The accumulation and release of polysaccharides by planktonic cells and the subsequent bacterial response during a controlled experiment

Seawater from the northern Adriatic, with low phosphorus (0.03 mu M) and ni trogen (1.0 mu m NO3 and 1.0 mu M NH4) concentration, was incubated for 12 days in 20-1 polycarbonate carboys. The addition of a nutrient mixture (0.6 mu M PO4, 5.1 mu M NO3, 1.8 mu M NH4, 10.6 mu M SiO2) induced a strong dia tom bloom, reaching 25 mu g l(-1) Chi rr, Primary and bacterial production were stimulated by the initial enrichment of nitrogen and phosphorus but ce ased when N and P depletion occurred after 4 days, inorganic N exhaustion r esulted in a significant production land accumulation of dissolved and part iculate carbohydrates. The initial accumulation of carbohydrates (CHO) in t he particulate phase was followed 2 days later by a significant release of dissolved CHO. The bacterial response to this organic carbon source, as ref lected by glucosidase activity, was probably inhibited by the severe P limi tation following the phytoplankton bloom, In the exponential phase, when P concentration was sufficiently high to sustain a significant glucosidase ac tivity, no increase in either dissolved organic carbon or dissolved total C HO was observed. We hypothesise that the periodic accumulation of dissolved organic carbon in the northern Adriatic is due to an excessive nitrogen en richment followed by a concurrent N and P limitation. (C) 1999 Federation o f European Microbiological Societies. Published by Elsevier Science B.V. Al l rights reserved.