GROWTH-KINETICS AND NITROGEN-NUTRITION OF THE MARINE DIATOM PHAEODACTYLUM-TRICORNUTUM IN CONTINUOUS DIALYSIS CULTURE

The growth kinetics and nitrogen (N)-nutrition of the marine pennate diatom Phaeodactylum tricornutum Bohlin were determined in continuous dialysis culture at different cell densities. Inflow nutrient medium was supplied as natural unenriched estuarine seawater to a dialysis culture system with a high ratio of membrane surface area/culture volume (Am/Vc). Under the experimental conditions, the supply of inorganic macronutrients (NO3- + NO4- and PO4(-3) by diffusion (Nd) was markedly greater than that provided by the dilution (FfCN) of the culture (Nd >> FfCN), thereby establishing an inverse relationship between the cell density and the dilution rate (D). This continuous dialysis system allows for the maintenance of prolonged growth (> two weeks) at various cell densities (1.4 to 27.2 x 10(9) cells l-1) within a range of dilution rates between 0.30 to 1.08 d-1. In high cell density cultures, where the extracellular medium was characterized as nutrient deficient, a lower growth rate (mu-e) was exhibited than in cultures with lower cell densities. The growth rate (mu-e) remained equivalent to the dilution rate (D) throughout the culture cycle, indicating that equilibrated growth was achieved. High cell density cultures yielded higher productivity (P), relative to that of cultures grown at lower cell densities, in terms of cell-N and -C produced per unit time. However, cell quotas of both N and C declined with increasing cell concentrations. Denser cultures were characterized by an enhanced N-conversion efficiency (Y(N)) and a higher cellular N/C atomic ratio. The nutritional response of this diatom in dense cultures reveals an efficient use of N-nutrients, presumably as a result of cellular nutrient adaptation to oligotrophic conditions.