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 mum) 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 mugC/L) than in the deeper water layer (50 m-bottom) (on average 19.1 mugC/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. P-Glucosidase activity ranged between 0.03 and 0.92 nmol L(-1)h(-1); beta-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 K-m values were slightly higher for the proteolytic activity (on average 0.43 muM for ectoproteolytic activity and 0.58 muM for total proteolytic activity) than for the beta-glucosidase (on average 0.36 muM) and beta-N-acetylglucosaminidase (on average 0.17 muM), 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 V-max:K-m 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.