Effects of elevated pCO2 on the photosynthetic performance of the sea ice diatoms Navicula directa and Navicula glaciei

Sea ice algal communities are generally dominated by pennate diatoms, which commonly occur at the ice-water interface and in brine channels. They also make a significant contribution to higher trophic levels associated with sea ice habitats. Here, the photosynthetic responses of two sea ice diatom species, Navicula directa and Navicula glaciei, to changes in pCO₂ under controlled laboratory conditions were compared. pCO₂ (390 ppm and 750 ppm) was manipulated to simulate a shift from present levels (1990) to predicted “IPCC year 2100 worst-case scenario” levels. To investigate these effects, a pulse-amplitude modulation (PAM) fluorometer was used to measure the photosynthetic performance. The ability of the sea ice algae to grow and photosynthesize within physio-chemical gradients in the sea ice suggests that both sea ice species are likely to be well adapted to cope with changes in pCO₂ concentrations. Lower pH and higher pCO₂ for 7 days resulted in increased biomass, especially for N. directa. However, a decline in photosynthetic capacity (rETR_max) was observed for both species (highest value 11.375 ± 0.163, control; and 8.322 ± 1.282, treatment). Navicula glaciei showed significant effects of elevated pCO₂ (p < 0.05) on its photosynthetic response, while N. directa did not. Future changes in CO₂ and pH may thus not significantly affect all diatoms but may lead to changes in the photosynthetic activities in some species.