Electron acceptors arsenate

A facultative autotroph (lithotroph) strain MLHE-1 was able to oxidize arsenite under anaerobic conditions to arsenate using nitrate as electron acceptor (Oremland et al. 2002). 

Laverman AM, JS Blum, JK Schaefer, EJP Phillips, DR Lovley, RS Oremland (1995) Growth of strain SES-3 with arsenate and other diverse electron acceptors. Appl Environ Microbiol 61 3556-3561. 

Arsenite is also an intermediate in the fungal biomethylation of arsenic (Bentley and Chasteen 2002) and oxidation to the less toxic arsenate can be accomplished by heterotrophic bacteria including Alcaligenes faecalis. Exceptionally, arsenite can serve as electron donor for chemolithotrophic growth of an organism designated NT-26 (Santini et al. 2000), and both selenate and arsenate can be involved in dissimilation reactions as alternative electron acceptors. 

The chlorate reductase has been characterized in strain GR-1 where it was found in the periplasm, is oxygen-sensitive, and contains molybdenum, and both [3Fe-4S] and [4Fe-4S] clusters (Kengen et al. 1999). The arsenate reductase from Chrysiogenes arsenatis contains Mo, Fe, and acid-labile S (Krafft and Macy 1998), and the reductase from Thauera selenatis that is specific for selenate, is located in the periplasmic space, and contains Mo, Fe, acid-labile S, and cytochrome b (Schroeder et al. 1997). In contrast, the membrane-bound selenate reductase from Enterobacter cloacae SLDla-1 that cannot function as an electron acceptor under anaerobic conditions contains Mo and Fe and is distinct from nitrate reductase (Ridley et al. 2006). 

Blum et al. (1998) isolated a bacterial strain Bacillus arsenicoselenatis from muds of Mono Lake, ahypersaline alkaline lake in northern California (see Section 24.2). Under anaerobic conditions in saline water, over an optimum pH range of 8.5-10, the strain can respire using As(V), or arsenate, as the electron acceptor, reducing it to As(III), arsenite. 

Fig. 33.1. Results of a batch experiment (symbols) by Blum et al. (1998) in which Bacillus arsenicoselenatis grows on lactate, using arsenate [As(V)] as an electron acceptor. Solid lines show results of integrating a kinetic rate model describing microbial respiration and growth.

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... 

The recently isolated Desulfotomaculum strain Ben-RB is able to grow using lactate as a substrate and arsenate as the sole electron acceptor (Macy et al. 2000). It has been proposed that arsenate reductase is associated with the respiratory chain of this organism, because >98% of the arsenate reductase bound to the plasma membrane. 

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. 

POLYACETYLENE. A linear polymer of acetylene having alternate single and double bonds, developed in 1978. It is electrically conductive, but this property can be varied in either direction by appropriate doping either with electron acceptors (arsenic pentaflnoride or a halogen) or with electron donors (lithium, sodium). Thus, it can be made to have a wide range of conductivity from insulators to n- or >-type semiconductors to strongly conductive forms, Polyacetylene can be made in both cis and trans modifications in the form of fibers and thin films, the conductivity... 

Following consumption of dissolved O2, the thermodynamically favored electron acceptor is nitrate (N03-). Nitrate reduction can be coupled to anaerobic oxidation of metal sulfides (Appelo and Postma, 1999), which may include arsenic-rich phases. The release of sorbed arsenic may also be coupled to the reduction of Mn(IV) (oxy)(hydr)oxides, such as birnessite CS-MnCb) (Scott and Morgan, 1995). The electrostatic bond between the sorbed arsenic and the host mineral is dramatically weakened by an overall decrease of net positive charge so that surface-complexed arsenic could dissolve. However, arsenic liberated by these redox reactions may reprecipitate as a mixed As(III)-Mn(II) solid phase (Toumassat et al., 2002) or resorb as surface complexes by iron (oxy)(hydr)oxides (McArthur et al., 2004). The most widespread arsenic occurrence in natural waters probably results from reduction of iron (oxy)(hydr)oxides under anoxic conditions, which are commonly associated with rapid sediment accumulation and burial (Smedley and Kinniburgh, 2002). In anoxic alluvial aquifers, iron is commonly the dominant redox-sensitive solute with concentrations as high as 30 mg L-1 (Smedley and Kinniburgh, 2002). However, the reduction of As(V) to As(III) may lag behind Fe(III) reduction (Islam et al., 2004). 

Redox reactions in soils are affected by a number of parameters, including temperature, pH (see Chapter 7), and microbes. Microbes catalyze many redox reactions in soils and use a variety of compounds as electron acceptors or electron donors. For example, aerobic heterotrophic soil bacteria may metabolize readily available organic carbon using NO3, NOj, N20, Mn-oxides, Fe-oxides and compounds such as arsenate (As04 ) and selenate (Se04 ) as electron acceptors. Similarly, microbes may use reduced compounds or ions as electron donors, for example, NH4, Mn2+, Fe2+, arsenite (AsCXj), and selenite (SeO ). 

The greatest likelihood for As release in soils and sediments typically occurs upon transition from oxidizing to reducing conditions. Under saturated conditions, the rapid consumption of O2 by aerobic microbes combined with the low solubility of O2 induces anaerobic bacteria to utilize alternative electron acceptors. Arsenic may be displaced either through reduction of arsenate to arsenite or through mineralogical transformations (inclusive of dissolution) of the soil matrix. 

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). 

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