Synergism between elevated pCO₂ and temperature on the Antarctic sea ice diatom Nitzschia lecointei

Abstract. Polar oceans are particularly susceptible to ocean acidification and warming. Diatoms play a significant role in sea ice biogeochemistry and provide an important food source to grazers in ice-covered oceans, especially during early spring. However, the ecophysiology of ice-living organisms has received little attention in terms of ocean acidification. In this study, the synergism between temperature and partial pressure of CO₂ (pCO₂) was investigated in relationship to the optimal growth temperature of the Antarctic sea ice diatom Nitzschia lecointei. Diatoms were kept in cultures at controlled levels of pCO₂ (∼390 and ∼960 μatm) and temperature (−1.8 and 2.5 °C) for 14 days. Synergism between temperature and pCO₂ was detected in growth rate and acyl lipid fatty acid (FA) content. Optimal growth rate was observed around 5 °C in a separate experiment. Carbon enrichment only promoted (6%) growth rate closer to the optimal growth, but not at the control temperature (−1.8 °C). At −1.8 °C and at ∼960 μatm pCO₂, the total FA content was reduced relative to the ∼390 μatm treatment, although no difference between pCO₂ treatments was observed at 2.5 °C. A large proportion (97%) of the total FAs comprised on average of polyunsaturated fatty acids (PUFA) at −1.8 °C. Cellular PUFA content was reduced at ∼960 relative to ∼390 μatm pCO₂. Effects of carbon enrichment may be different depending on ocean warming scenario or season, e.g. reduced cellular FA content in response to elevated CO₂ at low temperatures only, reflected as reduced food quality for higher trophic levels. Synergy between warming and acidification may be particularly important in polar areas since a narrow thermal window generally limits cold-water organisms.