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.

Virus-like particles (VLP), collected from the permanently anoxic Cariaco Basin between 10 October 1996 and 2 November 1999, were enumerated in water column and sediment trap samples. Vertical distributions of VLP from 18 depths between 7 and 1310 m generally corresponded to those of bacterial abundance and bacterial net production (BNP), with primary maxima consistently found in surface waters and midwater maxima near the O 2 /H 2 S interface. Temporal variations in VLP concentrations (0.81 to 630 × 10 8 VLP l -1 ) were highly correlated with chlorophyll a (chl a), bacterial abundance and BNP in the upper 250 m. In the redox transition zone (RTZ = 250 to 450 m), VLP abundance covaried with bacterial growth rates, but not bacterial abundance nor chemoautotrophic production. In the anoxic layer (> 450 m), temporal variations in VLP abundance were not significantly correlated with any measured variable. In the RTZ, the median VLP:bacteria ratio (VBR = 3) was significantly lower than in the oxic (VBR = 16) and anoxic (VBR = 31) layers for all observations, suggesting varying relationships between viruses, hosts and environment among these layers. Vertical fluxes of VLP associated with sedimenting debris varied between 0.39 and 520 × 10 ). VBRs in the sinking inventories were very low, varying from 0.01 to 1.2 and averaging 0.60, suggesting that VLP are not as numerically important in sinking particles as they are in suspended communities. Comparisons of sinking fluxes with suspended VLP inventories indicate that vertical transport is relatively unimportant in redistributing viruses in the water column. Estimated removal rates by sinking from the oxic, transition and anoxic layers averaged 0.11% mo -1 (n = 16) for apparent VLP.KEY WORDS: Viruses · Bacteria · Anoxia · Bacterial production · Microbial loop · Cariaco Basin Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 30: [103][104][105][106][107][108][109][110][111][112][113][114][115][116] 2003 levels, thereby increasing supply of respiratory carbon to the lowest trophic levels (bacteria and protozoa) and diminishing overall system efficiency by introducing additional trophic iterations for biologically essential elements (Murray & Eldridge 1994). Using electron microscopic and epifluorescent microscopic techniques, free virioplankton and viralinfected microplankton have been shown to be plentiful in a variety of lacustrine, estuarine, coastal, oceanic and hypersaline environments from the tropics to the poles (e.g. Proctor & Fuhrman 1990, Bird et al. 1993, Cochlan et al. 1993, Oren et al. 1997, Weinbauer & Höfle 1998a, Steward et al. 2000. With few exceptions, surveys have been confined to oxic surface waters with 1 oceanic profile extending down to 5000 m (Hara et al. 1996). In oceanic profiles, virioplankton concentrations correlate with distributions of the most abundant hosts (bacteria and phytoplankton) and their productivity, generally decreasing dramatically through the photic zone, and attaining ...

The Cariaco Basin, off the northeast coast of Venezuela, has long been the center of attention of scientists trying to explain paleoclimate. This peculiar anoxic basin records climate change over several dozen millennia within layers of sediment [Black eta., 1999]. A joint U.S.‐Venezuelan research effort launched in 1995—the Carbon Retention in a Colored Ocean (CARIACO) Program— provides a link between the sediment record and processes near the surface of the ocean for this basin. Sediment traps maintained by the program show that over 5% of the organic carbon contained in particles formed near the surface through primary production (photosynthesis) by phytoplankton reaches 275 m depth, and nearly 2% reaches 1,400 m.This flux is significant, because it represents a sink for carbon dioxide, which is a greenhouse gas, and because it helps explain the record of ancient climate stored at the bottom of the Cariaco Basin.