Extracellular Enzymes in Aquatic Environments: Possible Role of Non-Specific Peptidases in Microcystin Degradation and Effects of Assay Protocol on Calculated Activities

Extracellular enzyme assays are widely used methods to probe the interactions between microbes and complex organic matter. Microbes produce extracellular enzymes to degrade macromolecules into smaller molecules that can be transported across cell membranes. Enzyme assays provide a quantitative understanding of the rates and specificities of extracellular enzymes toward these macromolecules. This study explored 1) the biodegradation pathways of microcystin-LR (MC-LR), a cyanobacterial peptide toxin, by measuring the activities of extracellular peptidases produced by putative MCLR degraders and 2) the effects of enzyme assay protocol on activity measurements, which involved the creation of ezmmek, an R package designed to analyze enzyme assay data reproducibly under different protocols. Lactobacillus rhamnosus GG, an MC-LRdegrader that employs an unknown pathway, produces L-Leucine aminopeptidases. Future work can test whether these same peptidases are capable of degrading MC-LR. Two enzyme assay protocols were applied to the same freshwater sample, but resulted in significantly different activity measurements when analyzed with ezmmek. Widespread adoption of ezmmek could standardize enzyme analytical pathways performed by otherresearchers, and will make results more comparable among MC-LR and other organic matter degradation studies.