Increased temperature, rather than elevated CO2, modulates the carbon assimilation of the Arctic kelps Saccharina latissima and Laminaria solidungula

Ocean acidification and warming are affecting with special intensity the Arctic Ocean. Arctic coastal ecosystems are dominated by kelp forests with a high biomass production, which are expected to be directly affected by the increases in CO2 and temperature. This study presents the different physiological responses of the Arctic kelps Saccharina latissima and Laminaria solidungula from Kongsfjorden (Svalbard) cultured at 4 and 9 °C in combination with current (390 ppm) and increased (1200 ppm) levels of atmospheric CO2. Both species were largely unaffected by increased CO2 conditions. Carbon fixation was not influenced by CO2, indicating that photosynthesis was C-saturated at present levels, and 13C isotopic discrimination values from algal tissue suggested no deactivation of carbon concentrating mechanisms at increased CO2 levels. Inhibition of photosynthesis by carbonic anhydrases (CAs) inhibitors highlighted the important role of external CAs in inorganic carbon acquisition in Arctic kelps. Saccharina latissima showed a significantly higher growth rate at 9 °C than at 4 °C, probably due to the decrease in the dark respiration rate observed. Growth rate of L. solidungula was not affected by temperature or CO2, and increases in photosynthesis at 9 °C could be partially related to a higher dissolved organic carbon release rate. The photochemical performance of both species was not altered by any of the treatments. These results suggest that S. latissima might be more benefited than L. solidungula in a future warmer Arctic, while both populations seem to be resilient to higher CO2 concentrations. In order to allow full comparability with other ocean acidification data sets, the R package seacarb (Gattuso et al, 2016) was used to compute a complete and consistent set of carbonate system variables, as described by Nisumaa et al. (2010). In this dataset the original values were archived in addition with the recalculated parameters (see related PI). The date of carbonate chemistry calculation is 2017-01-12.