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

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 (pCO(2)) 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 pCO(2) (similar to 390 and similar to 960 mu atm) and temperature (-1.8 and 2.5 degrees C) for 14 days. Synergism between temperature and pCO(2) was detected in growth rate and acyl lipid fatty acid (FA) content. Optimal growth rate was observed around 5 degrees 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 degrees C). At -1.8 degrees C and at similar to 960 mu atm pCO(2), the total FA content was reduced relative to the similar to 390 mu atm treatment, although no difference between pCO(2) treatments was observed at 2.5 degrees C. A large proportion (97 %) of the total FAs comprised on average of polyunsaturated fatty acids (PUFA) at -1.8 degrees C. Cellular PUFA content was reduced at similar to 960 relative to similar to 390 mu atm pCO(2). 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.