Decoupling of calcification and photosynthesis in the coccolithophore Emiliania huxleyi under steady-state light-limited growth

Carbon fixation of Emiliania huxleyi was studied in light-limited, steady-state, continuous cultures. Six growth rates were examined ranging from 0.24 to 1.0 d-' although the lowest may have been carbon-limited. Cell-specific and chlorophyll-specific rates of photosynthesis and calcification increased as a function of growth rate. The ratio of calcification to photosynthesis (C/P) increased from about 0.2 to 0.7 as the growth rate increased (from 0.24 to 0.75 d.'), then the C / P ratio decreased slightly as the growth rate approached washout at 1 d-' Extrapolation of the regression data at low growth rates suggested that there should be zero calcification at a growth rate of about 0.1 d-' Cells were also given a 30 S acidification/neutralization treatment to dissolve their coccoliths, and then carbon fixation was measured. Photosynthesis and calcification at all growth rates increased by about 0.1 pg C cell-' h-' following this treatment. Carbon fixation per unit chlorophyll a was predicted by multiplying the total carbon:chlorophyll a ratio by the respective culture dilution rate. These predictions were almost identical to ''C measurements of carbon flxat~on per unit chlorophyll. Nevertheless, if only total carbon incorporation data are available for a coccolithophore population, accurate predictions of just photosynthesis or calcification will require the functlon (presented in this paper) which relates the C/Pratio to growth rate. This function suggests a decoupllng of photosynthesis from calcification as growth becomes progressively more light-limited.