Abstract
Abstract. Little is known concerning the effect of CO2 on phytoplankton ecophysiological processes under nutrient and trace element-limited conditions, because most CO2 manipulation experiments have been conducted under elements-replete conditions. To investigate the effects of CO2 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 CO2 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 CO2 levels, suggesting a further decrease in iron bioavailability under the high-CO2 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 CO2 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 CO2 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-CO2 conditions can alter the biogeochemical cycling of nutrients through decreasing iron bioavailability in the iron-limited HNLC regions in the future.
Highlights
The production and elemental composition of marine phytoplankton play a crucial role in driving ocean biogeochemical cycling of nutrients (Redfield et al, 1963)
It has been reported that the ocean acidification affects the elemental composition of phytoplankton, suggesting that oceanic nutrient biogeochemistry will alter according to future high-CO2 conditions (Hutchins et al, 2009)
The values for dissolved Fe (D-Fe: < 0.2 μm) concentrations in the present study were intermediate in relation to the values
Summary
The production and elemental composition of marine phytoplankton play a crucial role in driving ocean biogeochemical cycling of nutrients (Redfield et al, 1963). The elemental composition of phytoplankton is affected by changing ambient conditions such as nutrient concentrations and partial pressure of CO2 (pCO2) (e.g., Takeda, 1998; Burkhardt et al, 1999; Kudo, 2003), and difference in community compositions (Arrigo et al, 1999; Sugie et al, 2010a). This evidence suggests that the biogeochemical cycling of nutrients could change in response to future global climate change (Hutchins et al, 2009). There is a distinct lack of knowledge regarding the effect of CO2 on phytoplankton ecophysiology under nutrientor trace element-limited conditions, despite the fact that the phytoplankton production in oceanic environments is often limited to at least one element such as nitrogen and iron (Saito et al, 2008)
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