Response of a natural phytoplankton community from the Qingdao coast (Yellow Sea, China) to variable CO₂ levels over a short-term incubation experiment

Since marine phytoplankton play a vital role in stabilizing earth's climate by removing significant amount of atmospheric CO₂, their responses to increasing CO₂ levels are indeed vital to address. The responses of a natural phytoplankton community from the Qingdao coast (NW Yellow Sea, China) was studied under different CO₂ levels in microcosms. HPLC pigment analysis revealed the presence of diatoms as a dominant microalgal group; however, members of chlorophytes, prasinophytes, cryptophytes and cyanophytes were also present. δ¹³C_POM values indicated that the phytoplankton community probably utilized bicarbonate ions as dissolved inorganic carbon source through a carbon concentration mechanism (CCM) under low CO₂ levels, and diffusive CO₂ uptake increased upon the increase of external CO₂ levels. Although, considerable increase in phytoplankton biomass was noticed in all CO₂ treatments, CO₂-induced effects were absent. Higher net nitrogen uptake under low CO₂ levels could be related to the synthesis of CCM components. Flow cytometry analysis showed slight reduction in the abundance of Synechococcus and pico-eukaryotes under the high CO₂ treatments. Diatoms did not show any negative impact in response to increasing CO₂ levels; however, chlorophytes revealed a reverse tend. Heterotrophic bacterial count enhanced with increasing CO₂ levels and indicated higher abundance of labile organic carbon. Thus, the present study indicates that any change in dissolved CO₂ concentrations in this area may affect phytoplankton physiology and community structure and needs further long-term study.