Jean Garcin

Biopolymer hydrolysis and bacterial production under ambient hydrostatic pressure through a 2000 m water column in the NW Mediterranean

Kinetic parameters for aminopeptidase, phosphatase, and bacterial production rates were studied during spring and fall through a 2000 m water column in the NW Mediterranean. Bacterial production ranged from 60.4 ng at 30 m to 0.2 ng C l � 1 h � 1 at 2000 m. For both ectoenzymatic activities, the Km values ranged from 0.44 to 1.13mM for aminopeptidase activity and from 0.05 to 1.23mM for phosphatase activity. Depth profiles of the potential activity of aminopeptidase and phosphatase activity drastically decreased below depths of 100 m. At 1000 m, hydrolytic activities were one order of magnitude lower than the maximal rate measured in the surface layer. Despite this decrease, depthintegrated rates through the thickness of different water masses showed that the potential hydrolysis fluxes within the productive surface layer (10–200 m), through the twilight zone (200–1000 m depth), and through the deep water mass (1000–2000 m) were roughly the same order of magnitude. This study used the first assay for measuring ectoenzymatic activities of deep-sea microbial populations where pressure stresses were eliminated during sampling and incubation. The results showed that prokaryotic induced ectoenzymatic activities are affected by pressure conditions. Generally, aminopeptidase and phosphatase rates measured in samples maintained under in situ pressure conditions were 2.3 times higher than those measured in their decompressed counterparts. r 2002 Elsevier Science Ltd. All rights reserved.

Microbial activities at the benthic boundary layer in the Aegean Sea

Abstract During the Aegean Sea component of the EU MTP-MATER project, benthic samples were acquired along a depth gradient from two continental margins in the Aegean Sea. Sampling was undertaken during spring and summer 1997 and the microbial metabolic activities measured (Vmax for aminopeptidase activity, 14 C-glutamate respiration and assimilation) displayed seasonal variability even in deep-sea conditions. The metabolic rates encountered in the North Aegean (average depth 566±234 m), were approximately five-fold higher than in the deeper (1336±140 m) Southern part of the Aegean. The aminopeptidase rates, however, were the exception with higher values recorded in the more oligotrophic sediments of the Southern stations (1383±152 vs. 766±297 nmol MCA cm −2 h −1 ). A discrepancy in bacterial metabolism also appeared in the near bottom waters. In the Southern stations, 80% of the glutamate uptake was used for energy yielding processes and only 20% devoted to biomass production, while in the North Aegean, most of the used glutamate was incorporated into bacterial cells. During the early burial stages, bacterial mineralization rates estimated from 14 C-glutamate respiration decreased drastically compared to the rates of biopolymer hydrolysis estimated by aminopeptidase assays. Thus, at the 2-cm depth layer, these rates were only 32 and up to 77% of the corresponding average values, respectively, in the superficial layer. Such a discrepancy between the evolution of these two metabolic activities is possibly due to the rapid removal of readily utilizable monomers in the surface deposits. The correlation between bacterial respiration and total organic carbon, or total organic nitrogen, is higher in the surficial sediment (0–2 and 2–4 cm) than in the underlying layer. Conversely, it is only at 4-cm depth layer that the hydrolysis rates appear correlated with organic carbon and nitrogen concentrations. This pattern confirms the drastic degradation of organic matter during the early burial stages.

Bacterial utilization of glucose in the water column from eutrophic to oligotrophic pelagic areas in the eastern North Atlantic Ocean

Vertical profiles of glucose utilization rates were compared at three sampling stations in the eastern part of the tropical North Atlantic Ocean. The investigation area was along 20–21 °N and the three sampling sites, characterised by differences in their primary productivity, were located at 18 °W, 21 °W and 31 °W. In the superficial waters, maximum (Vmax) glucose utilization (respiration plus incorporation) depended on the nutritional load being 20-fold higher in the eutrophic, compared to the oligotrophic zone. Due to these variations, natural turnover times for this labile compound were approximately 1 day in the eutrophic area, and up to 435 days in the oligotrophic area. Bacterial activity showed a steep decline immediately below the mixed layer in the mesotrophic and eutrophic areas and below the deep chlorophyll maximum in the oligotrophic area. Discrepancies between microbial activities in the three areas decreased with increasing depth: at depths below 250 m potential utilization rates of glucose were similar whatever the nutrient richness of the photic layer. Nevertheless, the distribution of microbial activities through the whole water column depended greatly on the productivity of superficial waters. In nutrient-rich areas 73% of glucose utilization activity was realized in the productive upper layer, whereas only 4% was metabolized at depths below 250 m. Conversely, in the oligotrophic area, more than 40% of the glucose utilized in the whole water column was processed in the intermediate and deep-water masses. Integration of Vmax values for the whole water column, suggested potential carbon fluxes due to bacterial utilization of glucose of 6 and 34 mg C m−2 d−1 in the oligotrophic and eutrophic areas, respectively. The fate of the metabolised carbon depended on the nutrient availability. In the mixed-water layer the glucose respiration percentage (%R) increased from 30% in nutrient-rich areas to 60% under oligotrophic conditions, moreover %R increased with depth. This infers that at lower nutritional loads, a greater proportion of highly labile compounds is used for energetic purposes, and therefore return to the inorganic carbon pool, but with very low turnover rates.

Microbial activity at the deep water sediment boundary layer in two highly productive systems in the Western Mediterranean: the Almeria-Oran front and the Malaga upwelling

Microbiological and biogeochemical studies were carried out in sediments and near bottom waters in the upwelling area off the Malaga coast and in the Almeria-Oran frontal zone. In these nutrient-rich conditions, metabolic activity is potentially limited by oxygen availability through the sediment depth. In the surficial sediments of the frontal zone, however, oxygen penetrated to a depth of 4 cm, allowing oxic mineralisation processes to occur throughout the layer. In the surficial sediments of the upwelling area, oxygen penetration was limited to the top 2.5 cm, leading to anoxic conditions. Glutamate respiration and global oxygen uptake rates were clearly higher than in the frontal zone. In the superficial sediments of the frontal zone, bacteria were less abundant and showed the lowest potential rate for mineralisation processes, but the highest rate for proteolysis. This discrepancy is probably due to differences in the quality of organic inputs into the two areas, with labile organic compounds reaching the sea bottom in the shallower upwelling zone. Such inputs enhance the mineralisation of low molecular weight monomers, whilst inhibiting the polymer hydrolysis processes. Conversely, in the deeper frontal zone, labile monomers become exhausted, decreasing the mineralisation rates. Concomitantly, bacteria have to develop ectoenzymatic activities in order to extract their carbon and energy from the available semi-labile polymers. Consequently, the theoretical relationship between the mineralisation and hydrolysis processes are tightly coupled in the upwelling area, and uncoupled in the frontal zone.