The fresh water discharged by large rivers such as the Amazon is transported hundreds to thousands of kilometers away from the coast by surface plumes. The nutrients delivered by these river plumes contribute to enhanced primary production in the ocean, and the sinking flux of this new production results in carbon sequestration. Here, we report that the Amazon River plume supports N 2 fixation far from the mouth and provides important pathways for sequestration of atmospheric CO 2 in the western tropical North Atlantic (WTNA). We calculate that the sinking of carbon fixed by diazotrophs in the plume sequesters 1.7 Tmol of C annually, in addition to the sequestration of 0.6 Tmol of C yr ؊1 of the new production supported by NO 3 delivered by the river. These processes revise our current understanding that the tropical North Atlantic is a source of 2. diatom diazotroph associations ͉ nitrogen fixation ͉ new production ͉ river plumes ͉ Richelia D ownward vertical transport of organic carbon produced by phytoplankton, referred to as the biological pump, is a mechanism that transfers carbon from the surface to the deep ocean and regulates atmospheric CO 2 (1). The flux of nitrate (NO 3 ) from deep water to the photic zone can stimulate new phytoplankton production and export (2), but because the upwelling or diffusive flux of NO 3 is accompanied by a corresponding upward flux of CO 2 , its net contribution to removal of carbon from the atmosphere is much reduced. However, the sinking flux due to new production associated with nitrogenous inputs from rivers, atmospheric deposition, and N 2 fixation (diazotrophy), results in the net transport of atmospheric carbon to the deep ocean (3), or ''carbon sequestration'' (4).The Amazon River has the largest discharge of any river and accounts for 18% of all of the riverine input to the oceans. Between May and September, the Amazon plume covers up to 1.3 ϫ 10 6 km 2 with a freshwater lens of salinity Ͻ35 [supporting information (SI) Table S1], which accounts for 20% of the WTNA. Our understanding of the influence of the Amazon River on the carbon cycle in the WTNA has evolved significantly since Ryther et al. (5) first suggested that the Amazon River depressed the productivity of the region influenced by its plume. Several studies have focused on the nutrients delivered by the river to the inner shelf, the subsequent river-supported new production of 0. Fig. 1 and Table S2) complement earlier studies by examining the region of the plume starting 300 km north of the mouth of the river. We classified the stations into three categories based on sea surface salinity (SSS).¶ ¶ The ''low salinity'' group contained all of the stations with SSS Ͻ30. Stations that had SSS between 30 and 35 were classified as ''mesohaline,'' whereas those with SSS Ͼ35 were classified as ''oceanic.'' Surface NO 3 concentrations were below detection at most stations, with the highest value of 0.50 M recorded at the station with the lowest salinity of 24. DeMaster and Pope (7) found when plotting NO 3 vs...

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The Antarctic and Arctic regions offer a unique opportunity to test factors shaping biogeography of marine microbial communities because these regions are geographically far apart, yet share similar selection pressures. Here, we report a comprehensive comparison of bacterioplankton diversity between polar oceans, using standardized methods for pyrosequencing the V6 region of the small subunit ribosomal (SSU) rRNA gene. Bacterial communities from lower latitude oceans were included, providing a global perspective. A clear difference between Southern and Arctic Ocean surface communities was evident, with 78% of operational taxonomic units (OTUs) unique to the Southern Ocean and 70% unique to the Arctic Ocean. Although polar ocean bacterial communities were more similar to each other than to lower latitude pelagic communities, analyses of depths, seasons, and coastal vs. open waters, the Southern and Arctic Ocean bacterioplankton communities consistently clustered separately from each other. Coastal surface Southern and Arctic Ocean communities were more dissimilar from their respective open ocean communities. In contrast, deep ocean communities differed less between poles and lower latitude deep waters and displayed different diversity patterns compared with the surface. In addition, estimated diversity (Chao1) for surface and deep communities did not correlate significantly with latitude or temperature. Our results suggest differences in environmental conditions at the poles and different selection mechanisms controlling surface and deep ocean community structure and diversity. Surface bacterioplankton may be subjected to more short-term, variable conditions, whereas deep communities appear to be structured by longer water-mass residence time and connectivity through ocean circulation. bipolar | biodiversity | next-generation sequencing | microbial ecology

Despite the high abundance of Archaea in the global ocean, their metabolism and biogeochemical roles remain largely unresolved. We investigated the population dynamics and metabolic activity of Thaumarchaeota in polar environments, where these microorganisms are particularly abundant and exhibit seasonal growth. Thaumarchaeota were more abundant in deep Arctic and Antarctic waters and grew throughout the winter at surface and deeper Arctic halocline waters. However, in situ single-cell activity measurements revealed a low activity of this group in the uptake of both leucine and bicarbonate (<5% Thaumarchaeota cells active), which is inconsistent with known heterotrophic and autotrophic thaumarchaeal lifestyles. These results suggested the existence of alternative sources of carbon and energy. Our analysis of an environmental metagenome from the Arctic winter revealed that Thaumarchaeota had pathways for ammonia oxidation and, unexpectedly, an abundance of genes involved in urea transport and degradation. Quantitative PCR analysis confirmed that most polar Thaumarchaeota had the potential to oxidize ammonia, and a large fraction of them had urease genes, enabling the use of urea to fuel nitrification. Thaumarchaeota from Arctic deep waters had a higher abundance of urease genes than those near the surface suggesting genetic differences between closely related archaeal populations. In situ measurements of urea uptake and concentration in Arctic waters showed that small-sized prokaryotes incorporated the carbon from urea, and the availability of urea was often higher than that of ammonium. Therefore, the degradation of urea may be a relevant pathway for Thaumarchaeota and other microorganisms exposed to the low-energy conditions of dark polar waters.amoA | ureC | Beaufort Sea | Ross Sea | Amundsen Sea

A typical marine bacterial cell in coastal seawater contains only B200 molecules of mRNA, each of which lasts only a few minutes before being degraded. Such a surprisingly small and dynamic cellular mRNA reservoir has important implications for understanding the bacterium's responses to environmental signals, as well as for our ability to measure those responses. In this perspective, we review the available data on transcript dynamics in environmental bacteria, and then consider the consequences of a small and transient mRNA inventory for functional metagenomic studies of microbial communities.