Abstract
Microbial biofilms in gravity-driven membrane (GDM) filtration systems can efficiently degrade the cyanotoxin microcystin (MC), but it is unclear if this function depends on the presence of MC-producing cyanobacteria in the source water habitat. We assessed the removal of MC from added Microcystis aeruginosa biomass in GDMs fed with water from a lake with regular blooms of toxic cyanobacteria (ExpL) or from a stream without such background (ExpS). While initial MC removal was exclusively due to abiotic processes, significantly higher biological MC removal was observed in ExpL. By contrast, there was no difference in MC degradation capacity between lake and stream bacteria in separately conducted liquid enrichments on pure MC. Co-occurrence network analysis revealed a pronounced modularity of the biofilm communities, with a clear hierarchic distinction according to feed water origin and treatment type. Genotypes in the network modules associated with ExpS had significantly more links to each other, indicating that these biofilms had assembled from a more coherent source community. In turn, signals for stochastic community assembly were stronger in ExpL biofilms. We propose that the less “tightly knit” ExpL biofilm assemblages allowed for the better establishment of facultatively MC degrading bacteria, and thus for higher overall functional efficiency.
Highlights
Harmful cyanobacterial blooms are increasingly threatening water quality in freshwater and coastal marine ecosystems (Huisman et al, 2018; Paerl, 2018)
One aerobic pathway of MC-LR degradation mediated by the mlr gene cluster has been identified in Sphingomonas sp. (Bourne et al, 1996)
This gene encodes for three enzymes (MlrA, MlrB, and MlrC) that are responsible for the degradation process: first the cyclic peptide is linearized by cleavage of the Adda-Arg bond, and subsequently the MlrB and MlrC enzymes hydrolyze the linear peptide into smaller ones (Bourne et al, 2001; Dziga et al, 2013)
Summary
Harmful cyanobacterial blooms (cyanoHABs) are increasingly threatening water quality in freshwater and coastal marine ecosystems (Huisman et al, 2018; Paerl, 2018). Mature GDM biofilms can be an effective means of removing MCs from toxic cyanobacteria in the feed water (Kohler et al, 2014) This degradation process could be rapidly induced even without extended prior exposure to the toxin, indicating that the original establishment of MC degraders in the biofilm communities was unrelated to this specific metabolic trait (Silva et al, 2018). We hypothesized that potential differences might not merely be related to the presence or absence of MCdegrading bacteria in the source water, but might instead be rooted in community-related aspects of the biofilms For this purpose, we studied two types of microbial consortia generated with water from Lake Zurich and from a stream without recorded occurrence of toxic cyanobacteria. We produced planktonic enrichments with pure MC at aerobic and anaerobic conditions
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