S0 s2o2 s4o2 SO2

A wide variety of organisms are capable of oxidizing S in a wide variety of environments (Table 15.8). As with all soil organisms, the classification of these is strongly affected by developing molecular techniques. The names of some of these organisms may change and new ones may be added, but the basic physiology and enzymes utilized will still apply. These organisms can be divided into three groups: (1) photoautotrophs, including species of green and purple sulfur bacteria; (2) chemolithotrophs, such as members of the thiobacilli; and (3) heterotrophs, including a wide range of bacteria and fungi. While the first two occur generally in extreme environments such as hot sulfur springs, the last groups are largely responsible for oxidizing S0 in aerobic soils (Germida and Janzen, 1993).

Hydrogen sulfide oxidation has been observed in many unicellular and filamentous cyanobacteria (Garlick et al., 1977). The filamentous cyanobacterium Oscilla-toria limnetica can perform anaerobic photosynthesis and uses HS~ as the electron acceptor. The ecological significance of phototrophic HS~ oxidation is evident in the flexibility of cyanobacteria to adjust to changing environmental conditions of aerobiosis and anaerobic phototrophy in HS-rich environments. Depending on the environmental conditions present, the resulting reductant is transferred to: (1) CO2 to yield cell material, (2) protons to yield H2 in the absence of CO2, or (3) N2 to yield ammonia when combined N is absent. The green sulfur and purple sulfur bacteria represent a diverse morphological group, including cocci, vibrio, rods, spirals, and budding and gliding organisms. Both groups are commonly found in mud and stagnant waters containing H2S and exposed to light. They are also found under extreme conditions of high salinity or temperature and occur in S hot springs and saline lakes as a colored layer under salt deposits. The green S bacteria are strictly anaerobic and obligately phototrophic. They are able to use HS~ and S0 as photosynthetic electron donors, which are oxidized to SO4~. Thiosulfate (S2O|~) is also oxidized by the green S bacteria Chlorobaculum thio-sulfatophilum (formerly Chlorobium limicola subsp. thiosulfatophilum). These organisms have external S granules in the presence of HS~ and a homogeneous 16S rRNA sequence only distantly related to the purple S bacteria. The purple S bacteria are Proteobacteria ranging in color from bluish violet through purple, deep red, and orange because of the various carotenoid pigments that dominate the bacteriochlorophylls in color. These bacteria have a distinctive 16S rRNA composition that is shared by the nonphototrophic S oxidizers such as Beggiatoa and Thiobacillus. The phototrophic Proteobacteria include the Chromatiaceae, Ectothiorhodospiraceae, and Rhodospirillaceae families. The Chromatiaceae genera Thiospirillum and Thiocapsa store elemental S internally in the presence of HS~. Allochromatium vinosum (formerly Chromatium vinosum) is a facultative

TABLE 15.8 Characteristics of Some Sulfur-Oxidizing Archaea and Bacteria"



Inorganic substrates'


Temp. (°C)

Enzymes involved

Acidianus ambivalens

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