Physiology of diatom Skeletonema costatum (Grev.) Cleve photosynthetic extracellular release: evidence for a novel coupling between marine bacteria and phytoplankton

The photosynthesis of cellular materials by phytoplankton is accompanied by release of organic molecules from the algal cells into the water. The patterns of carbon fixation in particulate and dissolved pools were investigated in Skeletonema costatum cultured under 12 light/12 h dark cycles. The short-term production (1-15 min) of particulate organic carbon (POC) and extracellular organic carbon (EOC) compounds was studied by measuring the uptake of C-14-labelled sodium bicarbonate and its subsequent incorporation and release into organic compounds. Slightly modified traditional C-14 radiotracer protocols were used, including separation by electrophoresis and thin-layer chromatography and detection by autoradiography. Results indicated that there was a distinct difference between radiolabelled compounds in the POC and EOC pools. Several metabolites found in the EOC pool were not present in the POC pool, indicating the active release of these products from the cells into the ambient water during short-term incubations, and indicating that inorgamc carbon fixation pathways in marine autotrophs might be partly extracellular.