Rising temperatures may increase growth rates and microcystin production in tropical Microcystis species

Rising temperatures (1.4-6 °C) due to climate change have been predicted to increase cyanobacterial bloom occurrences in temperate water bodies; however, the impacts of warming on tropical cyanobacterial blooms are unknown. We examined the effects of four different temperatures on the growth rates and microcystin (MC) production of five tropical Microcystis isolates (M. ichthyoblabe (two strains), M. viridis, M. flos-aquae, and M. aeruginosa). The temperature treatments are based on current temperature range in Singapore's reservoirs (27 °C and 30 °C), as well as projected mean (33 °C) and maximum temperatures (36 °C) based on tropical climate change estimates of +6 °C in air temperature. Increasing temperatures did not significantly affect the maximum growth rates of most Microcystis strains. Higher growth rates were only observed in one M. ichthyoblabe strain at 33 °C and M. flos-aquae at 30 °C where both were isolated from the same reservoir. MC-RR and MC-LR were produced in varying amounts by all four species of Microcystis. Raised temperatures of 33 °C were found to boost total MC cell quota for three Microcystis strains although further increase to 36 °C led to a sharp decrease in total MC cell quota for all five Microcystis strains. Increasing temperature also led to higher MC-LR:MC-RR cell quota ratios in M. ichthyoblabe. Our study suggests that higher mean water temperatures resulting from climate change will generally not influence growth rates of Microcystis spp. in Singapore except for increases in M. ichthyoblabe strains. However, toxin cell quota may increase under moderate warming scenarios depending on the species.