Extracellular enzyme activity and dynamics of bacterial community in mucilaginous aggregates of the northern Adriatic Sea.

Bacterial degradation of mucilaginous aggregates (creamy layers, stringers and macroflocs) collected during two summer events (2001-2002) was tested. The objective was to describe the temporal trend of the bacterial activity, abundance and composition in the aggregated and dissolved organic matter under different trophic conditions. In the native aggregates proteins and organic phosphorous were actively hydrolyzed as aminopeptidase and alkaline phosphatase activities represented up to 87% and 25% of total activity, respectively; polysaccharides were less hydrolyzed and the highest activities were observed for beta-glucosidase (5% of the total). This hydrolysation pattern tends to a progressive accumulation of long persistent polysaccharides. During short term incubations nutrient addition (P, N and Glucose) differently stimulated bacterial growth in the seawater: P played the main role in stimulating bacterial production from 3 to 6 folds higher than in the control, whereas a secondary C-limitation was observed only for bacteria growing on seawater from macroflocs. This scarce dissolved organic carbon (DOC) bioavailability was confirmed by the lower DOC removal (13% macroflocs, 36% stringers). The total amount of carbon incorporated by bacteria living on aggregates was similar (0.58 mg C L(-1)) both in the control and under P enrichments showing a more balanced condition with respect to the seawater. Hence the well-known P limitation in the Northern Adriatic Sea affects only dissolved organic carbon uptake without influencing the uptake of aggregated organic matter. Organic matter limitation was observed only on stringers--total C incorporated raised to 0.96 mg C L(-1) after PNG addition. Macroflocs release of refractory compounds leads to DOC accumulation (73 microM DOC) contributing to inflate the pool of refractory DOC in the surrounding waters. Several evidences, including different monosaccharide composition of stringers and macroflocs (glucose 15% and 56% on stringers and macroflocs, respectively), bring to the conclusion that stringers are in an older stage in comparison with macroflocs. Community composition described by fluorescence in situ hybridization did not show significant differences between free-living and attached bacteria but it was modified by the different enrichment conditions: Cytophaga-Flavobacteria increased after inorganic nutrients enrichments while organics advantaged gamma-Proteobacteria.

Bacterial uptake of DOM released from P-limited phytoplankton.

The growth and the structure of a coastal bacterioplankton community were monitored in short-term bottle experiments in order to investigate the bacterial uptake of extracellular organic carbon released by the diatom Cylindrotheca closterium grown under P-balanced and P-depleted conditions. Bacterial specific growth rates and carbon demand were significantly lower in the exudates from P-depleted algae (24% and 30% reduction, respectively). The origin of the extracellular carbon appeared also to affect the taxonomic composition of the bacterioplankton assemblage, mainly reducing the development of gamma-Proteobacteria. This pattern of bacterial carbon uptake could contribute to a longer persistence of the exudates released in P-depleted conditions affecting the dynamics of the carbon cycle in marine environments.