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

Abstract

Seawater from the northern Adriatic, with low phosphorus (0.03 μM) and nitrogen (1.0 μM NO3 and 1.0 μM NH4) concentration, was incubated for 12 days in 20-l polycarbonate carboys. The addition of a nutrient mixture (0.6 μM PO4, 5.1 μM NO3, 1.8 μM NH4, 10.6 μM SiO2) induced a strong diatom bloom, reaching 25 μg l−1 Chl a. Primary and bacterial production were stimulated by the initial enrichment of nitrogen and phosphorus but ceased when N and P depletion occurred after 4 days. Inorganic N exhaustion resulted in a significant production (and accumulation) of dissolved and particulate carbohydrates. The initial accumulation of carbohydrates (CHO) in the particulate phase was followed 2 days later by a significant release of dissolved CHO. The bacterial response to this organic carbon source, as reflected by glucosidase activity, was probably inhibited by the severe P limitation following the phytoplankton bloom. In the exponential phase, when P concentration was sufficiently high to sustain a significant glucosidase activity, no increase in either dissolved organic carbon or dissolved total CHO was observed. We hypothesise that the periodic accumulation of dissolved organic carbon in the northern Adriatic is due to an excessive nitrogen enrichment followed by a concurrent N and P limitation.