Respiratory arsenate reduction

Table 1 Novel Bacterial and Archaeal Isolates That Can Grow by Respiratory Arsenate Reduction...

Chrysiogenes arsenatis is the only known organism capable of using acetate as the electron donor and arsenate as the terminal electron acceptor for growth. This reduction of arsenate to arsenite is catalyzed by an inducible respiratory arsenate reductase, which has been isolated and characterized by Kraft and Macy (1998). Arsenate reductase (Arr) from C. arsenatis is a... 

FIGURE 72.2. Arsenic detoxification mechanisms (reduction, oxidation, methylation, and resistance) in prokaryotes. (A) Respiratory arsenate reductase (Arr) is involved in the reduction of As(V) by the dissimilatory arsenate respiring organisms. (B) Arsenite oxidase (Aox/Aso) is responsible for oxidation of As(III) by chemoautotrophic or heterotrophic arsenite oxidizers. 

Desorption on Iron and Arsenic Reduction The effects of biological (dissimilatory) Fe(III) and As(V) reduction on As transport under dynamic flow conditions in an Fe (hydr)oxide-dominated system illustrate the complexity of reactions influencing arsenic dynamics within soils. We examined the impact of a model freshwater anaerobic bacterium, Sulfurosprillum harnesii strain SES-3, that reduces both Fe(III) and As(V) through respiratory processes (Oremland et al., 1994 Zobrist et al., 2000), on the desorption of arsenic from ferrihydrite-coated sands (Herbel and Fendorf, 2005, 2006). As noted above for As(III)-loaded... 

Figure 12 Vertical depth profiles of arsenate with rates of respiratory arsenate and sulfate reduction in the water column of meromictic Mono Lake, California, made during October, 1999. Sulfate-reduction profiles from the last period of meromixis (1986) when the lake was 4 m shallower are shown for comparison. (From Ref. 57.)...

Preliminary biochemical studies of the enzyme that catalyzes arsenate reduction in Sulfurospirillum barnesii have been conducted and the information only cited in two review articles (12,13). This enzyme is an integral membrane protein with a calculated mass of 100 kDa consisting of three different subunits 65, 31, and 22 kDa. The equivalent enzyme from Desulfomicrobium sp. str. Ben-RB (9) is discussed in Section III.D. The only respiratory arsenate reductase that has been studied in detail is that of C. arsenatis (14), discussed in Section II.D. 

This chapter concentrates on arsenate respiration by Chrysiogenes arsenatis and Desulfomicrobium sp. str. Ben-RB. The evidence indicates that they have specific respiratory arsenate reductases involved in energy generation. The isolation, phytogeny, physiology, and biochemistry of arsenate reduction are described separately for each organism. 

Anaerobic metabolism occnrs nnder conditions in which the diffusion rate is insufficient to meet the microbial demand, and alternative electron acceptors are needed. The type of anaerobic microbial reaction controls the redox potential (Eh), the denitrification process, reduction of Mu and SO , and the transformation of selenium and arsenate. Keeney (1983) emphasized that denitrification is the most significant anaerobic reaction occurring in the subsurface. Denitrification may be defined as the process in which N-oxides serve as terminal electron acceptors for respiratory electron transport (Firestone 1982), because nitrification and NOj" reduction to produce gaseous N-oxides. hi this case, a reduced electron-donating substrate enhances the formation of more N-oxides through numerous elechocarriers. Anaerobic conditions also lead to the transformation of organic toxic compounds (e.g., DDT) in many cases, these transformations are more rapid than under aerobic conditions. 

Microbial arsenate respiration contributes to the mobilization of arsenic from the solid to the soluble phase in various locales worldwide. In particular, microbial respiratory reduction of arsenate, As(V), to arsenite, As(III), is thought... 

Desorption on Metal Reduction Many bacteria and archaea can respire on Mn(lll/tV) and Fe(lll) oxides, leading to their dissolution, with the potential for concomitant displacement of arsenic into the aqueous phase (Cummings et al., 1999). In fact, within most soils and sediments, total As levels correlate with Fe content rather than Al or clay content (Smedley and Kinniburgh, 2002), and thus reductive dissolution—transformation of Fe(lll) phases should have a major impact on arsenic. Respiratory reduction of Fe in sediments generally occurs in zones where O2, NOs , and Mn(lV) [all being oxidants of Fe(ll) and alternative electron acceptors] are diminished (Lovley, 2000). 

Figure 1 Bacterial reduction of arsenate and oxidation of arsenite. (A). Cytoplasmic arsenate reductase (ArsC) as encoded by bacterial ars operons along with chromosomaUy encoded Pit and Pst phosphate transport systems with arsenate as an alternative substrate. After reduction from arsenate to arsenite, arsenite is removed from the cell by the ArsB membrane protein. (B). Anaerobic periplasmic arsenate reductase. (C). Aerobic periplas-mic arsenite oxidase, hnked via azurin to the respiratory chain.

Figure 8 Schematic representation of three possible microbial mechanisms for mobilization of arsenic oxyanions adsorbed to ferrihydrite surfaces by respiratory reduction. Bottom left Shewanella alga reduces Fe(lll) to Fe(It), thereby releasing As(V) into solution (41). Lower right bacterially reduced electron shuttle molecules pass electrons to solid-phase As(V) and Fe(III) (48). Top Sulfiirospirillum bamesii directly reduces both As(V) and Fe(III) (43).

These experiments pointed out that respiratory reduction of As(V) sorbed to solid phases can indeed occur in nature, but its extent and the degree of mobilization of the As(III) product is constrained by the type of minerals present in a given system. What remains unclear is whether micro-organisms can actually reduce As(V) while it is attached to the mineral surface, or if they attack a mono-layer of aqueous As(V) that is in equilibrium with the As(V) adsorbed onto the surface layer. If, as is the case for dissimilatory metal-reducing bacteria such as Geobacter sulfurreducens and Shewanella oneidensis (44,45), components of the electron transport chain are localized to the outer-membrane of some arsenate-respiring bacteria, direct reductive dissolution of insoluble arsenate minerals may be possible by attached bacteria. Too little is known at present about the topology... 

The reduction of arsenate [As(V)] to arsenite [As(III)] is known to occur in anoxic environments (1,2). Until recently, however, the organisms responsible for this reduction were not known. A number of different bacteria have been isolated that are able to respire with arsenate, reducing it to arsenite. With one exception, these organisms use the nonrespiratory substrate lactate as the electron donor (3-6) and are listed in Table 1. Two of them, Desulfotomaculum auripig-mentum str. OREX-4 (7,8) md Desulfomicrobium sp. str. Ben-RB (9), also respire with sulfate as the terminal electron acceptor. None are able to use the respiratory substrate acetate as the electron donor for arsenate respiration. The only organism known able to do so is Chrysiogenes arsenatis (10). 

Chapter 11 covers the exciting discovery of respiratory reduction of arsenate by prokaryotes, including seven new and highly diverse species of Eubact-eria and one new and one previously isolated species of Cremoarchae. Detailed biochemical and genetic characterization of the enzymes involved in these organ-... 

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