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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.

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.

Toxic cyanobacteria can affect several organisms, including herbivorous zooplankton, as cladocerans. In this study we tested the hypothesis that cladoceran species of different body size, origin and degree of exposure to toxic cyanobacteria would respond differently to a saxitoxin(STX)-producing strain of the cyanobacterium Cylindrospermopsis raciborskii (CYRF-01). Newborns were exposed to increasing concentrations of the strain CYRF-01 mixed to a fixed proportion of green algae for 14 days and survivorship, age at first reproduction, clutch size, total offspring and the intrinsic rate of population increase (r), as a measure of fitness, were estimated. Different responses to the strain CYRF-01 were observed, with Daphnia similis (temperate, large size) being more sensitive, followed by Moina micrura (tropical, small size), both presenting symptoms of decreased fitness and paralysis. While Ceriodaphnia richardi (tropical, small size) D. gessneri (tropical, medium size) and Diaphanosoma spinulosum (tropical, small size) showed neither of these symptoms, the later species showed even increased fitness in the cyanobacterial treatments relative to control with green algae, suggesting not only a greater resistance to this strain, but also that it can serve as a nutritional supplement for this cladoceran. In conclusion, the results showed that while some cladocerans species may be negatively affected in their fitness, others may be not affected at all or even utilize STX-producer cyanobacteria as a complementary resource. This suggests that responses of cladocerans to toxic cyanobacteria in nature may vary with species and that the presence of toxic cyanobacteria may shape zooplankton communities to the dominance of more tolerant herbivorous species.

The present study aimed to test the effects of raw water samples from a eutrophic reservoir and of a saxitoxin-producing strain of Cylindrospermopsis raciborskii on the swimming behavior of 2 key herbivore species of Daphnia. Two complementary approaches were used, acute bioassays and behavioral assays using an automated movement tracking system for measuring the following activity parameters: swimming time, resting time, distance traveled, and mean velocity. In both assays, animals were exposed to field samples or to toxic filaments in different concentrations and observed for 2 h to 3 h. In the acute bioassays, there was a decrease in the number of swimming individuals during the exposure period and a recovery following removal from toxic algae. A significant relationship was found between median effective concentration and the saxitoxin content of seston (r2 = 0.998; p = 0.025) in the acute bioassays with raw water samples. Behavioral assays also showed significant effects in the activity parameters with both field samples and the strain of C. raciborskii, with some recovery during the exposure period. Both approaches corroborated previous research on the effects of neurotoxic C. raciborskii on the swimming activity of Daphnia, and these effects are compatible with the mechanism of action of saxitoxins. The present study showed that activity parameters of aquatic organisms may be a useful tool in the evaluation of sublethal toxicity and detection of neurotoxins in raw water. Environ Toxicol Chem 2014;33:223–229. © 2013 SETAC

Toxic cyanobacteria in aquatic environments have been implicated in many poisoning incidents of livestock, wildlife, and domestic animals. Microcystins (MCYSTs) in water supplies represent a risk to public health. This work investigated the effect of water composition on the quantitation and biological activity of MCYSTs analyzed by different methods (HPLC, ELISA, and protein phosphatase 1 inhibition assay). Different MCYST concentrations were added to deionized water and quantified, confirming the efficiency of these analytical methods. MCYST concentrations diluted in drinking water had reduced detection by all methods tested. The drinking water used contained a free chlorine concentration of 2.5 mg/L and an Fe concentration of 0.45 mg/L, and the conductivity was 69.8 μS cm−1, whereas in deionized water, free chlorine and Fe were not detectable, and the conductivity was 1.6 μS cm−1. Drinking water also interfered with the biological activity of MYCSTs, as these toxins showed reduced protein phosphatase-1 inhibition. A free chlorine concentration of 2.5 mg/L in deionized water was completely effective in preventing any detection of 10 μg/L of added MCYSTs. Fe and Al ions also were very effective in reducing MCYST detection. The chemical composition of drinking water thus affected MCYST detection, indicating a significant reduction in quantitation of this molecule either because of its decomposition or through complexation with metal ions. © 2005 Wiley Periodicals, Inc. Environ Toxicol 20: 126–130, 2005.

In order to understand accumulation and depuration of microcystins (MCYSTs) in Tilapia rendalli, three experiments with juveniles were done. The experiments simulated the fish diet during a Microcystis aeruginosa bloom in three different situations. In the first one each fish received daily, during 15 days, fish food plus toxic cells of M. aeruginosa (20.4 microg MCYSTs fish(-1) day(-1)). In the following 15 days they were fed without toxic cells. In the second experiment, fish were fed only with toxic cells during 28 days (14.6 microg MCYSTs fish(-1) day(-1)) and in the third experiment, during 42 days, fish were fed with fish food plus toxic cells (29.2 microg MCYSTs fish(-1) day(-1)) previously disrupted (to simulate a senescent bloom). MCYSTs analyses were done by enzyme-linked immunosorbent assay (ELISA) in liver and muscle samples in all experiments and in faeces in the first one (only in the depuration period). The results demonstrated different profiles of MCYSTs accumulation in liver and muscle of T. rendalli. Comparing the experiments, the highest MCYSTs accumulation in the liver (2.8 microg g(-1)) occurred in the second one, where fish had only toxic cells as feeding source. In the first experiment, the highest MCYSTs accumulation in liver (0.6 microg MCYSTs g(-1)) was observed during the accumulation period, while in muscle, interestingly, the highest concentration (0.05 microg MCYSTs g(-1)) occurred in the depuration period. In this same period, it was also observed elimination of toxins through faeces. The second and third experiments showed almost the same average concentrations in tissues although fish have received more MCYSTs in third one. With respect to implications of the fish comsumption, MCYSTs accumulation in muscle of T. rendalli in all three experiments reached concentrations that would represent an intake of these toxins above the tolerable limit for humans and these results confirmed our previous observations from a field study. In conclusion, in this study it was observed that T. rendalli is able to accumulate MCYSTs and the availability of other feeding sources, besides toxic cells, probably interferes with the accumulation rate. Therefore, the occurrence of toxic cyanobacterial blooms produncing MCYSTs in aquaculture ponds could represent a risk to the quality of fish to the consumers.