Physiological and phylogentic studies of thermophilic hydrogen oxidizing bacteria from Icelandic hot-springs

Abstract

Twenty seven thermophilic hydrogen oxidizing cultures (purecultures or co-cultures) were obtained from various hot springs inGrensdalur, Hveragerði in SW-Iceland. The cultures were investigated indetail with respect to phylogenetics, physiology, hydrogen uptake rates andbiomass yield.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- andfull 16S rRNA analysis and DGGE analysis. Four main groups of hydrogenoxidizing bacteria were detected in the enrichment cultures obtained. Theclosest similarities were to species within the genera of Hydrogenophilus,Thiomonas (moderate thermophiles), Hydrogenobacter andSulfurihydrogenibium (true thermophiles). Three enrichments revealedbacteria close to Hydrogenophilus denitrificans and H. thermoluteolus.Three enrichments resulted in bacteria closely related to several species ofThiomonas. Fourteen enrichments gave closest relationship to strictchemolithotrophs belonging to Hydrogenobacter and five enrichmentsshowed the closest relationship to Sulfurihydrogenibium sp. Phylogeneticstudies also showed that heterotrophic bacteria were very persistent. Amongcontaminants were Meiothermus, Anoxybacillus and Geobbacillus sp.Batch experiments on all enrichments showed great variation inhydrogen oxidation rate and biomass yield. The hydrogen oxidation ratevaried from 0,16 to 1,64 mol H2L-1h-1 and the biomass yield from 0,21 – 3,18g DCW per mol H2 consumed. In general, the moderate thermophilicenrichments oxidized hydrogen faster as compared with the truethermophiles and produced more biomass.Cultivation in fed batch cultures of selected enrichments resulted inhigher hydrogen uptake rates and biomass formation compared with batchcultures. The maximum hydrogen uptake rate and biomass yield forRalstonia eutropha was 15,05 H2L-1h-1 and 4,9 g DCW per mol H2 utilized.Similar values for the moderate thermophilic enrichments (containingviiiHydrogenophilus and Thiomonas) 22 and 31 mmol H2L-1h-1; 4,5 and 3,4 gDWC per mol H2 utilized, respectively. The true thermophilic enrichmentsgave less biomass but similar hydrogen uptake rates as compared to themoderate thermophilic enrichments and the mesophilic control.Finally, four enrichment cultures were tested for oxidation of varioussulphur compounds. Two thermophilic enrichments could utilize thiosulphateand produced sulphur or sulphuric acid. Enrichment containing Thiomonassp. could oxidize thiosulphate to sulphuric acid and enrichment containingHydrogenophilus sp. utilized sulphur.Keywords: Thermophilic; Hydrogen oxidizing; Aerobic; Biomass yield;Hydrogen uptake; Bacteria; Phylogeny