PHYTOPLANKTON EXTRACELLULAR RELEASE AND BACTERIAL-GROWTH - DEPENDENCE ON THE INORGANIC N-P RATIO

Batch cultures of Chaetoceros affinis grown under nitrogen- (N:P = 5) and phosphorus- (N:P = 100) limited conditions exhibited 30 and 100% higher rates of photosynthetic extracellular release (PER), respectively, than C. affinis grown under balanced nutrient conditions (N:P = 16). Inoculations of natural bacterial assemblages to PER derived from phytoplankton grown under these different nutrient regimes exhibited highest potential bacterial growth yields (similar to 46 %) on PER derived from balanced growth. Analysis of the dissolved monomeric (MCHO) and polymeric (PCHO) carbohydrate pool and of the dissolved free amino acid (DFAA) fraction of the PER indicated significantly higher specific release rates of these components for phytoplankton grown under P limitation. Based on the fluctuation of the MCHO, PCHO and DFAA pool in the batch cultures and on bacterial extracellular enzymatic activity (EEA) and bacterial growth yield we have evidence that under P-limited conditions bacteria are not taking up the available carbon efficiently. Bacteria growing on PER derived from N-limited conditions exhibited high bacterial alpha- and beta-glucosidase activity, while under P limitation low EEA was detectable. Extrapolating our results to the severely P-limited summer conditions of the Northern Adriatic Sea, we might conclude that P limitation provokes elevated PER in exponentially growing phytoplankton which is not utilized efficiently by the natural bacterial consortia. This would allow the PER to coagulate and eventually form mucilage.