Synergism between elevated <i>p</i>CO<sub>2</sub> and temperature on the Antarctic sea ice diatom <i>Nitzschia lecointei</i>

Abstract

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 CO 2 (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 (∼ 390 and ∼ 960 µatm) and temperature (−1.8 and 2.5 • 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 • 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 pCO 2 , the total FA content was reduced relative to the ∼ 390 µatm treatment, although no difference between pCO 2 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 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 CO 2 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.

Long-term acclimation to elevated p CO 2 alters carbon metabolism and reduces growth in the Antarctic diatom Nitzschia lecointei

“…These results are complementary to a recent short-term study where no change in specific growth rate was detected in N. lecointei after 14 days exposure to 960 matm pCO 2 at 21.88C, although increased pCO 2 had a positive effect on growth at higher temperatures [9]. Another recent short-term study also concluded that natural sea ice algal communities could be very tolerant to changes in pCO 2 and pH [22].…”

“…Antarctic sea ice algae are known to have widespread range of carbon concentrating mechanisms (CCMs) [8] that facilitate carbon fixation [9]. Furthermore, up to 40% of marine phytoplankton primary production is generally released as dissolved organic carbon (DOC) [10], although values over 70% have been observed [11].…”

“…Short-term responses in sea ice algae to increased pCO 2 are highly variable, ranging from negative [21,22] to positive effects [9,22]. Results may also depend on temperature conditions [9] and manipulation technique [22]. Generally, effect sizes from ocean acidification experiments are quite small for sea ice algae.…”

“…More specifically, short-term changes in pCO 2 affect growth, fatty acid concentration and species competition of Antarctic microalgae [9,22,24,25]. Still, no long-term study (longer than one month) has previously been published on polar microalgae.…”

“…) of synthetic air (Air Liquide, Malmö, Sweden) with different pCO 2 (280, 390 and 960 matm), using a system similar to that in Torstensson et al [9]. figure S1) with pre-equilibrated f/2 medium from respective CO 2 -treatment.…”