Microcystin-LR exposure enhances toxin-degrading capacity and reduces metabolic diversity of sediment microbial communities.

Substantial outbreaks of cyanobacterial harmful algal blooms (cyanoHABs) are accompanied by the prevalent existence of microcystins (MCs) in eutrophic freshwater worldwide, which can compromise the safety of aquatic ecosystems and public health. However, the response of the sediment microbial community to MCs exposure remains unclear. In this study, the sediment microbial communities of Lake Taihu were continuously exposed to environmentally relevant concentration (10 μg/L) of microcystin-LR (MC-LR) in microcosms for three months to mimic MCs contamination during cyanoHABs. Meanwhile, the MC-LR degradation characteristics, metabolic profiles, functional genes, and structures of microbial communities were analyzed periodically. The results showed that the MC-LR degradation capacities of microbial communities significantly increased with prolonged toxin exposure time. The enhanced MC-LR degradation rate was positively correlated with the increase in the abundance of mlr genes, which corresponds to the MC-degrading bacterial community. However, the MC-LR degradation pathway was maintained, and one new biodegradation intermediate (Adda-Glu) was first identified. Moreover, the metabolic profiles of the microbial communities indicated that overall carbon metabolic activity displayed a stimulation-suppression trend, and the exposure led to different carbon utilization patterns. Furthermore, microbial community structure analysis showed that long-term MC-LR exposure enriched microbes associated with MC degradation (Pseudomonas, Stenotrophomonas, Sphingopyxis and Sphingomonas) but inhibited MC-sensitive bacterial genera. This study clearly elucidates that MC-LR exposure alters the function, metabolism, and structure of the sediment microbial community to adapt to polluted environments.