Plankton dynamics across the freshwater, transitional and marine research sites of the LTER-Italy Network. Patterns, fluctuations, drivers.

A first synoptic and trans-domain overview of plankton dynamics was conducted across the aquatic sites belonging to the Italian Long-Term Ecological Research Network (LTER-Italy). Based on published studies, checked and complemented with unpublished information, we investigated phytoplankton and zooplankton annual dynamics and long-term changes across domains: from the large subalpine lakes to mountain lakes and artificial lakes, from lagoons to marine coastal ecosystems. This study permitted identifying common and unique environmental drivers and ecological functional processes controlling seasonal and long-term temporal course. The most relevant patterns of plankton seasonal succession were revealed, showing that the driving factors were nutrient availability, stratification regime, and freshwater inflow. Phytoplankton and mesozooplankton displayed a wide interannual variability at most sites. Unidirectional or linear long-term trends were rarely detected but all sites were impacted across the years by at least one, but in many case several major stressor(s): nutrient inputs, meteo-climatic variability at the local and regional scale, and direct human activities at specific sites. Different climatic and anthropic forcings frequently co-occurred, whereby the responses of plankton communities were the result of this environmental complexity. Overall, the LTER investigations are providing an unparalleled framework of knowledge to evaluate changes in the aquatic pelagic systems and management options.

Assemblages' structure and activity of bacterioplankton in northern Adriatic Sea surface waters: a 3-year case study.

The bacterial community, both in terms of community structure (denaturing gradient gel electrophoresis fingerprinting) and activity (exoenzymatic hydrolysis of proteins, polysaccharides and phosphorylated molecules and leucine uptake), was investigated seasonally for 3 years (2004-2006) in a large-scale grid in the northern Adriatic Sea. A high variability characterized the spatial structure of bacterial assemblages and a scarce seasonality was found in all the nine studied stations. Bacterial communities were substantially diverse in the same season of the 3 years, in contrast to what was reported previously for oceanic sites. Assemblages were in general strongly affected by river inputs, especially in spring, when freshwater loads were higher. Finally, a close relationship was found between given assemblages and their patterns of degradation/production activities by applying a multivariate analysis (linear discriminant analysis) to the dataset. The high variability of bacterial community structures and patterns of activity may indicate an ecological response to the high dynamism that characterizes the basin both on a physical and on a biological basis.

Detection of pathogenic Vibrio parahaemolyticus through biochemical and molecular-based methodologies in coastal waters of the Gulf of Trieste (North Adriatic Sea).

Culturable vibrios were isolated from seawater collected during an annual sampling study performed along the Gulf of Trieste coast (Northern Adriatic Sea), and conventional culturing and identification methods were used to investigate the presence of Vibrio parahaemolyticus. Biochemically selected Vibrio strains were subjected to phenotypical identification performed using Alsina's scheme, API 20E and API 20NE. PCR and sequence analysis of the 16S rRNA gene and detection of the species-specific toxR and tlh genes were carried out on strains presumptively identified as V. parahaemolyticus and on a set of unidentified strains to confirm biochemical characterizations. In addition, PCR assays targeting the virulence genes, tdh and trh, were carried out to detect pathogenic strains. PCR results were compared with phenotypic characterizations to evaluate the accuracy of the biochemical methods applied. False-negative identifications were obtained by all phenotypic-based procedures, while API 20E yielded only one false positive. Because the amplification of the 16S rRNA gene produced uncertain results, toxR and tlh gene detections were necessary to confirm the biochemical identifications. Finally, molecular characterization demonstrated the presence of V. parahaemolyticus trh-positive strains and underlined the difficulty in the recognition of the pathogenic environmental organism using conventional methods.

Organic-matter degradative potential of Halomonas glaciei isolated from frazil ice in the Ross Sea (Antarctica).

Halomonas glaciei isolated from frazil ice in the Ross Sea (Antarctica) during austral summer 2003 was phenotypically characterized and its capability of degrading organic matter was tested. We evaluated specific bacterial growth rates (mu) to understand at which temperatures bacterial growth shows a linear and direct relationship with the available substrate (4-22 degrees C) and afterwards we tested H. glaciei growth curves and degradative potential at 0, 10 and 37 degrees C using two different media (one enriched and one depleted in PO(4)). The strain grew exponentially only at 10 degrees C. The fastest hydrolysis rates were expressed by enzymes aimed at polysaccharide degradation (alpha-D-glucosidase, beta-D-glucosidase and beta-D-galactosidase) while alkaline phosphatase and aminopeptidase activities were rather low. Our data suggest a preferential demand for carbon derived from carbohydrates rather than from proteins: ectoenzyme activities transformed into carbon mobilization from organic polymers, showed that the total carbon potentially released from polysaccharides can be almost one order of magnitude higher than the protein carbon mobilization. Principal component analysis of the enzyme affinity separated the six experimental conditions, highlighting how different physical (temperature) and chemical (PO(4) enrichment or depletion) features actively lead to a differentiation in the efficiency of the ectoenzymes produced, resulting in preferential degradation of diverse kinds of organic substrates.

Annual dynamics of bacterioplankton assemblages in the Gulf of Trieste (Northern Adriatic Sea).

Bacterioplankton community diversity was investigated monthly in coastal waters of the Gulf of Trieste (NE Adriatic Sea) throughout 2003. Superficial bacterial assemblages of two differently freshwater influenced stations were studied using PCR-DGGE fingerprinting techniques. Bacterial genetic diversity of the sampled area, as estimates of the number of DGGE bands was high (36-64) compared to that reported in other studies employing this fingerprint technique. The similarity index (Sorensen Index) between assemblages showed a defined operational taxonomic units (OTUs) succession pattern in the more typically marine station with stable winter communities and quickly changing summer ones. On the contrary in the station affected by riverine inputs no clear pattern was detected. In both sites, according to cluster analyses performed on the DGGE banding pattern, three seasonal assemblages were identified: winter-spring, summer and fall. Sequence analysis of fifty-six among the brightest gel bands led to the observation of bacteria affiliated to Gram positive, Cyanobacteria, Cytophaga-Flavobacteria-Bacteroides (CFB) lineages and the alpha-, gamma- and delta- subdivisions of the Proteobacteria. Gamma-Proteobacteria constituted the main fraction (60%) of sequences in the more typically marine station, whereas the river-influenced station was characterised by more heterogeneous assemblages (39% alpha-Proteobacteria, 32% Flavobacteria).