Background, aims and scope: Microcystins (MCs) are a family of natural toxins produced by cyanobacteria (blue-green algae). As a result of eutrophication, massive cyanobacterial blooms occur more frequently and MCs represent important contaminants of freshwater ecosystems. Bacterial biodegradation is considered a main mechanism for MC breakdown in environmental conditions. While existing studies were mostly focused on MC biodegradation by planktonic bacteria, our experiments examined the fate and kinetics of MC degradation in river-originated phototrophic biofilms and investigated factors influencing the rate of MC removal.
Methods: The fate of dissolved MCs was studied in laboratory microcosms with different composition (containing water only, water with phytoplankton and/or phototrophic biofilms). Biofilms originated from river ecosystem were pre-incubated under various conditions (with/without presence of cyanobacterial biomass or model organic substrates: glucose and protein--casein). Changes in MC concentration (0-14 days) in water columns were measured by HPLC-DAD after external additions of purified MCs (160 microg L(-1), MC-LR and MC-YR), and halftimes (t1/2) of MC removal were estimated.
Results and discussion: The slow degradation of MC was revealed in tap water (t1/2 approximately 14 days) and river water without cyanobacteria (t1/2 approximately 8 days). Enhanced removal occurred in the presence of natural planktonic cyanobacteria (t1/2 approximately 44 h), most probably due to microorganisms associated with the biomass of cyanobacterial bloom. More rapid MC elimination occurred in the variants containing phototrophic biofilms, and was particularly pronounced at those biofilms pre-cultivated in the presence of cyanobacterial blooms (t1/2 approximately 20 h). Much slower removal was observed in the variants simulating possible substrate-dependent induction of microorganism metabolism (biofilms pre-incubated with glucose: t1/2 approximately 35 h, and casein: t1/2 approximately 80 h). After termination of experiments, total amounts of MCs accumulated in the biofilms were below 5% of the initial toxin level revealing significant biodegradation processes.
Conclusion: The microcosm studies contributed to understanding of the environmental fate of MCs and revealed a rapid biodegradation by phototrophic biofilms. The rate of MC elimination depends on history of biofilm community, previous contact with cyanobacteria seems to be a selective factor improving the biodegradation potential.
Recommendation and outlook: Our results experimentally showed a positive role of biofilms in MC elimination during water treatment processes such as bank filtration or slow sand filtration, and could eventually serve for further research of biofilm-based technological applications for MCs removal in small-scale drinking water treatment facilities.