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

Abstract

Carbon fixation of Emiliania huxleyi was studied in light-limited,steady-state, continuous cultures. Six growth rates were examined rangingfrom 0.24 to 1.0 d-1 although the lowest may have been carbon-limited.Cell-specific and chlorophyll-specific rates of photosynthesis andcalcification increased as a function of growth rate. The ratio ofcalcification to photosynthesis (C/P) increased from about 0.2 to 0.7 as thegrowth rate increased (from 0.24 to 0.75 d-1), then the C/P ratio decreasedslightly as the growth rate approached washout at 1 d-1. Extrapolation ofthe regression data at low growth rates suggested that there should be zerocalcification at a growth rate of about 0.1 d-1. Cells were also given a 30s acidification/neutralization treatment to dissolve their coccoliths, andthen carbon fixation was measured. Photosynthesis and calcification at allgrowth rates increased by about 0.1 pg C cell-1 h-1 following thistreatment. Carbon fixation per unit chlorophyll a was predicted bymultiplying the total carbon:chlorophyll a ratio by the respective culturedilution rate. These predictions were almost identical to 14C measurementsof carbon fixation per unit chlorophyll. Nevertheless, if only total carbonincorporation data are available for a coccolithophore population, accuratepredictions of just photosynthesis or calcification will require the function(presented in this paper) which relates the C/P ratio to growth rate. Thisfunction suggests a decoupling of photosynthesis from calcification as growthbecomes progressively more light-limited.