 μm, 150-300 μm, and > 300 μm) and compared their survivability and toxic potential during culturing in sediment. The results showed that Microcystis colonies in sediments disappeared quickly at 25℃ but survived for long periods at 5℃. The survivability of smaller Microcystis colonies (< 300 μm) was significantly higher than that of larger ones (> 300 μm). The activities of catalase (CAT) were significantly increased in large colonies compared to small colonies at 15℃ and 25℃. Real-time PCR indicated that smaller colonies had higher proportion of potential toxic genotype, and Microcystis colonies cultured at 15℃ and 25℃ showed higher percentage of microcystin-producing genotype. These results indicate that Microcystis colonies survived longer at low temperature and that larger Microcystis colonies are more susceptible to oxidative stress in sediments. The difference of toxic potential of Microcystis colonies of different sizes in sediments may be related to their survival ability in sediments.

Different environmental factors drive the concentrations of microcystin in particulates, dissolved water, and sediments peaked at different times in a large shallow lake

Global distribution and health threats of microcystins (MCs) have received much more attention, but there are still significant knowledge gaps in the peak periods and driving factors of MC in different phases of freshwater ecosystems. Thus, we systematically analyzed the annual variation of different MC congeners (-LR, -RR, and -YR, where L, R, and Y respectively represent leucine, arginine, and tyrosine) in particulates, dissolved water, and sediments in three eutrophic bays of Lake Taihu, China. The results indicated that particulate MCs concentration was the highest, followed by dissolved and sediment MC, with the mean concentration of 7.58 μg/L, 1.48 μg/L, and 0.15 μg/g (DW), respectively. Except for particulate MC, the concentrations of the other two types of MC showed significant differences among the three bays. The dominant congeners of the three types of MCs were different, with the highest proportion of MC-LR being observed in sediment MCs and the lowest in particulate MCs. The peak period of the three types of MC was also different, with particulate MCs reaching their peak in July and October, dissolved MCs in May to July and October, and sediment MCs reaching their peak in September. Consistent with our hypothesis, the dynamics of different types of MCs were driven by different environmental factors. Particulate MCs were primarily related to biological parameters, followed by TP and dissolved carbon. By contrast, dissolved MCs strongly correlated with water temperature and dissolved oxygen. While sediment MCs were primarily driven by properties of sediments, followed by different forms of nitrogen in the water column. Our results suggested that particulate and dissolved MCs in northern Lake Taihu pose high health threats, especially in the peak period. Moreover, a more detailed and targeted risk management strategy should be designed to prevent the possible hazards posed by different types of MC.

Ecological health evaluation of rivers based on phytoplankton biological integrity index and water quality index on the impact of anthropogenic pollution: A case of Ashi River Basin

Based on the phytoplankton community matrices in the Ashi River Basin (ASRB), Harbin city, we developed an evaluation method using the phytoplankton index of biotic integrity (P-IBI) to evaluate ecological health while investigating the response of P-IBI to anthropogenic activities. We compared the effectiveness of P-IBI with that of the water quality index (WQI) in assessing ecological health. Between April and October 2019, phytoplankton and water samples were collected at 17 sampling sites in the ASRB on a seasonal basis. Our results showed that seven phyla were identified, comprising 137 phytoplankton species. From a pool of 35 candidate indices, five critical ecological indices (Shannon–Wiener index, total biomass, percentage of motile diatoms, percentage of stipitate diatom, and diatom quotient) were selected to evaluate the biological integrity of phytoplankton in the ASRB. The ecological status of the ASRB as measured by the P-IBI and WQI exhibited a similar spatial pattern. It showed a spatial decline in ecological status in accordance with the flow of the river. These results highlighted that P-IBI was a reliable tool to indicate the interaction between habitat conditions and environmental factors in the ASRB. Our findings contribute to the ecological monitoring and protection of rivers impacted by anthropogenic pollution.

Humic acid inhibits colony formation of the cyanobacterium Microcystis at high level of iron

Colony formation is a key process for the occurrence of Microcystis blooms. In order to inhibit colony formation of Microcystis at high level of iron using humic acid, unicellular Microcystis aeruginosa was cultivated in laboratory treated with varying concentrations of iron and humic acid. Our results showed that the extracellular polysaccharides (EPS) content and average colony size increased from 0.57 pg cells^-1 and 4.0 μm to 0.93 pg cells^-1 and 26.1 μm, respectively, while iron concentration increased from 0.68 mg L^-1 to 6.8 mg L^-1, suggesting that high level of iron stimulated EPS secretion and induced unicellular Microcystis to form colonies. Transcriptome analysis showed that two genes described as glycosyltransferases (BH695-2217 and BH695-3696) were significantly up-regulated while EPS content increased with increasing iron concentration indicating that iron may regulate the expression of genes involved in polysaccharide synthesis. When treated with 10 mg C L^-1 humic acid at high level of iron, the EPS content and average colony size decreased by 35.5% and 56.3%, respectively, revealing that humic acid inhibited EPS secretion under high level of iron condition, and ultimately inhibited colony formation of Microcystis. Our results suggested that humic acid could be used as an agentia inhibiting large colony formation of Microcystis and thereby reducing the occurrence of Microcystis blooms.

Feedback regulation of surface scum formation and persistence by self-shading of Microcystis colonies: Numerical simulations and laboratory experiments

Light availability is an important driver of algal growth and for the formation of surface blooms. The formation of Microcystis surface scum decreases the transparency of the water column and influences the vertical distribution of light intensity. Only few studies analysed the interactions between the dynamics of surface blooms and the light distribution in the water column. Particularly the effect of light attenuation caused by Microcystis colonies (self-shading) on the formation of surface scum has not been explored. In the present study, we simulate the effect of variable cell concentration of Microcystis colonies on the vertical distribution of light in the water column based on experimental estimates of the extinction coefficient of Microcystis colonies. The laboratory observations indicated that higher cell concentration of Microcystis enhance the light attenuation in water column and promotes surface scum formation. We extended an existing model for the light-driven migration of Microcystis by introducing the effect of self-shading and simulated the dynamics of vertical migration for different cell concentrations and different colonial morphologies. The simulation results show that high cell concentrations of Microcystis promote surface scum formation, as well as its persistence throughout diel photoperiods. Large and tight Microcystis colonies facilitate scum formation, while small and loose colonies increase scum stability and persistence. This study reveals a positive feedback regulation of Microcystis surface scum formation and stability by self-shading and provides novel insights into the underlying mechanisms.

Interaction of cyanobacteria with calcium facilitates the sedimentation of microplastics in a eutrophic reservoir

Low-density microplastics are frequently found in sediments of many lakes and reservoirs. The processes leading to sedimentation of initially buoyant polymers are poorly understood for inland waters. This study investigated the impact of biofilm formation and aggregation on the density of buoyant polyethylene microplastics. Biofilm formation on polyethylene films (4 × 4 × 0.15 mm) was studied in a eutrophic reservoir (Bautzen, Saxony, Germany). Additionally, aggregation dynamics of small PE microplastics (~85 µm) with cyanobacteria were investigated in laboratory experiments. During summer phototrophic sessile cyanobacteria (Chamaesiphon spp. and Leptolyngbya spp.) precipitated calcite while forming biofilms on microplastics incubated in Bautzen reservoir. Subsequently the density of the biofilms led to sinking of roughly 10% of the polyethylene particles within 29 days of incubation. In the laboratory experiments planktonic cyanobacteria (Microcystis spp.) formed large and dense cell aggregates under the influence of elevated Ca²⁺ concentrations. These aggregates enclosed microplastic particles and led to sinking of a small portion (~0.4 %) of polyethylene microplastics. This study showed that both sessile and planktonic phototrophic microorganisms mediate processes influenced by calcium which facilitates densification and sinking of microplastics in freshwater reservoirs. Loss of buoyancy leads to particle sedimentation and could be a prerequisite for the permanent burial of microplastics within reservoir sediments.

Nitrogen availability affects the dynamics of Microcystis blooms by regulating the downward transport of biomass

Nitrogen availability is one of the key factors affecting the dynamics of non-diazotrophic cyanobacterial blooms in eutrophic lakes. While previous studies mainly focused on the promoting effect of nitrogen on the growth of cyanobacteria, this study aimed to investigate the role of nitrogen availability in the downward transport of biomass and its effects on the dynamics of Microcystis blooms. We performed field enclosure experiments which demonstrated that nitrogen availability negatively affects the downward transport of biomass. With a nitrogen loading of 0.02 g N m^-2 d^-1, the Microcystis biomass in the water column decreased by 56.2% over a 4-day period. During the same period of time, the average sinking ratio was 0.23 d^-1; moreover, the termination of biomass growth was detected. At the notably higher nitrogen loading of 0.5 g N m^-2d^-1, the downward transport of biomass could still compensate for the biomass growth, although the average sinking ratio was lower at 0.16 d^-1. Additional laboratory culture experiments demonstrated that the increase in the downward transport of Microcystis occurred in parallel to an increase in the carbohydrate content and a decrease in gas vesicle content. Further proteomic analysis indicated that the carbohydrate accumulation induced by nitrogen deficiency was a result of the slowing down of catabolic consumption, especially the downregulation of glycolysis. Thus, our study suggests that increased intracellular carbohydrate accumulation at low nitrogen availability causes a higher sinking ratio of Microcystis, indicating that nitrogen limits the duration of Microcystis blooms; thus, decreased nitrogen availability may lead to increased sinking of biomass out of the water column, accelerating the dissipation of Microcystis blooms.

Calcium promotes formation of large colonies of the cyanobacterium Microcystis by enhancing cell-adhesion

Large Microcystis colonies can lead to the rapid formation of surface accumulations, which are a globally significant environmental issue. Laboratory studies have shown that Ca²⁺ can quickly promote non-classical Microcystis colony formation via cell-adhesion, but our knowledge of the changes in the morphology of these colonies during subsequent long-term culture with Ca²⁺ is limited. In this study, a 72-day cultivation experiment was conducted to determine the long-term effects of Ca²⁺ on Microcystis colony formation. Laboratory results indicate that Ca²⁺ causes Microcystis to rapidly aggregate and form a colony through cell adhesion, then colony formation by cell-adhesion lost dominance, owing to the decrease in Ca²⁺ concentrations caused by precipitation/complexation. Although the initial colony morphology by cell adhesion is sparse, the newly divided cells, without separating from the mother cells, constantly fill the gaps in the original colony at Ca²⁺ concentrations >40 mg L^-1 for a long time, which creates colonies on day 72 with a morphology similar to that of M. ichthyoblabe in Lake Taihu. If the Ca²⁺ levels in Lake Taihu continue to increase, Microcystis growth rate will decrease only slightly, while the colony proportion of total biovolume and biomass will increase. Moreover, higher Ca²⁺ concentrations do not affect microcystin content, but promote the content of bound extracellular polysaccharides (bEPS), enabling formation of larger colonies, which may promote Microcystis surface accumulation.