Mycosporine-like amino acids (MAAs)-producing Microcystis in Lake Erie: Development of a qPCR assay and insight into its ecology

Abstract

Mycosporine-like amino acids (MAAs) are UV-absorbing metabolites found in cyanobacteria. While their protective role from UV in Microcystis has been studied in a laboratory setting, a full understanding of the ecology of MAA-producing versus non-MAA-producing Microcystis in natural environments is lacking. This study presents a new tool for quantifying MAA-producing Microcystis and applies it to obtain insight into the dynamics of MAA-producing and non-MAA-producing Microcystis in Lake Erie. This study first developed a sensitive, specific TaqMan real-time PCR assay that targets MAA synthetase gene C (mysC) of Microcystis (quantitative range: 1.7 × 10¹ to 1.7 × 10⁷ copies/assay). Using this assay, Microcystis was quantified with a MAA-producing genotype (mysC⁺) in water samples (n = 96) collected during March-November 2013 from 21 Lake Erie sites (undetectable - 8.4 × 10⁶ copies/ml). The mysC⁺ genotype comprised 0.3-37.8% of the Microcystis population in Lake Erie during the study period. The proportion of the mysC⁺ genotype during high solar UV irradiation periods (mean = 18.8%) was significantly higher than that during lower UV periods (mean = 9.7%). Among the MAAs, shinorine (major) and porphyra (minor) were detected with HPLC-PDA-MS/MS from the Microcystis isolates and water samples. However, no significant difference in the MAA concentrations existed between higher and lower solar UV periods when the MAA concentrations were normalized with Microcystis mysC abundance. Collectively, this study's findings suggest that the MAA-producing Microcystis are present in Lake Erie, and they may be ecologically advantageous under high UV conditions, but not to the point that they exclusively predominate over the non-MAA-producers.

Keywords: Eutrophication; Harmful algal bloom; Porphyra; Shinorine; Sunscreen; UV irradiation.