Practical approaches to algal excretion

Release of dissolved organic carbon (DOC) by diatom cultures and by a mixed phytoplankton assemblage dominated by picophytoplankton was examined using filtration techniques; dialysis membrane based techniques were also investigated for their potential utility in studies of algal excretion. The inexpensive dialysis-diffusion system which was investigated shows promise as a method for obtaining phytoplankton exudate for physico-chemical characterization of exudate or for bacterial growth studies. Experimental results obtained using filtration techniques showed that at least a small amount of carbon fixed by photosynthesis is rapidly converted to DOC by rapidly growing cultures and that a larger proportion is converted to DOC by cells adapting to a change in irradiance, by senescent cells, and in a whole plankton community composed of a mixture of cells in unknown physiological condition. DOC production appears to occur simultaneously with photosynthesis and can be more than 10 % of total carbon fixation in cells which are not in complete metabolic equilibrium with the environment. Use of techniques which account for DOC release are, therefore, recommended for measurement of primary production in field situations unless the phytoplankton community is known to be in exponential growth