Decreased calcification to photosynthesis ratio in coccolithophores under reduced O2 and elevated CO2 environment

We examined the physiological performance in the most cosmopolitan coccolithophorid, Emiliania huxleyi and Gephyrocapsa oceanica, which were treated with 8.3 (ambient O 2 (AO)), 4.6 (medium O 2 (MO)), and 2.5 (low O 2 (LO)) mg L –1 O 2 under 400 ppm (ambient CO 2 (AC)) and 1000 (high CO 2 (HC)) ppm CO 2 conditions. Elevated CO 2 decreased the specific growth rate of cells cultured under AO and LO conditions in both species, but it increased the rate in the MO-grown E. huxleyi. Regardless of the CO 2 levels, diminished O 2 concentration inhibited the growth rate in E. huxleyi while accelerating the rate in G. oceanica. LO reduced the particulate organic carbon (POC) production rate compared to the AO treatment in both species. Additionally, the decrease was higher in the HC cultures than in the AC ones. LO also inhibited the production rate of particulate inorganic carbon (PIC) compared to the AO/AC treatment. Due to a higher reduction in the production rate of PIC than POC, the PIC/POC ratio was decreased in the LO treatment compared to the AO/AC treatment. The current study reveals that low O 2 can, individually or in combination with high CO 2 , considerably affect the physiology of marine photoautotrophic organisms.