Changes in the Top‐Down Control of Planktonic Bacteria in Response to Nutrient Addition and Warming in the Red Sea

Eutrophication and warming impacts on marine bacterioplankton and their top‐down controls (protistan grazers and viruses) are still little known. Here, we evaluated the seasonal variability of the joint impact of nutrient addition and temperature on the abundance of bacterioplankton, heterotrophic nanoflagellates (HNFs) and viruses in Red Sea coastal waters. We conducted four microcosm experiments in which samples were either incubated as such (control, C) or amended with phosphate and nitrate (inorganic, I), glucose (organic, O) or both types of nutrients (mixed, M). Each nutrient treatment was incubated at three temperatures spanning 6°C around ambient values (23°C–33°C). Microbial response ratios (RR, the ratio between the maximum abundance in each nutrient amendment treatment relative to the maximum abundance in the C treatment) were variable, with the most noticeable increases found in the I and M treatments, suggesting an effect mediated by increased primary production. Bacterioplankton showed weak responses to warming, but the RRs of HNFs and viruses in the I treatment tended to increase at higher temperatures. The response of HNFs to the increase in prey was stronger than that of viruses. Our results also suggest that the coupling between heterotrophic bacteria and HNFs will likely increase with future warming.