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Abstract. Light scattering properties of biogenic CaCO 3 particles [particulate inorganic carbon (PIC)]were determined on cultured calcifying algae and field-derived CaCO3 particles. The particles were separated from particulate organic carbon (POC) with a flow cytometer, volume-scattering functions were measured with a laser light-scattering photometer, and particle composition was measured using atomic absorption spectrometry. Small calcite coccoliths were best sorted by gating on the ratio of horizontally polarized forward light scattering and vertically polarized forward light scattering; plated coccolithophores could be sorted by gating on side scattering and forward angle light scattering. Normalized volume-scattering functions for the culture-derived calcite particles varied by a factor of 2 for the different species. Backscattering cross sections (m 2 particle -•) for calcite particles varied by -35 times and were generally a function of size. Backscattering efficiencies were -2-4 times higher for cells with CaCO3 than without it. CaCO3-specific backscattering showed much less variability across various species; the calcite-specific backscattering coefficient varied by only -38% for both cultured coccolithophores and field-derived CaCO• particles. Organic carbon-specific backscattering of "naked" coccolithophores was highly consistent within all coccolithophores used in our experiments, as well as with values in the literature. Our results suggest that both POC and PIC can be optically estimated, the former by measuring backscattering of alecalcified phytoplankton as well as their size distribution, and the latter is proportional to acid-labile backscattering. These results show the feasibility of a rapid optical technique for measuring two biogeochemically important carbon fractions in the sea.

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.

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