Climate Change Effects on Iron Availability to Arctic Phytoplankton

Phytoplankton, unicellular algae, are responsible for 50{\%} of earth's photosynthesis, and for a significant consumption of atmospheric CO$_{\mathrm{2}}$. Iron (Fe) is essential for phytoplankton, but is extremely depleted in seawater, limiting photosynthesis in 30{\%} of the global ocean. Oceanic Fe bioavailability is determined by physical and chemical processes. The Arctic Ocean is experiencing the greatest decrease in seawater pH (termed ocean acidification). Simultaneously, ice retreat is promoting higher light intensity in Arctic Ocean. We investigated the effects of ocean acidification and high light on Fe availability to Arctic phytoplankton. Iron uptake rates by plankton, using the radionuclide $^{\mathrm{55}}$Fe, were used as a proxy for Fe bioavailability. In an Arctic summer research cruise, we measured Fe uptake by two phytoplankton populations subjected to two light levels, as well as present CO$_{\mathrm{2}}$ levels (400ppm) or those expected by 2100 (1100 ppm). Our results demonstrated that high CO$_{\mathrm{2}}$ decreases Fe availability, while high light increases it, suggesting that future Fe bioavailability might be similar to present day. However, the detrimental effects of high CO$_{\mathrm{2}}$ were more pronounced in the plankton population exposed to higher seawater temperature. Future studies should investigate the interaction among light, CO$_{\mathrm{2}}$ and temperature on the Fe physiology of Arctic phytoplankton.