Impacts of algal organic matter and humic substances on microcystin-LR removal and their biotransformation during the biodegradation process

The application of bioaugmentation (i.e., injection of contaminant-degrading microorganisms) has shown its potential to remove harmful cyanotoxins like microcystin-LR (MC-LR) from drinking water sources. However, the natural organic matter (NOM) present in both natural and engineered water systems might affect the bacterial biodegradation of MC-LR. Therefore, for the successful application of bioaugmentation for MC-LR removal in water treatment, it is important to understand NOM effects on MC-LR biodegradation. In this study, the impact of NOM [algal organic matter (AOM) and humic substances (HS)] on MC-LR biodegradation was evaluated in the presence of varying concentrations of NOM by monitoring MC-LR biodegradation kinetics. The changes in NOM composition during MC-LR biodegradation were also characterized by a five-component Parallel factor (PARAFAC) model using 336 excitation-emission matrix (EEM) spectra collected at different sampling points. Our results showed decreases in MC-LR biodegradation rate of 1.6-and 3.4-fold in the presence of AOM and HS, respectively. The expression of the functional mlrA gene exhibited a similar trend to the MC-LR degradation rate at different NOM concentrations. EEM-PARAFAC analyses and NOM molecular size fractionation results indicated a relatively greater production of terrestrial humic-like components (57%) and a decrease of protein-like components. Two-dimensional correlation spectroscopy (2D-COS) analyses further confirmed that low molecular weight protein-like components were initially utilized by bacteria, followed by the formation of higher molecular weight humic-like components, likely due to microbial metabolism.