Responses of the diatom Asterionellopsis glacialis to increasing sea water CO₂ concentrations and the effect of turbulence

Abstract. Emissions of greenhouse gases, such as carbon dioxide (CO₂), are lead to increasing global and surface ocean temperatures. At the same time, as CO₂ equilibrates between the atmosphere and the surface ocean, it decreases sea water pH. As a result, the changes in physical and chemical properties of the ocean can affect marine primary producers in various ways. A number of researches have addressed the effects of ocean acidification on marine phytoplankton. However, phytoplankton responses to combined effects are still poorly understood. Here, we chose monospecific cultures of the cosmopolitan chain forming diatom Asterionellopsis glacialis (A. glacialis), grown semi-continuously under controlled laboratory conditions, to assess the combined effect of ocean acidification (~ 420 to 2800 µatm) and turbulence. At current CO₂ levels, growth rates of A. glacialis increased under enhanced turbulence. This was the result of an optimum shift towards lower CO₂ concentrations and accompanied by a prevalence of longer chains (more than 6 cells). For increasing CO₂ levels (up to ~ 2800 µatm) and decreased pH values, enhanced turbulence significantly decreased growth rates, chain length and organic matter production of A. glacialis. Thus, our study suggests that, even though A. glacialis benefited from enhanced turbulence, at present carbon dioxide concentration, at higher CO₂ levels, turbulence magnified the stress by acidification. If in the future, the ocean surface layer will be more frequently exposed to storm and wind events, then phytoplankton communities might be more sensitive to lower pH, with potential consequences for community composition and productivity.