Exposure to toxic Microcystis via intact cell ingestion and cell crude extract differently affects small-bodied cladocerans.

Cyanobacterial blooms are increasingly common in aquatic environments worldwide. These microorganisms cause concern due to their ability to produce cyanotoxins. Aquatic organisms, especially zooplankton, are exposed to cyanobacterial toxins by different routes, depending on the bloom phase. During cyanobacterial dominance, zooplankton is exposed to cyanotoxins through the ingestion of cyanobacterial cells, while at the bloom senescence, dissolved toxins are the most representative route. In this study, we assessed the effects of a microcystin-producing strain of Microcystis aeruginosa (NPLJ-4) on clones of the tropical small cladocerans Macrothrix spinosa (two clones) and Ceriodaphnia cornuta (one clone) exposed to intact cells and aqueous cell crude extracts. Short-term toxicity assays and life-table experiments were performed to assess the effects of the toxic M. aeruginosa on the survival and life history of the cladocerans. In the short-term toxicity assay, we found that cladocerans were more affected by intact cells. Both clones of M. spinosa were more affected when exposed to intact cells, while C. cornuta displayed about 5-fold more resistance. On the other hand, crude extracts had a low impact on cladocerans' survival. Also, we observed a significant decrease in survival, fecundity, and growth of animals exposed to sublethal and environmentally relevant concentrations of M. aeruginosa cellular biomass. However, even at high concentrations of dissolved microcystins, the crude extract did not have significant effects on the life history parameters of the cladocerans. Although they can be found during cyanobacterial bloom events, small-bodied cladocerans are still affected by toxic Cyanobacteria depending on the exposure route.

Ecological impacts of freshwater algal blooms on water quality, plankton biodiversity, structure, and ecosystem functioning.

Harmful algal blooms are among the emerging threats to freshwater biodiversity that need to be studied further in the Anthropocene. Here, we studied freshwater plankton communities in ten tropical reservoirs to record the impact of algal blooms, comprising different phytoplankton taxa, on water quality, plankton biodiversity, and ecosystem functioning. We compared water quality parameters (water transparency, mixing depth, pH, electrical conductivity, dissolved inorganic nitrogen, total dissolved phosphorus, total phosphorus, chlorophyll-a, and trophic state), plankton structure (composition and biomass), biodiversity (species richness, diversity, and evenness), and ecosystem functioning (phytoplankton:phosphorus and zooplankton:phytoplankton ratios as a metric of resource use efficiency) through univariate and multivariate analysis of variance, and generalized additive mixed models in five different bloom categories. Most of the bloom events were composed of Cyanobacteria, followed by Dinophyta and Chlorophyta. Mixed blooms were composed of Cyanobacteria plus Bacillariophyta, Chlorophyta, and/or Dinophyta, while non-bloom communities presented phytoplankton biomass below the threshold for bloom development (10 mg L-1, WHO alert level 2). Higher phytoplankton biomasses were recorded during Cyanobacteria blooms (15.87-273.82 mg L-1) followed by Dinophyta blooms (18.86-196.41 mg L-1). An intense deterioration of water quality, including higher pH, eutrophication, stratification, and lower water transparency, was verified during Cyanobacteria and mixed blooms, while Chlorophyta and Dinophyta blooms presented lower pH, eutrophication, stratification, and higher water transparency. All bloom categories significantly impacted phytoplankton and zooplankton structure, changing the composition and dominance patterns. Bloom intensity positively influenced phytoplankton resource use efficiency (R2 = 0.25; p < 0.001), while decreased zooplankton resource acquisition (R2 = 0.51; p < 0.001). Moreover, Cyanobacteria and Chlorophyta blooms negatively impacted zooplankton species richness, while Dinophyta blooms decreased phytoplankton richness. In general, Cyanobacteria blooms presented low water quality and major threats to plankton biodiversity, and ecosystem functioning. Moreover, we demonstrated that biodiversity losses decrease ecosystem functioning, with cascading effects on plankton dynamics.

Growth of Microcystisstrains isolated from environments with the presence and absence of submerged macrophytes in coexistence with Ceratophyllum demersum

Cyanobacterial blooms can cause severe ecological and health problems in drinking water reservoirs. To alleviate this problem, allelopathically active submerged macrophytes can be used to reduce cyanobacterial growth. Accordingly, this study aimed to evaluate the sensitivity of strains of the Microcystis aeruginosacomplex isolated from reservoirs with the presence and absence of submerged macrophytes to the allelochemicals of Ceratophyllum demersum.A coexistence experiment was carried out between the submerged macrophyte C. demersum and four Microcystisstrains, with two treatments for each strain, one in coexistence with the submerged macrophyte (7 g L-1) and control (in the absence of the macrophyte). Two strains of M. aeruginosa(BMIUFRPE-06 and BMIUFRPE-07) and two of M. panniformis(BMIUFRPE-08 and BMIUFRPE-09) were used, which were isolated from Cajueiro (with submerged macrophytes) and Tapacurá (without submerged macrophytes) reservoirs, respectively. The biomass of Microcystisstrains from the reservoir without macrophytes (BMIUFRPE-08 and BMIUFRPE-09) was significantly inhibited in 96% (T-test: p 0.05; growth rate -ANOVA: p > 0.05). These results suggest that strains isolated from environments with submerged macrophytes are less sensitive to allelochemicals of these plants,as these strains may be adapted to the coexistence with submerged macrophytes.

Modeling cyanobacterial blooms in tropical reservoirs: The role of physicochemical variables and trophic interactions.

Understanding the importance of environmental variables on the dominance of cyanobacteria is crucial for appropriately managing water resources. Although studies about temperate and subtropical regions show a high influence of nutrients and temperature on blooms, this relationship is still unclear for the tropics. Accordingly, we hypothesized that nutrients and temperature are the main factors driving cyanobacterial blooms in tropical reservoirs, and those relationships are intensified by the zooplankton. To test these hypotheses, we constructed a structural equation model based on the monitoring of ten reservoirs from Northeast Brazil. We analyzed the effects of physicochemical variables and zooplankton on cyanobacterial blooms and the biomass of four morphotypes. Cyanobacterial biomass varied within the reservoirs, with bloom records (0.2-268.4 mg L-1) in all of them, primarily constituted by the colonial morphotype, followed by picocyanobacteria, heterocyted, and non-heterocyted filaments. The cyanobacterial community was driven mainly by chemical variables (55.14% of the variation), followed by physical (48.28%), and zooplankton (39.47%). Through the structural equation model, we demonstrated that total cyanobacterial biomass, as well as the morphotypes, were mainly influenced by omnivorous crustaceans and total dissolved phosphorus. Solar radiation, air temperature, mixing zone, and salinity were important to explain the biomass of the morphotypes. The model explained most of the variation in the picocyanobacterial blooms (79.8%), followed by total cyanobacteria (62.4%), heterocyted filaments (59.1%), non-heterocyted filaments (58.2%), and coccoids (55.1%). Zooplankton groups were also influenced by the physicochemical variables, which presented direct and indirect effects on cyanobacteria. Given the predictions of increased eutrophication, warming, and salinization, cyanobacterial blooms will become more intense in tropical reservoirs. Thus, restoring measures must be adopted to reduce bloom development, such as external phosphorus and salt loadings, and biomanipulation.