MB6 -- Examining the relationship between nutrients, gene expression, and toxin production in Microcystis aeruginosa

The cyanobacterium Microcystis aeruginosa forms dense blooms and produces a group of toxins called microcystins. These toxins cause fish kills, fatalities of wildlife and livestock, and contamination of drinking water. Microcystins are cyclic heptapeptides produced by nonribosomal peptide synthesis. Their production varies with environmental conditions, such as temperature, nutrient availability, and pH. Although understanding the circumstances that increase M. aeruginosa toxicity is important for bloom prevention and mitigation strategies, the environmental factors that regulate microcystin production are not well understood. In this study, we investigated first, how nutrient availability influenced the expression of one of the genes involved in the biosynthesis of microcystins. Second, we investigated whether changes in gene expression correlated with increases in the microcystin concentration of M. aeruginosa. We cultured M. aeruginosa under 5 different nutrient conditions: control (cultured in BG-11 media), excess nitrogen (tripled nitrogen concentration), limited nitrogen (1/3 nitrogen concentration), excess phosphorus (tripled phosphorus concentration), and limited phosphorus (1/3 phosphorus concentration). We measured expression of mycD, one of the polyketide synthetase genes involved in microcystin biosynthesis, by qPCR. We also determined the intracellular concentration of microcystin by using a microcystin-specific ELISA. Relative to the control, we found that mycD expression was not significantly upregulated or downregulated in any of the treatments. However, we did find that there was a positive correlation between mycD expression and microcystin concentration. Overall, we could not detect an effect of nutrients on mycD gene expression, but we did find that higher gene expression led to increased microcystin concentration, suggesting that toxin production is controlled by transcription of the enzymes used for its biosynthesis, at least to some extent. In future studies, we plan to investigate the relationship between toxin concentration and gene expression in field samples