Temperature and salinity affect growth and toxin content of cyanobacterium Microcystis aeruginosa (PCC 7806) in estuarine environments

Microcystis aeruginosa is considered a harmful cyanobacterial species due to its ability to produce microcystins (MCs) and its increasing prevalence in estuarine environments. While previous studies have demonstrated the effects of individual environmental factors on either growth or toxin content of M. aeruginosa, potential interactive effects and resulting changes in its toxicity remain unclear. In this study, we first conducted an orthogonally designed growth experiment to assess potential effects of changes in temperature, salinity, pH, and nutrient conditions. Subsequently, we performed a full-factorial growth experiment focusing on temperature and salinity as key variables. Intracellular and extracellular MCs content, as well as phycocyanin levels, were measured during both exponential and stationary growth phases. Toxicity was further evaluated based on mortality and swimming behavior of the epibenthic copepod Nitokra spinipes and the planktonic copepod Acartia tonsa. Results showed that both growth rate and MCs content significantly increased with temperature (from 15 to 28 °C) but decreased with higher salinity (from 8 to 16 ppt). Moreover, cell density was significantly correlated with both intracellular and extracellular MCs contents. A significant interaction between temperature and salinity was observed. No correlation was found between intracellular MCs and phycocyanin contents. Finally, exposure to M. aeruginosa resulted in decreased swimming speed, increased inactivity, and higher mortality in A. tonsa, compared to the non-toxic Rhodomonas salina. Our study highlights the consequences of temperature and salinity on M. aeruginosa growth and toxin production, offering increased insights into the potential ecotoxicological risks of future blooms.