Temperature Modulates Coccolithophorid Sensitivity of Growth, Photosynthesis and Calcification to Increasing Seawater pCO2

Abstract

Increasing atmospheric CO2 concentrations are expected to impact pelagic ecosystem functioning in the near future by driving ocean warming and acidification. While numerous studies have investigated impacts of rising temperature and seawater acidification on planktonic organisms separately, little is presently known on their combined effects. To test for possible synergistic effects we exposed two coccolithophore species, Emiliania huxleyi and Gephyrocapsa oceanica, to a CO2 gradient ranging from ∼0.5–250 µmol kg−1 (i.e. ∼20–6000 µatm pCO2) at three different temperatures (i.e. 10, 15, 20°C for E. huxleyi and 15, 20, 25°C for G. oceanica). Both species showed CO2-dependent optimum-curve responses for growth, photosynthesis and calcification rates at all temperatures. Increased temperature generally enhanced growth and production rates and modified sensitivities of metabolic processes to increasing CO2. CO2 optimum concentrations for growth, calcification, and organic carbon fixation rates were only marginally influenced from low to intermediate temperatures. However, there was a clear optimum shift towards higher CO2 concentrations from intermediate to high temperatures in both species. Our results demonstrate that the CO2 concentration where optimum growth, calcification and carbon fixation rates occur is modulated by temperature. Thus, the response of a coccolithophore strain to ocean acidification at a given temperature can be negative, neutral or positive depending on that strain's temperature optimum. This emphasizes that the cellular responses of coccolithophores to ocean acidification can only be judged accurately when interpreted in the proper eco-physiological context of a given strain or species. Addressing the synergistic effects of changing carbonate chemistry and temperature is an essential step when assessing the success of coccolithophores in the future ocean.

Resistance of Arctic phytoplankton to ocean acidification and enhanced irradiance

“…Another possible reason for the lack of pCO 2 effects is the relatively high incubation temperature. High temperatures have been shown to shift phytoplankton pCO 2 optima to higher levels and could thus dampen OA effects on phytoplankton assemblages (Sett et al 2014;Holding et al 2015;Wolf et al 2017). Although the incubation temperatures were higher than in situ, we argue that our experimental set-up nonetheless reflects a realistic scenario for the current, and certainly the future Arctic Ocean.…”

“…Results from recent field studies have shown a potential for positive OAresponses in primary production and phytoplankton growth (Engel et al 2013;Coello-Camba et al 2014;Holding et al 2015). Increasing temperatures, however, seem to reduce potential benefits of increased CO 2 concentrations (Holding et al 2015), which could be due to a shift in CO 2 optima to higher levels (Sett et al 2014). Beyond changes in phytoplankton productivity, OA has been shown to influence Arctic phytoplankton community structure-i.e., the relative abundance of various taxonomic and functional groups (Newbold et al 2012;Brussaard et al 2013).…”

The Arctic picoeukaryote <i>Micromonas</i> pusilla benefits synergistically from warming and ocean acidification

“…growth increased with increasing pCO 2 from low to intermediate, but decreased 342 again under higher pCO 2 levels (Figure 1). Such an optimum behaviour can be expected for 343 most environmental drivers (Harley et al, 2017) and has previously been observed in response 344 to OA (Sett et al, 2014;Wolf et al, 2017). The response patterns in these studies were attributed 345 to a combination of beneficial effects of rising pCO 2 under potentially carbon-limiting 346 conditions for photosynthesis, and negative effects of declining pH on cellular homeostasis and 347 enzyme performance, which manifest mainly at high pCO 2 (Bach et al, 2013).…”

Reconsidering the role of carbonate ion concentration in calcification by marine organisms

“…A valuable measure to determine the potential of CO 2 to limit growth and photosynthesis is K 1/2 which denotes the CO 2 concentration where the process runs at half of its maximum. Available K 1/2 measurements suggest that CO 2 limitation mostly occurs well below CO 2 concentrations typically encountered by the organisms in their respective habitats (Rost et al, 2003;Sett et al, 2014 …”

“…Still others found that the response is not controlled by a single carbonate system parameter, but by the interplay of two or more. In coccolithophores, for example, calcification rates were repeatedly shown to increase from low to intermediate DIC but decrease again above certain thresholds (Langer et al, 2006;Bach et al, 2011Bach et al, , 2015Sett et al, 2014). This optimumcurve response pattern was explained by the interaction between HCO − 3 and protons (H + ), where HCO − 3 stimulates calcification as substrate and H + functions as inhibitor (Bach et al, 2011(Bach et al, , 2013.…”