Multiple strategies of bloom-forming Microcystis to minimize damage by solar ultraviolet radiation in surface waters

The occurrence of bloom-forming cyanobacteria is one of the most obvious sign of eutrophication in freshwaters. Although in eutrophic lakes water transparency in the ultraviolet (UV) region is strongly reduced, bloom-forming cyanobacteria are exposed to high solar UV radiation at the surface. Here, we show that, in a natural phytoplankton community from a very eutrophic lake, Microcystis synthesizes UV sunscreen compounds identified as mycosporine-like amino acids (MAAs). The biomass-specific MAA concentration was significantly correlated with the occurrence of Microcystis but not with other algal groups, even though they were dominant in terms of biomass. Based on a photo-optical model, we estimated that the maximum MAA concentration per cell observed (2.5% dry weight) will confer only approximately 40% of internal screening to a single layer of Microcystis cells. Thus, the formation of a colony with several layers of cells is important to afford an efficient UV screening by internal self-shading. Overall, we propose that Microcystis uses a combination of photoprotective strategies (MAAs, carotenoids) to cope with high solar UV radiation at the water surface. These strategies include also the screening of UV radiation by D-galacturonic acid, one of the main chemical components of the slime layer in Microcystis.