Interactive Effects of Ocean Acidification and Nitrogen-Limitation on the Diatom Phaeodactylum tricornutum

Climate change is expected to bring about alterations in the marine physical and chemical environment that will induce changes in the concentration of dissolved CO₂ and in nutrient availability. These in turn are expected to affect the physiological performance of phytoplankton. In order to learn how phytoplankton respond to the predicted scenario of increased CO₂ and decreased nitrogen in the surface mixed layer, we investigated the diatom Phaeodactylum tricornutum as a model organism. The cells were cultured in both low CO₂ (390 μatm) and high CO₂ (1000 μatm) conditions at limiting (10 μmol L⁻¹) or enriched (110 μmol L⁻¹) nitrate concentrations. Our study shows that nitrogen limitation resulted in significant decreases in cell size, pigmentation, growth rate and effective quantum yield of Phaeodactylum tricornutum, but these parameters were not affected by enhanced dissolved CO₂ and lowered pH. However, increased CO₂ concentration induced higher rETR_max and higher dark respiration rates and decreased the CO₂ or dissolved inorganic carbon (DIC) affinity for electron transfer (shown by higher values for K_{1/2 DIC} or K_{1/2 CO2}). Furthermore, the elemental stoichiometry (carbon to nitrogen ratio) was raised under high CO₂ conditions in both nitrogen limited and nitrogen replete conditions, with the ratio in the high CO₂ and low nitrate grown cells being higher by 45% compared to that in the low CO₂ and nitrate replete grown ones. Our results suggest that while nitrogen limitation had a greater effect than ocean acidification, the combined effects of both factors could act synergistically to affect marine diatoms and related biogeochemical cycles in future oceans.