Effects Of Ocean Acidification On The Calcifying Phytoplankton Coccolithophore

Elevating atmospheric CO₂ concentration increases dissolution of COB2 Binto upper surfaceocean, alters the carbonate system and reduces the pH (ocean acidification) and the saturate sateof calcium carbonate. Such an environmental change would lead to significant impacts oncalcifying organisms and marine ecosytems. Coccolithophores, the phytoplanktonic calcifyer,play important roles in oceanic carbon cycle. We, therefore, investigated the effects of elevatedCO₂ (ocean acidification) on the coccolithophore Emiliania hxuleyi, and examined itsphysiological response to ultraviolet radiation (UVR) under the acidified conditions.The pH of culture medium was controlled at 8.2, 7.9 and 7.6, respectively, by bubbling theambient air (380 ppmv) or the air with enriched CO₂ at 780 or 1460 ppmv. During thesemi-continuous cultures, we examined the effects of lowered pH on growth, morphology, andphotosynthesis and calcification of E. huxleyi with or without UVR. Cells grown at different pHregimes, showed varied rates of growth (μ), ranging within 0.7-1.1 d^-1. No significant differencewas found in the growth rate (about 0.9 d^-1) after the acclimation of about 8 days. The rate ofcalcification was reduced by 42.3-89.4ï¼…and the coccolith layer became significantly thinner(by 21.2-33.3ï¼…) under the acidified conditions, though their photosynthetic carbon fixation wasnot significantly affected. The cells with less amount of coccoliths showed higher inhibitioncaused by UVR for carbon fixation, increasing from about 45% (5%) at pH 8.2 to about 90%(15%) at pH 7.6 for acidification (photosynthesis), respectively. Since UVR reduced inorganicproduction to much higher percentage than the organic production, leading a tremendousreduction of the C/P ratio. Analysis of the optical transmission between the cells with or withoutcoccoliths indicated that the cells with coccoliths received about 20-25% less UVR.The coccoliths of E. huxleyi as well as other cocolithophores play a protective role againstharmful UVR. Thinning coccolith shell due to progressive ocean acidification will increase theexposure of the cells to harmful UVR and the sensitivity of calcification and photosynthesis tosolar UVR, and decrease C/P ratio. Therefore, biogeochemical and ecological processes relatedto these calcifying phytoplankton will be affected by ocean acidification.