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.