In the framework of the VECTOR DIVCOST Project, a 2‐year investigation was started in 2006, with the aim of testing the sensibility of microbial parameters to environmental changes and of assessing whether they can provide information about functional changes in the carbon cycle. The investigation was performed in the surface waters of two small brackish ponds (Ganzirri and Faro), located in the Cape Peloro transitional area (Sicily, Italy). The seasonal changes in both the microbial compartment [bacterioplankton, vibrios, exoenzymatic hydrolysis of proteins and polysaccharides, bacterial secondary production (HBP) and community respiration] and the trophic state of suspended matter [total suspended matter (TSM), particulate organic carbon (POC), particulate organic nitrogen (PON), C/N] were analysed in relation to the hydrological characteristics [temperature, salinity, oxygen, fluorescence, NH4, NO2, NO3, PO4]. Despite marked differences in the nutritional input and the diversification in both carbon budget and trophic level, the two ponds show similar trends in many of the investigated factors, hardly influenced by seasonal variations. Temporally coupled trends were observed for some parameters (enzyme activities, vibrios abundances, respiratory activity), whereas others (POC, PON, heterotrophic bacterial production, bacterioplankton) showed a seasonal shift between the two lakes. The different behaviour found for the some biotic parameters suggests that their response to environmental conditions may be modulated differently between the two lakes, which, despite their spatial proximity and reciprocal connection, do not always show contemporaneous functional processes.

Background: In aquatic environments, organic polymers such as proteins, polysaccharides and organic phosphates are cleaved and up taken by microorganisms through the expression of specifi c enzymes such as Leucine Aminopeptidase (LAP), beta Glucosidase (GLU) and Alkaline Phosphatase (AP), respectively. Microbial enzymatic activities are a fundamental step in the organic matter utilization and turnover into simple monomers.The context and purpose of the study: An experiment was carried out by combining the enzymatic assay using fl uorogenic substrates with microscopical counts obtained using the viability marker 5-cyano-2,3 ditolyl-tetrazolium chloride (CTC). The main objective was to verify whether the enzyme activity played by microbial cells depends of the cell viability or rather whether the hydrolytic activity is directly stimulated by the availability of organic substrates for microbial metabolism. Results:This study provides evidence that in oligotrophic waters the fl uorescence signal often decreases after incubation with the substrate analogue used for the enzymatic assay. For such samples, the extension of the incubation period from 2 to 5 hours has allowed the detection of a positive fl uorescence signal. Simultaneous counts of the abundance of CTC+ cells revealed that only a fraction close to 30% of the total bacterioplankton was actively respiring, suggesting that the observed increase was related to the presence of inducible enzymes rather than of actively metabolising cells.Main fi ndings: Since the synthesis of hydrolytic enzymes by the microbial community can be induced by the presence of the organic substrates, adapting the incubation period to the trophic condition of the examined area is required for accurate enzymatic measurements, especially for oligotrophic environments.Conclusions: This is the fi rst contribution to link the enzyme activity rates with the viability properties (in terms of actively metabolising components) of bacterioplankton inhabiting pelagic Mediterranean waters.Brief summary: Evidence of inducible enzymes in oligotrophic Mediterranean areas was provided by the increase of fl uorescence recorded after incubation up to 5 hours with specifi c fl uorogenic substrates, in concomitance with the detection of a low fraction of actively respiring cells. While microbial cells were quite active for AP synthesis, LAP and GLU were mostly inducible enzymes, activated by the addition of their specifi c polymeric organic substrate. Any potential implications:In the examined oligotrophic waters the enzymatic activities seem to be stimulated mostly by the availability of metabolisable organic substrates. https://www.peertechz.com classes (bacterio-, phyto-and zoo-plankton) perform autotrophic and heterotrophic activities that drive marine biogeochemical processes. Knowledge of the specifi c role played by different planktonic size organisms and of their biogeochemical implications in different ecological contexts attracts increasing attention by the scientifi c community

In aquatic ecosystems, the determination of the different physiological states coexisting within the bacterial assemblage is of great significance, as it may provide information on the effective role played by the viable component in the ecological processes. In May 2007, during a ship survey of the Messina Strait, a highly hydrodynamic system, an investigation was undertaken to study the abundance and distribution of the actively respiring and dead components of the bacterioplankton community. A dual-labelling procedure was applied, using SyBR Green as a probe of the total bacterioplankton community, in combination with the viability stains cyanotetrazolium chloride (CTC) or propidium iodide (PI), selective markers of actively respiring or membrane-damaged bacterial cells, respectively. Surface water samples were analysed onboard by a flow cytometer (FCM) and FCM counts were compared to the microscopic ones, obtained with the epifluorescence (EPI) method further performed in the laboratory. The study also pointed out the feasibility of the FCM approach as a rapid tool allowing the identification of the viable component and its discrimination from the dead or damaged one, also providing quantitative estimates correlated significantly with microscopic counts.