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

<p>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, <i>Navicula directa</i> and <i>Navicula glaciei</i>, 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 <i>N. directa</i>. 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). <i>Navicula glaciei</i> showed significant effects of elevated pCO₂ (<i>p</i> < 0.05) on its photosynthetic response, while <i>N. directa</i> 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.</p>