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Cyanobacterial blooms affect biotic interactions in aquatic ecosystems, including those involving heterotrophic bacteria. Ultra-small microbial communities are found in both surface water and groundwater and include diverse heterotrophic bacteria. Although the taxonomic composition of these communities has been described in some environments, the involvement of these small cells in the fate of environmentally relevant molecules has not been investigated. Here, we aimed to test if small-sized microbial fractions from a polluted urban lagoon were able to degrade the cyanotoxin microcystin (MC). We obtained cells after filtration through 0.45 as well as 0.22 μm membranes and characterized the morphology and taxonomic composition of bacteria before and after incubation with and without microcystin-LR (MC-LR). Communities from different size fractions ( 0.22 and 0.45 μm) were able to remove the dissolved MC-LR. The originally small-sized cells grew during incubation, as shown by transmission electron microscopy, and changed in both cell size and morphology. The analysis of 16S rDNA sequences revealed that communities originated from 0.22 and 0.45 μm fractions diverged in taxonomic composition although they shared certain bacterial taxa. The presence of MC-LR shifted the structure of 0.45 μm communities in comparison to those maintained without toxin. Actinobacteria was initially dominant and after incubation with MC-LR Proteobacteria predominated. There was a clear enhancement of taxa already known to degrade MC-LR such as Methylophilaceae. Small-sized bacteria constitute a diverse and underestimated fraction of microbial communities, which participate in the dynamics of MC-LR in natural environments.

Cyanobacterial blooms are increasingly reported worldwide, presenting a challenge to water treatment plants and concerning risks to human health and aquatic ecosystems. Advanced oxidative processes comprise efficient and safe methods for water treatment. Hydrogen peroxide (H2O2) has been proposed as a sustainable solution to mitigate bloom-forming cyanobacteria since this group presents a higher sensitivity compared to other phytoplankton, with no major risks to the environment at low concentrations. Here, we evaluated the effects of a single H2O2 addition (10 mg L−1) over 120 h in mesocosms introduced in a reservoir located in a semi-arid region presenting a Planktothrix-dominated cyanobacterial bloom. We followed changes in physical and chemical parameters and in the bacterioplankton composition. H2O2 efficiently suppressed cyanobacteria, green algae, and diatoms over 72 h, leading to an increase in transparency and dissolved organic carbon, and a decrease in dissolved oxygen and pH, while nutrient concentrations were not affected. After 120 h, cyanobacterial abundance remained low and green algae became dominant. 16S rRNA sequencing revealed that the original cyanobacterial bloom was composed by Planktothrix, Cyanobium and Microcystis. Only Cyanobium increased in relative abundance at 120 h, suggesting regrowth. A prominent change in the composition of heterotrophic bacteria was observed with Exiguobacterium, Paracoccus and Deinococcus becoming the most abundant genera after the H2O2 treatment. Our results indicate that this approach is efficient in suppressing cyanobacterial blooms and improving water quality in tropical environments. Monitoring changes in abiotic parameters and the relative abundance of specific bacterial taxa could be used to anticipate the regrowth of cyanobacteria after H2O2 degradation and to indicate where in the reservoir H2O2 should be applied so the effects are still felt in the water treatment plant intake.

Eutrophication is an ecological process that results in cyanobacterial blooms. Microcystin-LR is the most toxic variant of microcystins and may cause toxic effects in the organisms, mainly in hepatic tissues. The aims of this study were to use multiple biomarkers in order to evaluate the sublethal effects of a low concentration of MC-LR (1 μg/L) in fish Geophagus brasiliensis by waterborne exposure; and evaluate the depuration of this toxin during 15 days. A group of 30 fish was exposed to 1 μg/L of MC-LR solution for 96 h in a static bioassay. After this time, blood, brain, muscle, liver, gonad and gills were collected from half of the exposed fish group in order to evaluate chemical, biochemical, histological and genotoxic biomarkers. The rest of the fish group was submitted to the depuration experiment with free MC-LR water for 15 days. After this time the same tissues were collected and evaluated using biomarkers analysis. Toxic effects were found mostly in the fish liver from depuration time as alterations on the antioxidant system and histopathologies. The results showed that even low concentrations can cause sublethal effects to aquatic organisms, and cyanotoxins monitoring and regulation tools are required.

Harmful cyanobacterial blooms are increasingly common in aquatic environments. This can lead to higher concentrations of cyanotoxins, such as microcystins (MCs), posing a great risk to diverse organisms, including humans. MCs are among the most commonly reported cyanotoxins in freshwater environments worldwide, where they may have different fates. MCs can adsorb to suspended particles into the water column and deposit onto the sediment where they can be affected by physical factors (e.g. winds in shallow lakes causing sediment resuspension) or biological factors (e.g. biodegradation). Here we focused on the conditions of a coastal shallow lagoon contaminated by MCs aiming to estimate the return of pre-existing MCs from the sediment to the water column, to evaluate the adsorption of dissolved MC-LR to the sediment and to verify the occurrence of biodegradation. In experiments with sediment, desorption and adsorption were tested under the influence of temperature, pH and aeration, reproducing conditions observed in the lagoon. MC-desorption was not detected under the tested conditions. Spiking MC-LR into lagoon water samples in the presence of sediment resulted in a 50 % reduction of soluble MC-LR concentration in control conditions (25 °C, pH 8.0, no aeration). Increasing temperature (45 °C) or introducing aeration further stimulated MC-LR removal from the water. Biodegradation was observed in sediment samples and interstitial water (even with tetracycline). The composition of the bacterial community differed in sediment and interstitial water: major phyla were Chloroflexi, Proteobacteria, Firmicutes, and OP3. From the assigned OTUs, we identified genera already described as MC degrading bacteria. Thus, the sediment is a key factor influencing the fate of MC-LR in this shallow coastal lake contributing to stable adsorption and biodegradation.

The cyanobacterium Cylindrospermopsis raciborskii is a widespread species increasingly being recorded in freshwater systems around the world. It is of particular concern because strains in some geographic areas are capable of producing toxins with implications for human and animal health. Studies of this species have increased rapidly in the last two decades, especially in the southern hemisphere where toxic strains are prevalent. A clearer picture is emerging of the strategies adopted by this species to bloom and out-compete other species. This species has a high level of flexibility with respect to light and nutrients, with higher temperatures and carbon dioxide also promoting growth. There are two types of toxins produced by C. raciborskii: cylindrospermopsins (CYNs) and saxitoxins (STXs). The toxins CYNs are constitutively produced irrespective of environmental conditions and the ecological or physiological role is unclear, while STXs appear to serve as protection against high salinity and/or water hardness. It is also apparent that strains of this species can vary substantially in their physiological responses to environmental conditions, including CYNs production, and this may explain discrepancies in findings from studies in different geographical areas. The combination of a flexible strategy with respect to environmental conditions, and variability in strain response makes it a challenging species to manage. Our ability to improve bloom prediction will rely on a more detailed understanding of the complex physiology of this species.