Environmental drivers of bloom-forming cyanobacteria in the Baltic Sea: Effects of salinity, temperature, and irradiance

Abstract

Environmental changes, including hydrological modifications caused by global warming, are one of the major drivers of harmful cyanobacterial bloom expansion. The ecophysiological study of bloom-forming filamentous cyanobacteria Nodularia spumingena, Aphanizomenon sp. and Dolichospermum lemmermannii was conducted in a series of laboratory tests. Cyanobacterial cultures were grown at various combinations of environmental conditions (scenarios). These scenarios were combinations of irradiance in the sense of Photosynthetically Active Radiation (PAR) spectrum (10, 100, 190 and 280 μmol photons m−2s−1), temperature (15, 20 and 25 °C) and salinity (3, 8 and 13). The cell concentration, pigment content and photosynthetic performance of cyanobacteria were measured to analyze the environmental stress. Generally, a positive effect of high irradiance and temperature on the abundance of these organisms as well as a preference to low salinity were observed. Pigment concentration (chlorophyll a (Chl a), carotenoids (Car) and Car/Chl a ratio), Chl a fluorescence and photosynthetic irradiance response (P-E) curves were used to characterize photoacclimation capacity of the cyanobacterial strains. The highest Car/Chl a ratio was observed at a high irradiance (190–280 μmol photons m−2s−1) and the lowest at 10 μmol photons m−2s−1 and 25 °C. Chl a fluorescence of cyanobacterial strains indicated that the highest irradiance (280 μmol photons m−2s−1) and the highest temperature (25 °C) had a negative effect on both the maximum quantum yield (Fv/Fm) and the effective quantum yield of photosystem II (PSII) photochemistry (ΦPSII). This effect was more pronounced in the case of ΦPSII than Fv/Fm. Based on photosynthesis irradiance response (P-E) curves, two mechanisms of photoacclimation were recognized. The maximum value of the photosynthetic capacity (Pm) expressed per unit biomass for cells grown at 10 μmol photons m−2s−1 indicated a change in the number of Photosynthetic Units (PSU). The constant values of the initial slope of the P-E curve (α) and the maximum value of Pm expressed per Chl a unit at 280 μmol photons m−2s−1 indicated another mechanism, a change in PSU size. The study showed a wide range of filamentous cyanobacteria responses to the changing environment. This may explain the successful growth of freshwater and brackish filamentous cyanobacteria in the Baltic Sea and suggests further expansion with progressing climate change.