During the CARIACO time series program, microbial standing stocks, bacterial production, and acetate turnover were consistently elevated in the redox transition zone (RTZ) of the Cariaco Basin, the depth interval (ϳ240-450 m) of steepest gradient in oxidation-reduction potential. Anomalously high fluxes of particulate carbon were captured in sediment traps below this zone (455 m) in 16 of 71 observations. Here we present new evidence that bacterial chemoautotrophy, fueled by reduced sulfur species, supports an active secondary microbial food web in the RTZ and is potentially a large midwater source of labile, chemically unique, sedimenting biogenic debris to the basin's interior. Dissolved inorganic carbon assimilation (27-159 mmol C m Ϫ2 d Ϫ1 ) in this zone was equivalent to 10%-333% of contemporaneous primary production, depending on the season. However, vertical diffusion rates to the RTZ of electron donors and electron acceptors were inadequate to support this production. Therefore, significant lateral intrusions of oxic waters, mixing processes, or intensive cycling of C, S, N, Mn, and Fe across the RTZ are necessary to balance electron equivalents. Chemoautotrophic production appears to be decoupled temporally from short-term surface processes, such as seasonal upwelling and blooms, and potentially is more responsive to longterm changes in surface productivity and deep-water ventilation on interannual to decadal timescales. Findings suggest that midwater production of organic carbon may contribute a unique signature to the basin's sediment record, thereby altering its paleoclimatological interpretation.

Approximately half of the world's net annual photosynthesis occurs in the oceans (∼48 Pg C y−1). Areas bordering continents (bottom <2000 m) support 10–15% of this production. We used satellite data to compute annual global net primary production (1998–2001), and derived the global particulate organic carbon (POC) flux settling below the permanent thermocline and to the seafloor using an empirical model of POC remineralization. Approximately 0.68 Pg C y−1 sink below the thermocline on continental margins, compared to 1.01 Pg C y−1 in the deep ocean. Over 0.62 Pg C y−1 settles to the seafloor on margins, compared to 0.31 Pg C y−1 to deep ocean sediments. At least 0.06 Pg C y−1 may be buried in sediments on margins. Therefore, margins may be responsible for >40% of the carbon sequestration in the ocean. These regions must be accounted for in realistic models of the global carbon cycle and its linkages to climate change.

Microbial diversity and distribution are topics of intensive research. In two companion papers in this issue, we describe the results of the Cariaco Microbial Observatory (Caribbean Sea, Venezuela). The Basin contains the largest body of marine anoxic water, and presents an opportunity to study protistan communities across biogeochemical gradients. In the first paper, we survey 18S ribosomal RNA (rRNA) gene sequence diversity using both Sanger-and pyrosequencing-based approaches, employing multiple PCR primers, and state-of-the-art statistical analyses to estimate microbial richness missed by the survey. Sampling the Basin at three stations, in two seasons, and at four depths with distinct biogeochemical regimes, we obtained the largest, and arguably the least biased collection of over 6000 nearly full-length protistan rRNA gene sequences from a given oceanographic regime to date, and over 80 000 pyrosequencing tags. These represent all major and many minor protistan taxa, at frequencies globally similar between the two sequence collections. This large data set provided, via the recently developed parametric modeling, the first statistically sound prediction of the total size of protistan richness in a large and varied environment, such as the Cariaco Basin: over 36 000 species, defined as almost full-length 18S rRNA gene sequence clusters sharing over 99% sequence homology. This richness is a small fraction of the grand total of known protists (over 100 000-500 000 species), suggesting a degree of protistan endemism.

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[1] We report 15 N/ 14 N measurements of water column nitrate and ammonium, sinking particles, and sediments from the Cariaco Basin, an anoxic marine basin off the coast of Venezuela. Water column denitrification occurring in the basin has only a very small isotopic imprint on nitrate in the basin because nitrate consumption is nearly complete in the actively denitrifying water near the oxic/anoxic interface ($275 m). Being free of a large denitrification signal, the d 14 N of the thermocline nitrate, which, in turn, may record the input of newly fixed N to the upper ocean, be it local or more regional in origin.

This is the second paper in a series of three that investigates eukaryotic microbial diversity and taxon distribution in the Cariaco Basin, Venezuela, the ocean's largest anoxic marine basin. Here, we use phylogenetic information, multivariate community analyses and statistical richness predictions to test whether protists exhibit habitat specialization within defined geochemical layers of the water column. We also analyze spatio-temporal distributions of protists across two seasons and two geographic sites within the basin. Non-metric multidimensional scaling indicates that these two basin sites are inhabited by distinct protistan assemblages, an observation that is supported by the minimal overlap in observed and predicted richness of sampled sites. A comparison of parametric richness estimations indicates that protistan communities in closely spaced-but geochemically different-habitats are very dissimilar, and may share as few as 5% of total operational taxonomic units (OTUs). This is supported by a canonical correspondence analysis, indicating that the empirically observed OTUs are organized along opposing gradients in oxidants and reductants. Our phylogenetic analyses identify many new clades at species to class levels, some of which appear restricted to specific layers of the water column and have a significantly nonrandom distribution. These findings suggest many pelagic protists are restricted to specific habitats, and likely diversify, at least in part due to separation by geochemical barriers.

at 1225 m, with no seasonality in the proportion of vertical flux to primary production. In total, between 10 and 11 gC m -2 yr-*were delivered to the bottom sediment of Cariaco, which suggests that between 4 x l0 s and 1 x 10 6 t of C yr -• were delivered to sediments within the upwelling area of the Cariaco Basin. This represents permanent sequestration of carbon previously entrained in the North Atlantic gyre in the area of formation of SUW. Results suggests that upwelled inorganic nitrogen, rather than nitrogen fixation, is responsible for the large productivity and particulate carbon settling flux in the Cariaco Basin.