Relationships between organic carbon and microbial components in a Tyrrhenian area (Isola del Giglio) affected by mucilages.

The chemical and biological properties of the water column at a Tyrrhenian site (Isola del Giglio) were studied during a 3-year period. The results highlighted the oligotrophic features of the site, characterised by quite low concentrations of organic carbon (on average DOC 102 micromol/L and POC 9 micromol/L). Relevant bacterial biomass (on average 42.1 microg C/L) and a notable activity (in terms of frequency of dividing cells, on average more than 5%) were observed. However, remarkable changes for these parameters were seasonally recorded. The cyclic occurrence, generally during the late spring-summer period, of benthic mucilage indicated that localised distrophic processes may occur. In particular, the benthic mucilage events of 2000 and 2001 were investigated, although some comparative information was available also for 1999 and 2002. The mucilage aggregates generally showed high bacterial colonisation, which have remarkable effects on the organic matter cycle both inside the aggregates and in the surrounding seawater. During the benthic mucilage development, an increase of DOC and POC concentrations was observed (up to 129 and 18 micromol/L, respectively, in June 2000 and up to 145 and 10 micromol/L, respectively, in May and June 2001) in the water column adjacent to the bottom. However, a general decrease of the trophic value of particulate matter (in terms of C/N ratio) was also observed, especially in 2000 after the disappearance of the mucilage. The available energy and organic matter during the mucilage events led to an increased presence of bacteria in the bottom waters of the Isola del Giglio, with maximum biomass values in 2001. Similarly, the replicative activity of bacteria was higher in 2001 (frequency of dividing cells about 5% vs. 3% of 2000). The lower activity of 2000, in addition to the lower trophic value of organic matter and different environmental conditions (namely lower temperature), might be involved in the persistence of mucilage in 2000 with respect to the rapid disappearance observed in 2001.

Enzymatic activity on sandy beaches of the Ligurian Sea (NW Mediterranean).

Enzymatic activity was measured on two beaches of the Ligurian Sea (NW Mediterranean) during late spring and summer 2003. The detected activities (leucine aminopeptidase, beta-glucosidase, alpha-glucosidase, and beta-N-acetylglucosaminidase) were related to the available organic substrates (proteins and carbohydrates) and to the bacterial community (expressed in terms of abundance, biomass, and frequency of cell division). The very low chlorophyll a concentrations (never higher than 40 ng g(-1)) suggested that heterotrophic microorganisms play a major role in the beach ecosystem. Enzymatic activities devoted to organic matter degradation were lower in the emerged part of the beaches and higher in the sites covered, permanently or temporarily, by seawater, suggesting that sea action enlivens the degradation processes. Leucine aminopeptidase ranged from 0.26 to 13.02 nmol g(-1)h(-1), and beta-glucosidase (the most expressed glycolytic enzyme) from 0.03 to 4.51 nmol g(-1)h(-1). Strong changes in the proteolytic/glycolytic activity ratio were observed, with a sudden rise in glycolysis during summer, leading to ratio values from about 30 down to 1. Thus, beaches were identified as preferential degradation sites, where very refractory compounds such as cellulose may also be efficiently processed.

Ectoenzymatic ratios in relation to particulate organic matter distribution (Ross Sea, Antarctica).

The results of a study on ectoenzymatic activity (the enzyme activity bound to particles larger than 0.2 micro m) and its relation to organic particle concentration are reported here. The sampling was carried out during the 1994 Antarctic spring, at a fixed station (Station 11) in the polynya of the Ross Sea, an area characterized by quick changes in sea ice cover. The sampling was repeated 4 times over a 20-day time period. The particulate organic matter distribution followed the physical structure of the water column, which depends on ice dynamics and is mainly determined by salinity. In the mixed-water surface layer (0-50 m) the concentrations were higher (on average 65.6 micro gC/L) than in the deeper water layer (50 m-bottom) (on average 19.1 micro gC/L). This distribution and quality, expressed by the protein:carbohydrate ratio, linked the particulate organic matter to the phytoplanktonic bloom which was in progress in the area. We determined the kinetic parameters of the glycolytic and proteolytic ectoenzymes and also the total activity for the proteolytic enzyme, in order to evaluate the contribution of the particle-bound activity. We observed higher values in the surface layer than in the deeper layer. b-Glucosidase activity ranged between 0.03 and 0.92 nmol L(-1) h(-1); b-N-acetylglucosaminidase activity was in the range of 0.04-0.58 nmol (L-1) (h-1). The total proteolytic activity (leucine aminopeptidase) ranged between 0.85 and 33.71 nmol L(-1) (h-1). The ectoproteolytic activity was about 35-60% of the total. The Km values were slightly higher for the proteolytic activity (on average 0.43 micro M for ectoproteolytic activity and 0.58 micro M for total proteolytic activity) than for the b-glucosidase (on average 0.36 micro M) and b-N-acetylglucosaminidase (on average 0.17 micro M), showing no remarkable variations in the water column. The ectoenzymatic ratios and their relationship with particulate organic substrates confirm the close link between organic substrate availability and degradation system response. The significant and positive correlations are not specific and suggest a prompt and efficient systemic response to the input of trophic resources. Nevertheless, changes in ectoenzyme activity and synthesis may act as adaptive responses to changing features of the ecosystem. In particular, variations in the proteolysis:glycolysis ratio depend on the functional features of the ecological system. In our study area this ratio is higher (about 10 or more) during production (particularly autotrophic) and lower (about 5 or less) during degradation/consumption events. The analysis of previous data, collected over a larger area characterized by different environmental conditions due to the changes of the pack ice cover, during the same cruise, confirms the existence of a significant relationship. Furthermore, the analysis of enzyme-uptake systems, expressed as Vmax:Km ratio, suggests that glycolytic ectoenzymes, although poorly expressed, may encourage microconsumers to grow rapidly on a wide range of organic substrates, including the refractory ones such as cellulose and chitin. However, low ectoenzyme potential exploitation rates of available organic substrates (on average about 5% for glycolytic and 12% for proteolytic ectoenzymes) would suggest that, during spring, zooplankton grazing or vertical and lateral transport are likely to play an important role in the removal of organic materials from the system.