Optical backscattering by calcifying algae: Separating the contribution ofparticulate inorganic and organic carbon fractions

Light scattering properties of biogenic CaCO3 particles [particulate inorga nic carbon (PIC)] were determined on cultured calcifying algae and field-de rived CaCO3 particles. The particles were separated from particulate organi c carbon (POC) with a flow cytometer, volume-scattering functions were meas ured with a laser light-scattering photometer, and particle composition was measured using atomic absorption spectrometry. Small calcite coccoliths we re best sorted by gating on the ratio of horizontally polarized forward lig ht scattering and vertically polarized forward light scattering;plated cocc olithophores could be sorted by gating on side scattering and forward angle light scattering. Normalized volume-scattering functions for the culture-d erived calcite particles varied by a factor of 2 for the different species. Backscattering cross sections (m(2) particle(-1)) for calcite particles va ried by similar to 35 times and were generally a function of size. Backscat tering efficiencies were similar to 2-4 times higher for cells with CaCO3 t han without it. CaCO3-specific backscattering showed much less variability across various species; the calcite-specific backscattering coefficient var ied by only similar to 38% for both cultured coccolithophores and field-der ived CaCO3 particles. Organic carbon-specific backscattering of "naked" coc colithophores 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 decalcified phytoplankton as well as their size distribut ion, and the latter is proportional to acid-labile backscattering. These re sults show the feasibility of a rapid optical technique for measuring two b iogeochemically important carbon fractions in the sea.