Effects of elevated pCO2 on the response of coccolithophore Emiliania huxleyi to prolonged darkness

ABSTRACT Although numerous studies have examined the responses of coccolithophores to ocean acidification, less is known on the fate of those calcifying organisms when they sink to the ocean’s aphotic regions. In this study, the coccolithophore Emiliania huxleyi was first grown under a regular 12/12 light/dark cycle at 20°C, exposed to both high (1000 μatm) and ambient CO2 (410 μatm) levels. The cultures were then transferred to continuous darkness for 96 h at 20°C or 16°C. We found that elevated CO2 decreased the specific growth rate while increasing the cellular particulate organic carbon (POC) and nitrogen (PON) contents and the POC/PON ratio of E. huxleyi in the light/dark period. After 96 h of dark acclimation, the cell abundance decreased more obviously at 20°C than at 16°C but showed no significant difference between the two CO2 treatments. The decrease in volumetric POC concentration was most prominent in the high CO2/20°C treatment and least in the ambient CO2/16°C treatment. At 16°C, the PON concentration increased in the high CO2 cultures and exhibited no change in the ambient CO2 cultures. While at 20°C, the PON concentration decreased significantly both under high and ambient CO2 conditions. The final POC/PON ratio showed no significant differences among the different temperature and CO2 treatments. Overall, a higher percentage of POC relative to that of PON was lost in darkness with increasing CO2 concentration, with potential implications for the ocean’s nutrient cycle.