Ocean warming modulates the effects of acidification on Emiliania huxleyi calcification and sinking

Abstract

Ongoing ocean warming and acidification are tied to the rapid accumulation of human-induced carbon dioxide (CO 2 ) in the atmosphere and subsequent uptake of heat and CO 2 by the surface ocean. These processes are expected to drive large changes in marine ecosystems. While numerous studies have examined the effects of ocean acidification on coccolithophores, less is known on their combined effect. In this study, we investigate temperature modulation of the carbonate chemistry sensitivity of the coccolithophore Emiliania huxleyi (RCC1827 from the Western Mediterranean) in a culture experiment. We analyzed the responses of coccolith morphology, particulate inorganic and organic carbon production, and sinking rate of individual cells. E. huxleyi was exposed to three CO 2 levels (ca. 400 latm, 900 latm, and 1400 latm) at 158C and 208C. Temperature adds to the negative effect of increasing pCO 2 on coccolith morphology, suggesting that a significant number of E. huxleyi strains might suffer from a temperature increase, hampering their evolutionary success. Temperature amplified the positive effect of increasing pCO 2 on organic carbon production, while modulating the response of calcification rates, indicating that the response to increasing pCO 2 must be taken with caution depending on the temperature range studied. Sinking rates were positively correlated with temperature, whereas pCO 2 did not have any effect. The combined effect of carbonate chemistry and temperature on the E. huxleyi ratio between particulate inorganic carbon and particulate organic carbon (PIC/POC) might also lower the sinking rate of aggregates. In conclusion, in a warmer and more acidified ocean, individual coccolithophore cells might sink faster, while aggregates might sink slower.

Phosphorus limitation and heat stress decrease calcification in <i>Emiliania huxleyi</i>

“…In all of the above studies, however, growth rate increased from low to high temperature and none of the tested temperatures were therefore above the optimum for growth (Eppley, 1972). To our knowledge, this study is the first to show that heat stress is not only detrimental for coccolith morphology (Watabe and Wilbur, 1966;Langer et al, 2010;Milner et al, 2016) but also for coccolith production in E. huxleyi. Certainly, the potential for long-term adaptation needs to be considered, as temperature increases are unlikely to occur on timescales short enough to preclude adaptation in a rapidly growing species.…”

“…Although a similar decrease in growth rate was not observed in batch culture, measurements of growth rate in semi-continuous cultures are more robust because growth rate is measured as an average of numerous dilution cycles. The doubling in coccolith malformations provides further evidence that 24 • C cultures were heat-stressed (Watabe and Wilbur, 1966;Langer et al, 2010;Milner et al, 2016).…”

“…Several studies have contrastingly reported the PIC / POC ratio to be insensitive to temperature (Feng et al, 2008;Matson et al, 2016;Milner et al, 2016) or to increase with rising temperatures (Sett et al, 2014). In all of the above studies, however, growth rate increased from low to high temperature and none of the tested temperatures were therefore above the optimum for growth (Eppley, 1972).…”

“…Possibly, more coccoliths were shed under heat stress, underestimating the coccolith number of these cells. As E. huxleyi in general sheds many coccoliths, this effect can be considerable (Milner et al, 2016). We therefore cannot conclusively determine whether the effect of P limitation on the PIC / POC ratio was modified by heat stress in batch culture.…”

“…These studies suggest that higher than optimum temperature leads to an increase in malformations (Watabe and Wilbur, 1966;Langer et al, 2010). Although the effect of temperature on carbon production in E. huxleyi has been addressed in numerous studies (Sorrosa et al, 2005;Feng et al, 2008;Satoh et al, 2009;De Bodt et al, 2010;Borchard et al, 2011;Sett et al, 2014;Matson et al, 2016;Milner et al, 2016;Rosas- Navarro et al, 2016), none of these studies tested the effect of above-optimum temperature. To our knowledge, this study is the first to specifically test the impact of heat stress on carbon production in this species.…”