Strain-specific responses of toxic and non-toxic Microcystis aeruginosa to exudates of heterotrophic bacteria

Abstract

Although the cyanobacterium Microcystis is colonial during water blooms, isolated Microcystis strains become unicellular in culture. Studies indicate that heterotrophic bacteria can promote Microcystis colonies in culture, but little is known about the underlying mechanism or how widespread it is among bacteria. This study investigated the identity and colony-promoting effects of bacteria isolated from the 2014 Microcystis bloom in Lake Erie. Isolates were classified by their 16S rRNA gene sequences. Toxic and non-toxic cultures of M. aeruginosa were exposed to exudates of bacterial isolates, and their morphology, polysaccharide content, and reflectance spectra were compared to those of M. aeruginosa control cultures. Six isolates belonging to three genera enhanced the frequency or size of M. aeruginosa colonies in cultures where a dialysis barrier prevented direct contact between heterotrophic cells and M. aeruginosa cells. Toxic and non-toxic M. aeruginosa strains differed in how their morphology and optical properties responded to treatment. This study demonstrates that heterotrophic bacteria can promote colonial morphology in Microcystis without making physical contact with the Microcystis cells, as well as the first to indicate that toxic and non-toxic strains of the same morphospecies have different morphological and optical responses.