The Effect of Organic Carbon Addition on the Community Structure and Kinetics of Microcystin-Degrading Bacterial Consortia

Immobilization of Microcystin by the Hydrogel-Biochar Composite to Enhance Biodegradation during Drinking Water Treatment

Microcystin-LR (MC-LR), the most common algal toxin in freshwater, poses an escalating threat to safe drinking water. This study aims to develop an engineered biofiltration system for water treatment, employing a composite of poly(diallyldimethylammonium chloride)-biochar (PDDA-BC) as a filtration medium. The objective is to capture MC-LR selectively and quickly from water, enabling subsequent biodegradation of toxin by bacteria embedded on the composite. The results showed that PDDA-BC exhibited a high selectivity in adsorbing MC-LR, even in the presence of competing natural organic matter and anions. The adsorption kinetics of MC-LR was faster, and capacity was greater compared to traditional adsorbents, achieving a capture rate of 98% for MC-LR (200 μg/L) within minutes to tens of minutes. Notably, the efficient adsorption of MC-LR was also observed in natural lake waters, underscoring the substantial potential of PDDA-BC for immobilizing MC-LR during biofiltration. Density functional theory calculations revealed that the synergetic effects of electrostatic interaction and π-π stacking predominantly contribute to the adsorption selectivity of MC-LR. Furthermore, experimental results validated that the combination of PDDA-BC with MC-degrading bacteria offered a promising and effective approach to achieve a sustainable removal of MC-LR through an "adsorption-biodegradation" process.

Gut Microbiota of Freshwater Gastropod (Bellamya aeruginosa) Assist the Adaptation of Host to Toxic Cyanobacterial Stress

Gut microbes play a critical role in helping hosts adapt to external environmental changes and are becoming an important phenotype for evaluating the response of aquatic animals to environmental stresses. However, few studies have reported the role that gut microbes play after the exposure of gastropods to bloom-forming cyanobacteria and toxins. In this study, we investigated the response pattern and potential role of intestinal flora in freshwater gastropod Bellamya aeruginosa when exposed to toxic and non-toxic strains of Microcystis aeruginosa, respectively. Results showed that the composition of the intestinal flora of the toxin-producing cyanobacteria group (T group) changed significantly over time. The concentration of microcystins (MCs) in hepatopancreas tissue decreased from 2.41 ± 0.12 on day 7 to 1.43 ± 0.10 μg·g−1 dry weight on day 14 in the T group. The abundance of cellulase-producing bacteria (Acinetobacter) was significantly higher in the non-toxic cyanobacteria group (NT group) than that in the T group on day 14, whereas the relative abundance of MC-degrading bacteria (Pseudomonas and Ralstonia) was significantly higher in the T group than that in the NT group on day 14. In addition, the co-occurrence networks in the T group were more complex than that in the NT group at day 7 and day 14. Some genera identified as key nodes, such as Acinetobacter, Pseudomonas, and Ralstonia, showed different patterns of variation in the co-occurrence network. Network nodes clustered to Acinetobacter increased in the NT group from day 7 to day 14, whereas the interactions between Pseudomonas and Ralstonia and other bacteria almost changed from positive correlations in the D7T group to negative correlations in the D14T group. These results suggested that these bacteria not only have the ability to improve host resistance to toxic cyanobacterial stress by themselves, but they can also further assist host adaptation to environmental stress by regulating the interaction patterns within the community. This study provides useful information for understanding the role of freshwater gastropod gut flora in response to toxic cyanobacteria and reveals the underlying tolerance mechanisms of B. aeruginosa to toxic cyanobacteria.

Degradation of Three Microcystin Variants in the Presence of the Macrophyte Spirodela polyrhiza and the Associated Microbial Communities

Cyanobacteria, which form water blooms all over the world, can produce a wide range of cyanotoxins such as hepatotoxic microcystins (MCs) and other biologically active metabolites harmful to living organisms, including humans. Microcystin biodegradation, particularly caused by bacteria, has been broadly documented; however, studies in this field focus mainly on strains isolated from natural aquatic environments. In this paper, the biodegradation of microcystin-RR (MC-RR), microcystin-LR (MC-LR), and microcystin-LF (MC-LF) after incubation with Spirodela polyrhiza and the associated microorganisms (which were cultured under laboratory conditions) is shown. The strongest MC biodegradation rate after nine days of incubation was observed for MC-RR, followed by MC-LR. No statistically significant decrease in the concentration of MC-LF was noted. Products of MC decomposition were detected via the HPLC method, and their highest number was found for MC-RR (six products with the retention time between 5.6 and 16.2 min), followed by MC-LR (two products with the retention time between 19.3 and 20.2 min). Although the decrease in MC-LF concentration was not significant, four MC-LF degradation products were detected with the retention time between 28.9 and 33.0 min. The results showed that MC-LF was the most stable and resistant MC variant under experimental conditions. No accumulation of MCs or their biodegradation products in S. polyrhiza was found. The findings suggest that the microorganisms (bacteria and algae) associated with S. polyrhiza could be responsible for the MC biodegradation observed. Therefore, there is a need to broaden the research on the biodegradation products detected and potential MC-degraders associated with plants.

Taxonomic and Genotypical Heterogeneity of Microcystin degrading Bacterioplankton in Western Lake Erie

Microcystins (MCs) are among the predominant cyanotoxins that are primarily degraded by heterotrophic bacteria in various freshwater environments, including Lake Erie, a Laurentian Great Lake. However, despite the prevalence of MCs in Lake Erie basins, our knowledge about the taxonomic diversity of local MC-degrading bacteria is largely limited. The current study obtained thirty-four MC-degrading bacterial pure isolates from Lake Erie surface water and characterized their taxonomical and phenotypic identities as well as their MC-degradation rates under different pH, temperature, availability of organic substrates and with other MC-degrading isolates. Obtained MC-degrading isolates included both Gram-positive (18 isolates of Actinobacteria and Firmicutes) and Gram-negative bacteria (16 isolates of Gamma-proteobacteria); and 7 of these isolates were motile, and 13 had the capacity to form biofilms. In general, MC-degradation rates of the isolates were impacted by temperature and pH but insensitive to the presence of cyanobacterial exudates. At the optimal temperature (30-35°C) and pH (7-8), individual isolates degraded MC-LR, the most abundant MC isomer, at an average of 0.20 µg/mL/hr. With additions of cyanobacterial exudates, only Pseudomonas sp. LEw-2029, a non-motile biofilm maker, showed increased MC degradation (0.25 µg/mL/hr). Five out of nine tested dual culture mixtures showed rises in MC degradation rates than their corresponding monocultures; the highest rate reached 0.40 µg/mL/hr for the pair LEw-(1132 + 2029). PCR amplification of mlrA genes yielded negative results for all isolates; subsequent enzyme assay-Mass Spectrum analysis identified no product associated with the mlr gene-based MC degradation pathway. Collectively, our results demonstrated that a diversity of indigenous Lake Erie bacteria can degrade MCs via a novel mlr-independent pathway. Obtained MC degraders, especially Pseudomonas sp. LEw-2029, may serve as candidates for the development of biological filters to remove cyanotoxins in water treatment systems.

A Mini Review on Microcystins and Bacterial Degradation

Microcystins (MCs) classified as hepatotoxic and carcinogenic are the most commonly reported cyanobacterial toxins found in the environment. Microcystis sp. possessing a series of MC synthesis genes (mcyA-mcyJ) are well documented for their excessive abundance, numerous bloom occurrences and MC producing capacity. About 246 variants of MC which exert severe animal and human health hazards through the inhibition of protein phosphatases (PP1 and PP2A) have been characterized. To minimize and prevent MC health consequences, the World Health Organization proposed 1 µg/L MC guidelines for safe drinking water quality. Further the utilization of bacteria that represent a promising biological treatment approach to degrade and remove MC from water bodies without harming the environment has gained global attention. Thus the present review described toxic effects and bacterial degradation of MCs.