Morphologically Conspicuous Sulfur-Oxidizing Eubacteria

Abstract

This chapter deals with the genera Achromatium, Macromonas, Thiobacterium, Thiospira, and Thiovulum. They all belong to the chemotrophic microbial population generally encountered in natural habitats characterized by the simultaneous presence of HzS and Oz, i.e., the border areas between aerobic and anaerobic zones in surface waters and the outflows of HzS-bearing springs. It will be useful to introduce this discussion with some general remarks on the special nature of this ecological niche to help explain the embarrassing paucity of knowledge we possess about its inhabitants. In nature, the coexistence of HzS and Oz can only be sustained in systems subject to continuous inputs of both, because HzS is not stable in the presence of Oz. It 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,b); reaction products range from sulfur and polysulfides to thiosulfate, sulfite, and sulfate. Thus, this habitat contains an array of reduced sulfur compounds which are all potential substrates for chemolithotrophic oxidation but which differ greatly in their stability in the presence of Oz. The chemotrophic segment of the microbial population occupying this special niche includes - in addition to accidental "interlopers" such as HzStolerant microaerophilic heterotrophs-a group of organisms called colorless sulfur bacteria, which appear to interact directly with the reduced sulfur compounds characterizing this habitat. These colorless sulfur bacteria include in the first place Thiobacillus, Sulfolobus, and Thiomicrospira (see Chapters 77 and 81) which have been shown to possess the capacity to chemolithotrophically oxidize reduced sulfur compounds. Secondly, the group includes the five genera to be discussed hereAchromatium, Macromonas, Thiobacterium, Thiospira, and Thiovulum -as well as Beggiatoa, Thiothrix, and Thioploca (see this Handbook, Chapter 22). 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. 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. Another argument for inclusion of this group is the observed absence of these genera in habitats devoid of HzS. Finally, there are an indefinite number of colorless sulfur bacteria that are not recognized in Bergey's Manual of Determinative Bacteriology, eighth edition (Buchanan and Gibbons, 1974) or discussed in this Handbook because they have been described poorly or no more than once. The works of the old masters like 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 usefully be recognized. Thus the 11 genera discussed in this book represent only the most accessible part of the inhabitants of this ecological niche; even so, only four 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, and Beggiatoa, of which only heterotrophic (or mixotrophic?) strains exist in pure culture. The other seven genera have not yielded pure cultures most probably because they are obligate sulfide-oxidizers, which, in view of the autoxidizability of HzS, renders the use of solid media impossible and that of liquid media extremely cumbersome, since continuous inputs of HzS and Oz have to be maintained. Thus, for the moment, these seven genera exist only by virtue of their morphological recognizability. The morphological characteristics are the only basis for the division into different genera. In nature, quite often the predominance of one or a few colorless sulfur bacteria can be observed. 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 HzS.