Effects of CO2 and iron availability on rbcL gene expression in Bering Sea diatoms

Iron (Fe) can limit phytoplankton productivity in approximately 40% of the global ocean, including high-nutrient, low-chlorophyll (HNLC) waters. However, there is little information available on the impact of CO2-induced seawater acidification on natural phytoplankton assemblages in HNLC regions. We therefore conducted an on-deck experiment manipulating CO2 and Fe using Fe-deficient Bering Sea waters during the summer of 2009. The concentrations of CO2 in the incubation bottles were set at 380 and 600 ppm in the non-Fe-added (control) bottles and 180, 380, 600, and 1000 ppm in the Fe-added bottles. The phytoplankton assemblages were primarily composed of diatoms followed by haptophytes in all incubation bottles as estimated by pigment signatures throughout the 7 day incubation period. At the end of incubation, the relative contributions of diatoms to chlorophyll a biomass decreased significantly with increased CO2 levels in the controls, whereas minimal changes were found in the Fe-added treatments. These results indicate that, under Fe-deficient conditions, the growth of diatoms was negatively affected by the increase in CO2 availability. To confirm this, we estimated the expression and phylogeny of rbcL (which encodes the large subunit of RubisCO) mRNA in diatoms by quantitative reverse transcription PCR and clone library techniques, respectively. Interestingly, regardless of Fe availability, the expression and diversity of rbcL cDNA decreased in the high CO2 treatments (600 and 1000 ppm). The present study suggests that the projected future increase in seawater pCO2 could reduce the RubisCO activity of diatoms, resulting in a decrease in primary productivity and a shift in the food web structure of the Bering Sea.

Endo H., Sugie K., Yoshimura T. & Suzuki K., 2014. Effects of CO2 and iron availability on rbcL gene expression in Bering Sea diatoms. Biogeosciences Discussions 11:18105-18143. Article.

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