Aerobic carboxydobacteria

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Abstract
Bacterial consumption of CO, a major atmospheric pollutant, has been described in the literature long ago, but often with controversial data. Attempts were made, therefore, to isolate and study a broader spectrum of aerobic CO bacteria in order to better understand their ecology. About 100 samples from various locations in the USSR (water, mud, junkyard, or garden, soil etc.) were inoculated into mineral medium and incubated under 80% CO and 20% O2. Such enrichments yielded a large number of strains representing 5–6 different bacterial species. The highest number of CO utilizers came from fresh samples of polluted, moist soil or mud. Some mixed cultures were composed of vitamin-dependent bacteria living together in a state of protocooperation. Seven strains (previously already partially described) were studied in greater detail. They are regarded as “carboxydobacteria.” Six carboxydobacteria also utilized hydrogen, but none of the 7 cultures grew with methane or formate. The optimal concentration of, and sensitivity to, CO varied with the strains. Reducedvs. oxidized spectra revealed in all cultures the presence ofb-, c-, anda-type cytochromes, indicating a functioning electron transport chain when grown under CO. Isotope experiments with14C indicated that CO was assimilated only after oxidation to CO2, the pathway being the normal Calvin cycle reaction sequence as observed in other chemolithotrophs. The key enzymes of this sequence were shown to be present and active. Organotrophic growth of carboxydobacteria was obtained with a number of compounds such as organic acids, primary alcohols, carbohydrates, and amino acids. A description of properties of the 7 test strains is given. Their ecological niche(s) seem to be where facultatively anaerobic bacteria produce organic intermediates as well as H2 and CO. Pellicle formation indicates their presence on the water surface (neuston layer). Relationships with hydrogen-oxidizing bacteria are discussed and explained on ecological grounds.

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