Synergistic effects of pCO2 and iron availability on nutrient consumption ratio of the Bering Sea phytoplankton community

[摘要] Little is known concerning the effect of CO₂ on phytoplanktonecophysiological processes under nutrient and trace element-limitedconditions, because most CO₂ manipulation experiments have beenconducted under elements-replete conditions. To investigate the effects ofCO₂ and iron availability on phytoplankton ecophysiology, we conductedan experiment in September 2009 using a phytoplankton community in the ironlimited, high-nutrient, low-chlorophyll (HNLC) region of the Bering Sea basin. Carbonate chemistry was controlled by the bubbling of the several levels ofCO₂ concentration (180, 380, 600, and 1000 ppm) controlled air, and twoiron conditions were established, one with and one without the addition ofinorganic iron. We demonstrated that in the iron-limited control conditions,the specific growth rate and the maximum photochemical quantum efficiency(F_v/F_m) of photosystem (PS) II decreased withincreasing CO₂ levels, suggesting a further decrease in ironbioavailability under the high-CO₂ conditions. In addition, biogenicsilica to particulate nitrogen and biogenic silica to particulate organiccarbon ratios increased from 2.65 to 3.75 and 0.39 to 0.50, respectively,with an increase in the CO₂ level in the iron-limited controls. Bycontrast, the specific growth rate, F_v/F_m values andelemental compositions in the iron-added treatments did not change inresponse to the CO₂ variations, indicating that the addition of ironcanceled out the effect of the modulation of iron bioavailability due to thechange in carbonate chemistry. Our results suggest that high-CO₂conditions can alter the biogeochemical cycling of nutrients throughdecreasing iron bioavailability in the iron-limited HNLC regions in thefuture.

[学科分类] 地球化学与岩石