OS25D-01
Prokaryotic Respiration and Production in the Meso- and Bathypelagic Realm of the Eastern and Western North Atlantic Basin
We measured prokaryotic production and respiration in the major water masses of the North Atlantic down to a depth of ~4000 m by following the progression of the two branches of North Atlantic Deep Water (NADW) in the oceanic conveyor belt. Prokaryotic abundance decreased exponentially with depth by 86% in the eastern basin and by 91% in the western basin. Prokaryotic production measured via 3H-leucine incorporation showed a similar pattern as prokaryotic abundance and decreased by 88% in the eastern and by 94% in the western basin. Prokaryotic respiration, measured via oxygen consumption, decreased by 80% from ~100 m depth to the NADW. Prokaryotic growth efficiencies of ~2% in the deep waters (depth range ~1200-4000 m) indicate that the prokaryotic carbon demand exceeds dissolved organic matter input and surface primary production by ~2 orders of magnitude. Cell-specific prokaryotic production was rather constant throughout the water column, in the eastern and in the western basin. Along with increasing cell-specific respiration towards the deep water masses and the relatively short turnover time of the prokaryotic community in the dark ocean (34-54 days), prokaryotic activity in the meso- and bathypelagic North Atlantic is higher than previously assumed. Preliminary data from a newly developed pressure retaining system will be presented to show whether depressurization of samples from the dark ocean affects prokaryotic rate measurements.
OS25D-02
Role of Archaeal and Bacterial Activity in the Carbon Cycling of the Meso- and Bathypelagic Realm of the North Atlantic Ocean
The abundance and cell production of Archaea in the meso- and bathypelagic North Atlantic were determined along a S-N transect from 65°N to 10°N following the North Atlantic Deep Water. Using an improved catalyzed reporter deposition-FISH (CARD-FISH) method and specific oligonucleotide probes, Archaea were found to be consistently more abundant than Bacteria below 100 m depth. Combining microautoradiography with CARD-FISH (MICRO-CARD-FISH) revealed a high fraction of metabolically active cells in the deep ocean. Even at 3000 m depth, about 16 % of the Bacteria were taking up leucine. The percentage of Eury- and Crenarchaeaota taking up leucine did not follow a specific trend with depth ranging from 6 - 35 % and 3 - 18 %, respectively. The fraction of Crenarchaeota taking up inorganic carbon increased with depth, while Euryarchaeota taking up inorganic carbon decreased from 200 m to 3000 m depth. The ability of Archaea to take up inorganic carbon was used as a proxy to estimate archaeal cell production and to compare this archaeal production with total prokaryotic production measured via leucine incorporation. We estimate that archaeal production in the meso- and bathypelagic North Atlantic contributes between 41-84 % in the Labrador Sea Water and declining to 10-20 % in the North Atlantic Deep Water. The recently emerging notion that Crenarchaeota are ammonia oxidizers was tested using q-PCR of the amoA gene, encoding a key enzyme in the ammonium oxidation, ammonium monooxygenase. Although Crenarchaeota are the dominating prokaryotic group in the bathypelagic waters of the North Atlantic, amoA abundance was barely detectable in the bathypelagic realm. AmoA abundance decreased from the bottom of the euphotic layer to the bathypelagic realm by three orders of magnitude while the abundance of Crenarchaeota as revealed by MICRO-CARD-FISH decreased only by one order of magnitude over this depth range. Thus, we conclude from this study that bathypelagic Crenarchaeota are actively growing in the dark ocean although at lower growth rates than Bacteria, however, it is likely that in contrast to recent observation on Crenarchaeota from the upper water column, bathypelagic Crenarchaeota might not oxidize ammonia in significant amounts. This would agree with the general scarcity of ammonia in the deep ocean.
OS25D-03
Prokaryotic activity and community structure in meso- and bathypelagic waters of the North Atlantic
The meso- and bathypelagic layers of the oceans are characterized by low biomass, low temperature and low concentrations of organic compounds. These low values make difficult to measure prokaryotic activity in the deep-ocean, thus leading many oceanographers to think that the deep layers of the ocean are populated with starving prokaryotes with no biogeochemically significant activity. Despite the scarcity of available data, there is growing evidence that deep-water prokaryotes are well adapted to their environment and far from wasting away. In this study, we assessed prokaryotic community composition and ectoenzyme activity in the different meso- and bathypelagic water masses of the North Atlantic. No significant decrease in OTU richness was observed with depth, indicating that deep prokaryotic communities are as diverse as those in surface waters. Moreover, the geographical distribution of both bacterial and archaeal ribotypes clearly matched that of water masses determined by physical parameters, showing that different water masses harbor distinct prokaryotic communities. Cell-specific ectoenzyme activities of deep-water prokaryotes were comparable to those found in surface waters and sometimes significantly higher, indicating that deep-waters microbes are actively processing DOM. A "hotspot" of microbial activity was detected in deep waters at the Charlie Gibbs Fracture-Zone (CGFZ) where there is significant exchange of deep waters between the eastern and western basins of the North Atlantic. As about one half of the prokaryotes in the water column live in the deep-ocean, these data suggest that their contribution to the global carbon cycles may be larger than expected. Moreover, the differences found among different prokaryotic communities from different water masses in terms of diversity and activity stress the need for more detailed characterization of microbial processes in deep waters.
OS25D-04
DOC Decay in the Interior of the North Atlantic Basin
Biogeochemical variables including dissolved organic carbon (DOC) were measured along the A16N, A20 and A22 meridional transects in the North Atlantic as part of the US Repeat Hydrography program in 2003. The data provide an expansive high-resolution spatial picture of DOC variability within the North Atlantic Ocean's interior. A decreasing gradient in DOC concentrations was observed from north to south within the North Atlantic Deep Water (NADW). In addition, geochemical tracers such as chlorofluorocarbon (CFC) concentrations were measured concomitantly and partial pressure of CFC ({\it p}CFC) in seawater was derived. We observed that DOC concentration variability in the mesopelagic and bathypelagic in the North Atlantic covaried with {\it p}CFC variability in certain portions of the transects. For example, within the bathypelagic of the North Atlantic A20 line we observed that low DOC concentrations (40 to 42 uM C) coincided with {\it p}CFC depleted waters, and high DOC concentrations (as high as 48 to 49 μM C) were observed in waters with elevated {\it p}CFC. The approximate time that a water mass last ventilated with the atmosphere, mean CFC-age, can be estimated from a polynomial fit of seawater {\it p}CFC to the global average atmospheric {\it p}CFC history (Walker et al., 2000; JGR, 105). In some depth horizons changes in DOC concentration was highly correlated to changes in relative CFC-age i.e an r2 of 0.65 was estimated for the NADW between 43\deg N to 25\deg N along 52\deg W. This slope of this relationship allows us to gain insight about removal rates of a portion of the DOC pool that turn over on longer time scales than can be estimated from short-term bioavailability assays or prokaryotic production rate measurements. We will present data from these North Atlantic transects that address the topic of DOC decay within the oceans mesopelagic and bathypelagic depth horizons.
OS25D-05
Bacterioplankton Distribution and Production in the Bathypelagic Ocean: Directly Coupled to Particulate Organic Carbon Export?
Bacterioplankton abundance and productivity in the bathypelagic North Pacific appear to be coupled with POC fluxes (Nagata et al. 2000). In that analysis, bacterial biomass and productivity were found to be several-fold greater in subarctic than subtropical waters, consistent with the basin-scale distribution of POC flux and suggestive of `sinking POC to DOC to bacteria' transformations of the carbon. To test this hypothesis, we sought to determine if the very strong spatial and temporal gradients in POC flux in the western Arabian Sea would force similar deep ocean gradients in bacterial variables. When viewed on a within- or between-cruise basis, we found variability in bacterial abundance and thymidine incorporation in the deep Arabian Sea, but correspondence was equivocal between these variables and several correlates to export: flux of biogenic carbon from the euphotic zone, state of the monsoon, proximity to productive coastal upwelling zones. However, when annual mean bacterial abundance at 2000 m was compared to annual POC flux at that depth, we found a strong correspondence - high annual flux supported high bacterial abundance (such a correspondence was not found for bacterial productivity). This finding suggests that bathypelagic bacterial abundance responds to the long-term mean inputs of organic matter, and evidently less to episodic inputs. A comparative evaluation with the North Pacific demonstrated that while the bathypelagic bacteria there showed correspondence to deep POC flux, POC flux alone would not account for the wide meridional variations in bacterial abundance that have been reported.
OS25D-06
Encounter of the First Kind: Bacterial Colonization of Marine Snow and Implications for DOM Dynamics in the Oceans
Marine snow particles are one of the primary vehicles for deep-sea carbon sequestration, contributing to the coupling of mesopelagic and benthic zones in the oceans. Bacterial degradation and remineralization of marine snow could regulate organic fluxes to depth and affect the dynamics of dissolved organic matter in the water column. Population dynamics of bacteria on marine snow particles depends on rates of colonization, detachment, growth, and mortality. Understanding bacterial dynamics on marine snow necessarily begins with describing encounter opportunities between bacteria and marine snow particles. In this study, colonization and detachment rates of bacteria of different physiological states were measured on model particles. Four bacterial isolates from the Wadden Sea and two from the York River were incubated in either nutrient-free water or marine broth overnight. Bacteria were then inoculated in experimental chambers with suspended model marine snow particles (agar spheres). Bacteria colonizing the model particles were sampled regularly and counted by DAPI staining and epifluorescence microscopy. At the end of the colonization, remaining model particles were transferred to nutrient-free water, and the rate at which bacteria detach from the particles was quantified in a similar way. Bacteria that were fed overnight colonized faster and achieved higher steady-state abundances than those that were starved. Fed bacteria initially detached at a higher rate for the first five minutes, but then behaved similarly to starved bacteria thereafter. The colonization and detachment behaviors of the bacteria in different nutrient regimes could potentially be explained by differences in motility. Fed bacteria could change their motility patterns to increase encounter rates with particles and, therefore, increase colonization. These observations suggest that marine snow particles could be turned over by microorganisms more rapidly in eutrophic environments than marine snow in oligotrophic areas.
OS25D-07
Comparison of the Molecular Composition of Marine Organic Matter From the Sargasso Sea and North Pacific
Heterotrophic processes ultimately regulate marine organic matter distributions and dynamics in the interior ocean. Understanding the chemical composition of marine organic matter will help unravel fundamental aspects and controls of heterotrophic transformations and remineralization. In this study, neutral sugars, amino sugars and D/L-amino acids were analyzed in various size fractions of particulate organic matter (POM) and dissolved organic matter (DOM) from the Sargasso Sea and North Pacific. Neutral sugars and amino sugars showed similar size- and depth-related distribution patterns and appeared to be tightly coupled. High molecular weight DOM (HMW-DOM) accounted for about 60% of these biochemicals throughout depth. In contrast, the distribution of amino acids was shifted to greater abundances in smaller size fractions. Amino sugars represented a substantial fraction (15%) of carbohydrates and likely play an important role in N cycling. Ratios of galactosamine and glucosamine between 1 and 2.8 in all size fractions indicated that chitin was not the primary source of glucosamine. Together with abundances of bacterially derived D-amino acids, the data provide evidence for a major prokaryotic source. Carbon-normalized yields of all biochemicals displayed significant gradients from surface to deep waters. Deep water DOM from the Sargasso Sea and North Pacific was clearly different. D-amino acids were selectively preserved compared to L-amino acids indicating that bacteria cell components are an important constituent of the very resistant fraction of DOM.