Microbial activities at the benthic boundary layer in the Aegean Sea

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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, 14C-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 14C-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.

Introduction

Part of the organic material produced in the euphotic zone of the ocean sinks or diffuses through the water column. The benthic boundary layer is the endpoint of the sedimenting material which stimulates high metabolic rates for microbial populations both in the near bottom waters and in the superficial sediment layers (Poremba and Hoppe, 1995, Boetius, Scheibe, Tselepides and Thiel, 1996). The fate of the organic material degraded by benthic microbial populations during the first burial stages is: (a) to return to the water column in an inorganic form to supply the photosynthetic process, or (b) to be immobilized below the sediment mixed layer, where it may be mineralized by bacterial populations in the deep subsurface layers, or undergo definitive sequestration within the geosphere.

The major reservoir of total organic matter in the water column is in the dissolved phase (Bolin, Rosswall, Richey, Freney, Ivanov, & Rodhe, 1983). On the other hand, rapidly sinking large particles or aggregates that pass through the pelagic biological filter constitute the main source of energy-rich material on the seafloor (Alldredge and Silver, 1988, Turley, Lochte and Lampitt, 1995, Tselepides, Polychronaki, Marrale, Akoumianaki, Dell’Anno, Pusceddu and Danovaro, 2000). As a result, most of the organic sedimentary material is composed of high molecular weight polymers (Boetius and Lochte, 1994, Poremba, 1995). Quantitative and qualitative composition of organic matter in the benthic boundary layer depends on phytoplankton biomass production in the photic zone and its residence time in the water column. Increasing the residence time decreases the readily utilizable dissolved organic mater that reaches the benthos, while increasing the fraction of high molecular weight organic polymers (Boetius & Lochte, 1994). Such polymers (>600 Da) cannot pass through the bacterial cell membrane, therefore, in order to be used as carbon and energy sources by heterotrophic bacteria they must first be hydrolyzed to low molecular weight compounds (Chrost, 1991, Meyer-Reil, 1991).

Technically, it is difficult to measure the actual metabolic rates for microbial populations in sediment samples (Tholosan & Bianchi, 1997). Nevertheless, studies on the distribution of microbial populations and quantification of their possible metabolic activities at the benthic boundary layer are particularly useful for understanding the coupling between pelagic and benthic nutrient fluxes (Smith, Kaufmann, & Baldwin, 1994). In a recent study of the Cretan Sea (Duineveld et al., 2000), sediment community oxygen consumption, microbial biomass and chlorophyll content of the surface layer of sediment were measured in an attempt to understand coupling between the pelagic and benthic environment. All of these variables decreased simultaneously with increasing water depth.

The goal of this study was to describe the distribution of bacterial populations and their metabolic activities involved in organic matter regeneration at the benthic boundary layer, comparing different depths and trophic conditions in the northern and southern parts of the Aegean Sea.

Section snippets

Sample collection

Sampling stations were located along the continental margins of the South and North Aegean Sea (Fig. 1) and visited during two cruises of the R/V AEGAEO in spring and summer 1997. In the north, stations ranged from 115 to 1271-m depth and in the south from 914 to 1705-m (Table 1). Sediments (nearly 40 cm deep) and overlying water (approximately 10–20 cm above the sea floor) were collected with a Bowers/Connelly multiple corer (Barnett, Watson, & Connelly, 1984). Only undisturbed sediment cores

Bacterial density

In the North Aegean, during late summer 1997 (MATER cruise II), samples were collected from stations ranging from 115 to 1271 m depth which allowed for the examination of the effect of depth on bacterial distribution and activity through the water-sediment boundary layer. In the near bottom waters, bacterial densities decreased from the shelf to the slope and then to the deep basin. At 1271-m depth bacterial density was one order of magnitude lower than at 115-m depth (Table 2).

In the surficial

Conclusions

During this study, the potential monomer remineralisation rates tended to be negatively correlated with depth, while bacterial hydrolysis rates were positively and significantly correlated with depth. Similarly, Bensoussan and Bianchi (1983) observed that in the coastal zone, metabolism of bacterial strains isolated from the NBW is directed to degrading large organic molecules, while in the deep sea, the bacterial metabolism is mainly directed to using low molecular weight compounds. In recent

Acknowledgements

This work was supported by the European Commission’s Marine Science and Technology (MAST) Program, under the MTP-MATER project (contract MAST III-CT96-0051). Authors sincerely thank the officers and crew of R/V AEGAEO for their kind contribution in the field.

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