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Twenty seven thermophilic hydrogen oxidizing cultures (pure
cultures or co-cultures) were obtained from various hot springs in
Grensdalur, Hveragerði in SW-Iceland. The cultures were investigated in
detail with respect to phylogenetics, physiology, hydrogen uptake rates and
Enrichments under hydrogen oxidation conditions showed that 10%
oxygen gave best results for obtaining stable pure- or co-cultures.
Phylogenetic studies of the enrichments were done with partial- and
full 16S rRNA analysis and DGGE analysis. Four main groups of hydrogen
oxidizing bacteria were detected in the enrichment cultures obtained. The
closest similarities were to species within the genera of Hydrogenophilus,
Thiomonas (moderate thermophiles), Hydrogenobacter and
Sulfurihydrogenibium (true thermophiles). Three enrichments revealed
bacteria close to Hydrogenophilus denitrificans and H. thermoluteolus.
Three enrichments resulted in bacteria closely related to several species of
Thiomonas. Fourteen enrichments gave closest relationship to strict
chemolithotrophs belonging to Hydrogenobacter and five enrichments
showed the closest relationship to Sulfurihydrogenibium sp. Phylogenetic
studies also showed that heterotrophic bacteria were very persistent. Among
contaminants were Meiothermus, Anoxybacillus and Geobbacillus sp.
Batch experiments on all enrichments showed great variation in
hydrogen oxidation rate and biomass yield. The hydrogen oxidation rate
varied from 0,16 to 1,64 mol H2L-1h-1 and the biomass yield from 0,21 – 3,18
g DCW per mol H2 consumed. In general, the moderate thermophilic
enrichments oxidized hydrogen faster as compared with the true
thermophiles and produced more biomass.
Cultivation in fed batch cultures of selected enrichments resulted in
higher hydrogen uptake rates and biomass formation compared with batch
cultures. The maximum hydrogen uptake rate and biomass yield for
Ralstonia eutropha was 15,05 H2L-1h-1 and 4,9 g DCW per mol H2 utilized.
Similar values for the moderate thermophilic enrichments (containing
Hydrogenophilus and Thiomonas) 22 and 31 mmol H2L-1h-1; 4,5 and 3,4 g
DWC per mol H2 utilized, respectively. The true thermophilic enrichments
gave less biomass but similar hydrogen uptake rates as compared to the
moderate thermophilic enrichments and the mesophilic control.
Finally, four enrichment cultures were tested for oxidation of various
sulphur compounds. Two thermophilic enrichments could utilize thiosulphate
and produced sulphur or sulphuric acid. Enrichment containing Thiomonas
sp. could oxidize thiosulphate to sulphuric acid and enrichment containing
Hydrogenophilus sp. utilized sulphur.
Keywords: Thermophilic; Hydrogen oxidizing; Aerobic; Biomass yield;
Hydrogen uptake; Bacteria; Phylogeny
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