Colourless sulfur bacteria and their role in the sulfur cycle |
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Authors: | J Gijs Kuenen |
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Institution: | (1) Laboratory of Microbiology, University of Groningen, Biological Centre, Haren (Gr.), The Netherlands |
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Abstract: | Summary The bacteria belonging to the families of the Thiobacteriaceae, Beggiatoaceae and Achromatiaceae are commonly called the colourless
sulfur bacteria. While their ability to oxidize reduced inorganic sulfur compounds has clearly been established, it is still
not known whether all these organisms can derive metabolically useful energy from these oxidations.
During the last decades research has mainly focussed on the genus Thiobacillus. Bacteria belonging to this genus can oxidize
a variety of reduced inorganic sulfur compounds and detailed information is available on the biochemistry and physiology of
these energy-yielding reactions. The thiobacilli, most of which can synthesize all cell material from CO2, possess a well-regulated metabolic machinery with high biosynthetic capacities, which is essentially similar to that of
other procaryotic organisms.
Although the qualitative role of colourless sulfur bacteria in the sulfur cycle is well documented, quantitative data are
virtually absent. Activities of colourless sulfur bacteria in nature must be related to direct and indirect parameters, such
as: the rate of oxidation of (S35) sulfur compounds, the rate of C14O2-fixation, the rate of acid production and numbers and growth rates of the bacteria. However, chemical reactions and similar
activities of heterotrophic organisms mask the activities of the colourless sulfur bacteria to various extents, depending
on the condition of the natural environment. This interference is minimal in regions where high temperature and/or low pH
allow the development of a dominant population of colourless sulfur bacteria, such as hot acid sulfur springs, sulfide ores,
sulfur deposits and some acid soils.
The oxidation of inorganic sulfur compounds is carried out by a spectrum of sulfur-oxidizing organisms which includes: 1)
obligately chemolithotrophic organisms 2) mixotrophs 3) chemolithotrophic heterotrophs 4) heterotrophs which do not gain energy
from the oxidation of sulfur compounds but benefit in other ways from this reaction, and 5) heterotrophs which do not benefit
from the oxidation of sulfur compounds. The spectrum is completed by a hypothetical group of heterotrophic organisms, which
may have a symbiotic relationship with thiobacilli and related bacteria. Such heterotrophs may stimulate the growth of colourless
sulfur bacteria and thereby contribute to the oxidation of sulfur compounds.
Future research should focus in the first place on obtaining and studying pure cultures of many of the colourless sulfur bacteria.
In the second place, studies on the physiological and ecological aspects of mixed cultures of colourless sulfur bacteria and
heterotrophs may add to a better understanding of the role of the colourless sulfur bacteria in the sulfur cycle.
Paper read at the Symposium on the Sulphur Cycle, Wageningen, May 1974. |
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