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广州快乐10分开奖结果:Complete arsenic-based respiratory cycle in the marine microbial communities of pelagic oxygen-deficient zones
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Marine oxygen-deficient zones (ODZs) are naturally occurring midlayer oxygen-poor regions of the ocean, sandwiched between oxygenated surface and deep layers. In the absence of oxygen, microorganisms in ODZs use other compounds, such as oxidized forms of nitrogen and sulfur, as terminal electron acceptors. We identified the presence and expression of genes for both arsenic reduction and oxidation in marine ODZs, suggesting the microbial community in these waters is also cycling arsenic for respiratory gain. The existence of an arsenic respiratory cycle in pelagic waters suggests microbial arsenic metabolisms may be underestimated in the modern ocean and were likely an even more significant contributor to biogeochemical cycles in the anoxic ancient oceans when arsenic concentrations were higher.
Microbial capacity to metabolize arsenic is ancient, arising in response to its pervasive presence in the environment, which was largely in the form of As(III) in the early anoxic ocean. Many biological arsenic transformations are aimed at mitigating toxicity; however, some microorganisms can respire compounds of this redox-sensitive element to reap energetic gains. In several modern anoxic marine systems concentrations of As(V) are higher relative to As(III) than what would be expected from the thermodynamic equilibrium, but the mechanism for this discrepancy has remained unknown. Here we present evidence of a complete respiratory arsenic cycle, consisting of dissimilatory As(V) reduction and chemoautotrophic As(III) oxidation, in the pelagic ocean. We identified the presence of genes encoding both subunits of the respiratory arsenite oxidase AioA and the dissimilatory arsenate reductase ArrA in the Eastern Tropical North Pacific (ETNP) oxygen-deficient zone (ODZ). The presence of the dissimilatory arsenate reductase gene arrA was enriched on large particles (>30 um), similar to the forward bacterial dsrA gene of sulfate-reducing bacteria, which is involved in the cryptic cycling of sulfur in ODZs. Arsenic respiratory genes were expressed in metatranscriptomic libraries from the ETNP and the Eastern Tropical South Pacific (ETSP) ODZ, indicating arsenotrophy is a metabolic pathway actively utilized in anoxic marine water columns. Together these results suggest arsenic-based metabolisms support organic matter production and impact nitrogen biogeochemical cycling in modern oceans. In early anoxic oceans, especially during periods of high marine arsenic concentrations, they may have played a much larger role.
- ?1To whom correspondence may be addressed. Email: or .
?2Present address: Marine Chemistry & Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.
?3Present address: Horn Point Laboratory, University of Maryland, Cambridge, MD 21613.
Author contributions: J.K.S. and G.R. designed research; J.K.S., C.A.F., and C.M. performed research; J.K.S. analyzed data; and J.K.S. and G.R. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Data deposition: The sequence reported in this paper has been deposited in the GenBank database (GenBank Bio Project PRJNA350692).
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1818349116/-/DCSupplemental.
Published under the PNAS license.