Effect of Zinc on Microcystis aeruginosa UTEX LB 2385 and Its Toxin Production

Cyanobacteria harmful algal blooms (CHABs) are primarily caused by man-made eutrophication and increasing climate-change conditions. The presence of heavy metal runoff in affected water systems may result in CHABs alteration to their ecological interactions. Certain CHABs produce by-products, such as microcystin (MC) cyanotoxins, that have detrimentally affected humans through contact via recreation activities within implicated water bodies, directly drinking contaminated water, ingesting biomagnified cyanotoxins in seafood, and/or contact through miscellaneous water treatment. Metallothionein (MT) is a small, metal-sequestration cysteine rich protein often upregulated within the stress response mechanism. This study focused on zinc metal resistance and stress response in a toxigenic cyanobacterium, Microcystis aeruginosa UTEX LB 2385, by monitoring cells with (0, 0.1, 0.25, and 0.5 mg/L) ZnCl(2) treatment. Flow cytometry and phase contrast microscopy were used to evaluate physiological responses in cultures. Molecular assays and an immunosorbent assay were used to characterize the expression of MT and MC under zinc stress. The results showed that the half maximal inhibitory concentration (IC(50)) was 0.25 mg/L ZnCl(2). Flow cytometry and phase contrast microscopy showed morphological changes occurred in cultures exposed to 0.25 and 0.5 mg/L ZnCl(2). Quantitative PCR (qPCR) analysis of selected cDNA samples showed significant upregulation of Mmt through all time points, significant upregulation of mcyC at a later time point. ELISA MC-LR analysis showed extracellular MC-LR (µg/L) and intracellular MC-LR (µg/cell) quota measurements persisted through 15 days, although 0.25 mg/L ZnCl(2) treatment produced half the normal cell biomass and 0.5 mg/L treatment largely inhibited growth. The 0.25 and 0.5 mg/L ZnCl(2) treated cells demonstrated a ~40% and 33% increase of extracellular MC-LR(µg/L) equivalents, respectively, as early as Day 5 compared to control cells. The 0.5 mg/L ZnCl(2) treated cells showed higher total MC-LR (µg/cell) quota yield by Day 8 than both 0 mg/L ZnCl(2) control cells and 0.1 mg/L ZnCl(2) treated cells, indicating release of MCs upon cell lysis. This study showed this Microcystis aeruginosa strain is able to survive in 0.25 mg/L ZnCl(2) concentration. Certain morphological zinc stress responses and the upregulation of mt and mcy genes, as well as periodical increased extracellular MC-LR concentration with ZnCl(2) treatment were observed.