[1] Particulate organic carbon (POC) sinking out of the sunlit euphotic zone at the surface of the ocean feeds the deep sea and alters the CO 2 concentration of the atmosphere. Most of the sinking POC is reoxidized to dissolved inorganic carbon (DIC) before it hits the sea floor, but the mechanism for this is poorly understood. Here we develop a global model of the microbial loop in the aphotic zone based on new measurements of deep ocean bacterial metabolism. These together imply that a significant fraction of the decreasing POC flux with depth is converted to dissolved organic carbon (DOC) rather than directly to DIC as is commonly assumed, thereby providing the substrate for free-living bacteria in the deep ocean. The model suggests the existence of a substantial DOC-pool with a relatively fast turnover time in the deep sea. By implementing the microbial loop in a model of the global ocean circulation, we show that the observed gradient of DOC in the deep North Atlantic can be explained by the temperature dependence of bacterial metabolic activity in conjunction with the formation of deep-water at high latitudes.
Bacterial turnover of dissolved organic carbon (DOC) and the regulation of bacterial activity was investigated during 2 cruises in the Greenland Sea in June and August 1999, in order to provide information on the role of bacteria for the biogeochemical cycling of DOC in the northern North Atlantic. We measured the actual pool size of labile DOC (DOC-L) and in situ bacterial growth rate (production:biomass ratio; P:B) in vertical profiles in the Greenland Sea, covering a depth range from surface to 3662 m depth. In situ bacterial growth rates ranged from 0.0002 to 0.5 d -1 , and significant bacterial growth was observed down to the deepest samples. Approximately half of the variation in in situ growth rate could be described by a model with DOC-L (µM) and temperature (°C) as variables: log(P:B) = 0.37 × log(DOC-L) + 0.059 × temperature -2.41 (r 2 = 0.47, p < 0.0001). DOC-L concentration ranged from 13.5 µM (surface waters) to < 0.1 µM in some of the deep-water samples and showed a significant decrease with depth. The presence of DOC-L in samples from below 1000 m depth (average 0.28 ± 0.21 µM C (n = 12)) suggested an efficient transport of organic carbon from the productive layer, possibly by sedimenting particles. Growth experiments with bacterioplankton from surface waters and deep waters of the Greenland Sea showed significant influence of both temperature (°C) and DOC-L (µM) on bacterial growth rate (µ; d -1 ) as described by log(µ) = 0.25 × log(DOC-L) + 0.081 × temperature -1.14 (r 2 = 0.73, p < 0.0001). Inorganic nutrient concentration did not affect bacterial growth rate. Our findings suggest that bacterial DOC uptake in the Greenland Sea is controlled by a combination of temperature and the concentration of DOC-L.KEY WORDS: Bacteria · Dissolved organic carbon · Substrate · Temperature · Greenland Sea · Q 10 -factor
Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 32: [151][152][153][154][155][156][157][158][159][160][161][162][163] 2003 of the DOC consists of carbon components with turnover times that range from hours to years; however, the labile DOC (DOC-L) has operationally been defined as the amount of DOC that can be utilized by bacteria within 1 to 2 wk of incubation (Søndergaard & Middelboe 1995).The concentration of DOC-L is influenced by a number of factors, which affect bacterial DOC utilization. The affinity towards the given DOC-L of the dominant bacterial populations will influence the rate of DOC uptake, and low affinity to a given DOC-L pool of the bacterial community may lead to accumulation of DOC-L (Søndergaard & Middelboe 1995). Limitation of bacterial growth by the availability of inorganic nutrients or predation have been proposed as alternative mechanisms for accumulation of DOC-L (Thingstad et al. 1997(Thingstad et al. , 1998, since grazer control or inorganic nutrient limitation of bacterioplankton may cause periodic reduction of bacterial carbon uptake. Finally, temperature may be an important factor controlling bacter...
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