Microbial activities at the benthic boundary layer in the Aegean Sea
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.
References (47)
- et al.
Characteristics, dynamics and significance of marine snow
Progress in Oceanography
(1988) - et al.
Estimation of lipids in marine animals and tissue: detailed investigations of the sulfovanilin method for total lipids
Journal of Experimental Marine Biology and Ecology
(1973) - et al.
Microbial biomass and activities in deep-sea sediments of the Eastern Mediterranean: trenches are benthic hotspots
Deep-Sea Research
(1996) - et al.
Labile organic matter and microbial biomasses in deep-sea sediments (Eastern Mediterranean Sea)
Deep-Sea Research
(1993) - et al.
Biochemical composition of sedimentary OM and bacterial distribution in the Aegean Sea: trophic state and pelagic-benthic coupling
Journal of Sea Research
(1999) - et al.
Deep-Sea ecosystem response to climate changes: the eastern Mediterranean case study
Trends in Ecology and Evolution
(2001) - et al.
Benthic pelagic coupling in the oligotrophic Cretan Sea
Progress in Oceanography
(2000) Some studies on the distribution and composition of particulate organic carbon in the north Atlantic ocean
Deep-Sea Research
(1970)- et al.
Neutral monosaccharides in surface sediments of the Northwestern Mediterranean Sea
Organic Geochemistry
(2002) - et al.
Bacterial biomass and activity in the deep waters of the eastern Atlantic-evidence of a barophilic community
Deep-Sea Research
(1997)