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Sulfite:cytochrome c oxidoreductase

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"With permission from Friedrich (1998).

'FL, facultative lithotroph; FP, facultative phototroph; OL, obligate lithotroph; OP, obligate phototroph. CTS, thiosulfate; TT, tetrathionate.

photoautotroph, able to grow anaerobically in the light with HS-, SO2-, S2O2-, and H2, but not S4O6-. It is also capable of aerobic chemotrophic growth in the dark with S2O23- and CO2, provided the oxygen concentration is low. Sulfur oxidation by this organism proceeds in three steps: (1) oxidation of HS - or S2O3- to S0, which is deposited intracellularly; (2) oxidation of HS- or S0 to SO2-; and (3) oxidation of SO3- to the final product, SO4-. Members of the family Ectoth-iorhodospiraceae transiently store elemental S outside the cell and subsequently oxidize it to SO4- upon depletion of HS-. Some of the species in this family are marine, slightly halophilic curved gram-negative motile cells. Other species grow only in extremely salty alkaline conditions.

A wide, unrelated array of diverse chemolithitrophs have been grouped on the basis of their sulfur oxidation, including bacteria such as Thiobacillus, Beggiatoa, Thioploca, and Thiothrix and the archaeon Acidianus. Acidianus ambivalens (formerly Desulfurolobus ambivalens) exhibits the unique combination of dissimila-tory aerobic S oxidation and anaerobic S respiration in one strain. The organism is a strictly acidophilic, extreme thermophilic, and obligately chemolithotrophic archaeon able to obtain energy from aerobic S oxidation and also grows under anaerobic conditions by reducing SO4- with H2 as the electron donor. The thiobacilli represent the classic S-oxidizing organisms. The historical criterion for classifying organisms into the Thiobacillus genus was that all the species are rod-shaped eubacteria able to obtain energy for autotrophic growth by oxidizing inorganic S. Widespread application of 16S rRNA gene sequence analysis and DNA-DNA hybridization have provided tools for clarifying the taxonomy of this genus. Application of these tools has resulted in the reassignment of 14 species of thiobacilli into other genera (Kelly and Wood, 2000).

The majority of the thiobacilli are obligate aerobes, though some such as T. denitrificans can grow anaerobically by using nitrate as a terminal electron acceptor. Others use electron donors such as Fe2+ (Acidithiobacillus ferrooxidans) or NCS- (Thiobacillus thioparus), rather than S. The fact that they are facultative or obligate chemolithotrophs means they are able to oxidize S independent of the supply of available C. These bacteria are easily isolated from extreme environments such as hot, acid soils, S-polluted soils near sulfur piles, and soils subjected to high atmospheric deposition of S. Attempts to isolate these bacteria from agricultural soils have been more sporadic. The view that thiobacilli are the dominant players in S oxidation in soils is largely based on the observation that these bacteria are capable of much higher rates of S oxidation in culture than those achieved by heterotrophs growing under similar conditions. However, the isolation of these organisms from agricultural soils has not been very successful, and no consistent correlation between S oxidation rates and the presence of thiobacilli has been found except that these rates are generally low in soils that lack these organisms.

Sulfur oxidation is also mediated by a large number of heterotrophic soil microorganisms including bacteria such as Arthrobacter, Bacillus, Micrococcus, Mycobacterium, and Pseudomonas; some actinomycetes; and fungi such as Absidia, Alternaria, Fusarium, and Trichoderma. Lawrence and Germida (1991)

divided these organisms into those that oxidized S0 to produce primarily thiosul-fate, those that oxidized S0 to produce sulfate, and those capable of oxidizing thiosulfate to sulfate. The first group was found to be the most abundant population. No energy is gained by these organisms and the transformations appear to be incidental to the major metabolic pathways. The reason for the oxidation is, therefore, not known; but the possibility of protection against H2O2 has been suggested. It remains difficult to partition S oxidation between autotrophic and heterotrophic populations, and it is likely a mixed population that is responsible in most instances.

Reduction

Reduction of oxidized forms of S, particularly SO4-, by microorganisms occurs in two different ways. In the first, S is incorporated into cellular constituents such as the S in amino acids. This process is referred to as assimilatory sulfate reduction, or immobilization, as described above. In the other, the reduction leads to the formation of sulfide (e.g., H2S) as the end product. This is referred to as dissimilatory, or respiratory, sulfate reduction. This process is mediated by anaerobic, organotrophic organisms that use low-molecular-weight organic compounds or H2 as electron donors and the oxidized S compounds as terminal electron acceptors in a process similar to denitrification. These organisms are responsible for sulfide formation in waterlogged soils and sediments. Historically, the organisms involved in dissimilatory S or SO24- reduction were thought to represent a narrow physiological and ecological group that belonged to either Desulfovibrio or Desulfotomaculum. Sulfate reduction is now recognized in a number of bacterial genera (Castro et al., 2000; Table 15.9). Sulfate-reducing bacteria are found over an extensive range of pH and salt concentrations in saline lakes, evaporation beds, deep-sea sediments, and oil wells. The organisms can tolerate heavy metal and dissolved sulfide concentrations up to 2%. Because it is mediated mainly by anaerobic bacteria, sulfate reduction is not important in well-aerated soils, except in anaerobic microsites, but is a major component of the S cycle in periodically waterlogged or flooded soils such as rice paddies (Germida et al., 1992).

Two groups of sulfate-reducing bacteria have been recognized (Germida et al., 1992). The first group consists of bacteria that use organic C as an energy source, but do not completely oxidize it to CO2. This group includes species of Desulfovibrio and Desulfotomaculum, whose principle metabolic products are acetate and H2S, but some strains can also grow fermentatively on pyruvate in the absence of sulfate. The second group is more diverse and includes species of Desulfobacter, Desulfococcus, Desulfosarcina, and Desulfonema. These species can completely oxidize organic C to CO2 by using SO42- as the terminal electron acceptor.

Although sulfate-reducing bacteria are largely organotrophic, such that most of the C fixed is derived from organic matter, some organic molecules such as low-molecular-weight fatty acids (e.g., butyric, propionic, and acetic acids) are inhibitory

TABLE 15.9 Characteristics of Some Genera of Dissimilatory Sulfate-Reducing Bacteria"

Genus

Morphology

Growth temp. (°C) Substrate/comments

Gram-negative mesophilic Desulfobulbus Desulfomicrobium Desulfomonas

Desulfovibrio

Desulfobacter

Desulfococcus Desulfomonile Desulfonema Desulfosarcina

Gram-positive spore-forming Desulfotomaculum

Lemon to rod Ovoid to rod Rod

Oval to rod

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