In addition to natural aeration, when exposed to light, the medium also receives O 2 from photosynthesis, subject to diurnal fluctuations. The riddle they pose is further complicated by the complexity of their natural habitat. Thus we are faced with a vicious circle: lack of knowledge of the physiology of these organisms impedes the design of effective isolation procedures, which, in turn, are the very requirement for obtaining such physiological knowledge through pure culture study. Knowledge of the factors determining such predominance would obviously be of great help in isolation studies, but so far we have no clue as to the determinants of competition and survival of these diverse organisms, which have to compete not only among one another, but very likely also against the chemical oxidation of H 2S. In nature, quite often the predominance of one or a few colorless sulfur bacteria can be observed. The morphological characteristics are the only basis for the division into different genera. Thus, for the moment, these six genera exist only by virtue of their morphological recognizability. The other six genera have not yielded pure cultures most probably because they are obligate sulfide-oxidizers, which, in view of the autoxidizability of H 2S, renders the use of solid media impossible and that of liquid media extremely cumbersome, since continuous inputs of H 2S and O 2 have to be maintained. Only the marine Beggiatoa strains seem to be facultative autotrophs ( Nelson, 1989). Thus the 11 genera discussed in this book represent only the most accessible part of the inhabitants of this ecological niche even so, only five of them exist at the moment in pure culture: Thiobacillus, Sulfolobus, and Thiomicrospira, which can be cultivated with the stable compounds thiosulfate and/or sulfur as oxidizable substrates, Macromonas, of which only heterotrophic strains exist in pure culture, and Beggiatoa, from marine and freshwater habitats. The works of the old masters, such as Gicklhorn, Hinze, Kolkwitz, Lauterborn, Molisch, Nadson, Warming, and Winogradsky, offer clear indications that several genera and even more species of colorless sulfur bacteria exist that cannot now be recognized usefully.
ACHROMA BACTERIA SYMBIOSIS MANUAL
Finally, there are an indefinite number of colorless sulfur bacteria that are not recognized in the eighth edition of Bergey’s Manual of Determinative Bacteriology ( Buchanan and Gibbons, 1974) or discussed in this Handbook because they have been described poorly or no more than once. Another argument for inclusion of this group is the observed absence of these genera in habitats devoid of H 2S. The nutritional status of these genera is by no means certain and their relationship to reduced-sulfur compounds may range from obligate chemolithotrophy to protective, detoxifying sulfide oxidation, or to merely gratuitous sulfide oxidation. These eight genera are included among the colorless sulfur bacteria because the observed appearance and disappearance of sulfur inclusions suggest the possession of at least the capacity to oxidize sulfide and sulfur.
Secondly, the group includes the five genera to be discussed here- Achromatium, Macromonas, Thiobacterium, Thiospira, and Thiovulum-as well as Beggiatoa, Thiothrix, and Thioploca (see 16 and 166). These colorless sulfur bacteria include in the first place Sulfolobus, Thiobacillus, and Thiomicrospira (see The Order Thermoproteales and The Genera Thiobacillus, Thiomicrospira and Thiosphaera both in the second edition), which have been shown to possess the capacity to chemolithotrophically oxidize reduced-sulfur compounds. The chemotrophic segment of the microbial population occupying this special niche includes-in addition to accidental “interlopers” such as H 2S-tolerant microaerophilic heterotrophs-a group of organisms called colorless sulfur bacteria, which appear to interact directly with the reduced-sulfur compounds characterizing this habitat. Thus, this habitat contains an array of reduced sulfur compounds that are all potential substrates for chemolithotrophic oxidation but that differ greatly in their stability in the presence of O 2. H 2S can be rapidly oxidized without the intervention of living organisms, the rate of oxidation depending on pH, temperature, and the presence of catalysts and/or inhibitors ( Chen and Morris, 1972a, 1972b) reaction products range from sulfur and polysulfides to thiosulfate, sulfite, and sulfate. In nature, the coexistence of H 2S and O 2 can only be sustained in systems subject to continuous inputs of both substances, because H 2S is not stable in the presence of O 2. Bars = 10 µm-except for the typical colonies of the Thiobacterium species: (a) “puffball” shape, (b) dendroid shape, Bars = 5 µ. Composite drawing of the described organisms, redrawn from the original literature.
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