Effects of simultaneous increase in temperature and ocean acidification on biochemical composition and photosynthetic performance of common macroalgae from Kongsfjorden (Svalbard)

Increased temperature and ocean acidification are two main factors threatening Arctic coastal ecosystems. To explore the consequences of increased average temperature and ocean acidification in Kongsfjorden, the performance of six common species of macroalgae, the chlorophyte Monostroma arcticum, the rhodophytes Phycodrys rubens, and Ptilota plumosa, and the phaeophytes Alaria esculenta, Desmarestia aculeata and Saccorhiza dermatodea, was tested after 9 days of culture at 4 and 10 A degrees C in combination with current (ca. 390 ppmv) and future (1000 ppmv) levels of atmospheric CO2 under solar radiation in summer. Temperature affected mainly the photosynthetic performance as measured by PAM fluorescence, particularly the initial slope (alpha) of ETR curves, the light saturation parameter (E (k) ), and F (v) /F (m) values, as well as the protein content, especially in the phaeophytes. On the other hand, CO2 affected mainly the internal accumulation of carbohydrates and lipids. The C:N balance was largely unaffected. External carbonic anhydrase activity was not inhibited at high CO2, and nitrate reductase activity remained unaffected. The resulting growth rate was not altered by treatments in three out of the six species studied. On the other hand, P. rubens showed a positive effect of increasing temperature, D. aculeata was negatively influenced by CO2, and S. dermatodea was positively affected by CO2. Significant interactions between CO2 and temperature were found in 20 % of the analyses. Whether additive or synergistic, the co-occurrence of a higher temperature with other stressors such as elevated CO2 increases the probability of community changes by modifying the performance of these species.