Influence of nutrient ratios on the in vitro extracellular polysaccharide production by marine diatoms from the Adriatic Sea

Three different species of marine diatoms, Nitzschia closterium (Ehr) Smith , Chaetoceros sp. (Ehr) and Skeletonema costatum (Grev) Cleve, were isolate d from the Adriatic Sea and studied in vitro for the production of extracel lular carbohydrates. Variations of the nitrogen (N)/phosphorus (P)/silicon (Si) ratios in the growth medium affect the accumulation and release of car bohydrates. In the N. closterium cultures at high N/P ratio, the rate of ex tracellular polysaccharide release was higher both in rapidly growing cells (2.14 mu g per 10(6) cells day(-1)) and in stationary phase (1.0 mu g per 10(6) cells day(-1)) compared to S.costatum and Chaetoceros sp. Instead, at low NIP ratios, S.costatum and Chaetoceros sp. produced large amounts of e xtracellular polysaccharides during the logarithmic phase, compared to N.cl osterium, with values of 10 mu g per 10(6) cells day(-1) for S.costatum and 2.8 mu g per 10(6) cells day(-1) for Chaetoceros sp., respectively; in par ticular, only S.costatum was able to produce extracellular carbohydrates (1 .2 mu g per 10(6) cells day(-1)) during the stationary phase of growth. Und er Si limitation, any of the three diatom species produce extracellular pol ysaccharides both in the logarithmic and stationary phase of growth. The po tential ecological significance of these findings is discussed.