Thermomicrobia is a class of heterotrophic, obligate aerobes within the phylum Chloroflexi. At the time of writing, it contained four distinct species, Thermomicrobium roseum, Sphaerobacter thermophilus, Nitrolancetus hollandicus and WKT50.2, the novel strain described in this study. The physiology and ecology of this strain, and the description of the class Thermomicrobia was investigated as part of this study.
Strain WKT50.2 was isolated from the hot springs at Waikite, New Zealand. We investigated temperature and pH range and optima, analysed PLFA, and determined preferred carbon and energy sources, N-fixation, modes of growth and antibiotic sensitivity. As a general phenotype, WKT50.2 is a heterotrophic and thermophilic obligate aerobe with a preference for moderately alkaliphilic pH conditions. The possible ecology of WKT50.2 was discussed.
WKT50.2 was able to oxidise methane to gain energy. This was an unexpected result as methanotrophy is a highly specialised phenotype restricted to the phyla Proteobacteria and Verrucomicrobia. Methanotrophs also tend to be metabolic specialists with no capability to oxidise substrates other than C1 compounds. In contrast, WKT50.2 was also able to oxidise ammonia and nitrite, along with a range of saccharides and protein rich substrates. WKT50.2 was sensitive to most of the antibiotics tested in this study, but metronidazole and trimethoprim both stimulated growth in a dosage-dependent manner. WKT50.2 is clearly a distinct genus from T. roseum and S. thermophilus, based on 16S rRNA gene sequence similarities (91 % and 88 % respectively), G+C content and metabolic capabilities.
T. roseum and S. thermophilus were confirmed as heterotrophic, thermophilic, moderately alkaliphilic, obligate aerobes. The temperature and pH optima and ranges for T. roseum and S. thermophilus were amended, along with expanding the known metabolic capabilities of both species. Of particular note was the discovery that both T. roseum and S. thermophilus were also able to oxidise both methane and ammonia for energy, and that both species possessed cellulolytic activity.
In summary, the class Thermomicrobia can be redefined as comprising obligately aerobic, facultative heterotrophs. Cells are non-motile, non-spore forming rods. This study has shown that the cells have proteinaceous cell walls containing ornithine, and no lipopolysaccharide. The major fatty acid is 12-Me-18:0, and lipids include ester-linked diols. The major isoprenoid quinone is MK-8.
Several of the observations in this study merit further research. The discovery of methanotrophy in this class was unexpected and significant, and future work should include the identification of the enzymes involved in methanotrophy, and those involved in the oxidation of ammonia for energy. The mechanism for growth stimulation of WKT50.2 and T. roseum by antibiotics was also an interesting observation and warrants further analysis to determine the mode of action.
