The response of deep-sea vent microorganisms to mercury (Hg) was investigated through measurements of total Hg (THg) concentrations in hydrothermal fluids from diffuse and focused flow vents on the East Pacific Rise at 9uN and the estimations of the proportion of Hg-resistant chemosynthetic thiosulfate-oxidizing microbes in a representative subset of diffuse flow fluids. Fluids were enriched in THg, with concentrations ranging from 15 to 445 pmol L 21 and 3.5 6 0.1 to 11.0 6 0.8 nmol L 21 in diffuse and focused flow emissions, respectively. In diffuse flow samples, most probable numbers (MPN) indicated that 0.25-24.6% of the total chemosynthetic thiosulfateoxidizing bacteria were resistant to Hg. The highest resistance levels were observed in samples with the highest THg concentrations, indicating that adaptation to life with toxic Hg had occurred in the diffuse flow environment. High THg concentrations in vent emissions and adaptation to Hg among chemosynthetic vent microbes indicate that (1) Hg in vent emissions may contribute to the oceanic Hg mass balance, and (2) activities of chemosynthetic microbes may mobilize solid-phase Hg in metal sulfide and contribute to Hg detoxification in deep-sea diffuse flow vents. Thus, the activities of chemosynthetic microbes may be critical to the mobility, geochemical cycling, and toxicity of Hg in the vent ecosystem.Metal resistance is common among microorganisms and is a critical factor modulating the effects of metals in the environment (Nies 1999). Although the evolutionary origin of microbial metal resistance is unknown, geothermal environments, such as deep-sea hydrothermal vents, where heavy metal concentrations may be comparable to those in contaminated sites, could represent ecological niches in which metal-microbe interactions occurred early during microbial evolution. In these environments, metals often form complexes with reduced sulfur compounds (Dopson et al. 2003). Deep-sea hydrothermal vents are associated with spreading centers along mid-oceanic ridge systems, with hydrothermal fluids occurring as either low-temperature diffuse flows (in which cold, oxygenated seawater 1 Corresponding author (barkay@aesop.rutgers.edu). AcknowledgmentsWe thank the crew of R/V Atlantis and the crew and pilots of the deep-submergence vehicle Alvin for their skillful operations at sea. Gratitude is extended to Eileen Ekstrom for the communication of unpublished data. The contribution of two anonymous reviewers, whose comments on a previous version of the manuscript have greatly improved this paper, is gratefully acknowledged.

The concentrations of Ag, Cd, Cu, Hg(II), monomethylmercury (MeHg), Pb, and Zn were measured and modeled in calanoid copepods collected from the Hudson River plume in April 2005 and May 2006. Copepods in the buoyant plume were significantly (p , 0.05) enriched in Ag, Cu, Pb, Zn, and MeHg (2-to 25-fold), but depleted in Cd (by 25-40%) relative to oceanic copepods. Median burdens of Ag, Cu, and Hg(II) differed in plume copepods collected in April 2005 and May 2006 by factors of 1.5 to 3, reflecting the influence of river discharge and associated physical processes on plankton dynamics and metal bioaccumulation. Only the concentrations of Cu and Zn in plume copepods clearly showed the expected trend of highest levels near the mouth of the Hudson River and lower levels offshore. Spatial trends in concentrations of Ag and Hg(II) in plume copepods were strikingly different from those in suspended particles and the dissolved phase, apparently reflecting differences in metal bioavailability because of shifts in available food types. Modeled concentrations of Cd, Cu, Pb, and Zn in plume copepods were within less than a factor of two of measured values, but model results showed that Hg(II) was less bioavailable, and Ag and MeHg were more bioavailable in the Hudson River plume than laboratory results would predict. Zooplankton from urban river plumes may accumulate potentially toxic levels of trace metals or act as a trophic link between metal contamination in polluted estuaries and pelagic ecosystems.

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