Synergistic effects of pCO 2 and iron availability on nutrient consumption ratio of the Bering Sea phytoplankton community
Little is known concerning the effect of CO 2 on phytoplankton ecophysiological processes under nutrient and trace element-limited conditions, because most CO 2 manipulation experiments have been conducted under elements-replete conditions. To investigate the effects of CO 2 and iron availability on phytoplankton ecophysiology, we conducted an experiment in September 2009 using a phytoplankton community in the iron limited, high-nutrient, low-chlorophyll (HNLC) region of the Bering Sea basin . Carbonate chemistry was controlled by the bubbling of the several levels of CO 2 concentration (180, 380, 600, and 1000 ppm) controlled air, and two iron conditions were established, one with and one without the addition of inorganic iron. We demonstrated that in the iron-limited control conditions, the specific growth rate and the maximum photochemical quantum efficiency ( Fv/ Fm) of photosystem (PS) II decreased with increasing CO 2 levels, suggesting a further decrease in iron bioavailability under the high-CO 2 conditions. In addition, biogenic silica to particulate nitrogen and biogenic silica to particulate organic carbon ratios increased from 2.65 to 3.75 and 0.39 to 0.50, respectively, with an increase in the CO 2 level in the iron-limited controls. By contrast, the specific growth rate, Fv/ Fm values and elemental compositions in the iron-added treatments did not change in response to the CO 2 variations, indicating that the addition of iron canceled out the effect of the modulation of iron bioavailability due to the change in carbonate chemistry. Our results suggest that high-CO 2 conditions can alter the biogeochemical cycling of nutrients through decreasing iron bioavailability in the iron-limited HNLC regions in the future.