Phytoplankton extracellular release and bacterial growth: dependence on the inorganic N: P ratio

Abstract

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 affims 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 (- 46 %) on PER derived from balanced growth. Analysis of the dissolved monomeric (MCHO) and polymeric (PCHO) carbo- hydrate 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 extra- cellular 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-lmted conditions exhibited high bacterial a- and p-glucosidase activity, while under P limta- tion 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 m exponentially growing phytoplankton which is not utilized efficiently by the natural bacterial consortia. This would allow the PER to coagulate and eventually form mucilage.