Abstract
In nature, nitrification has been revealed across a wide temperature range of 4°C – 84°C. Whereas thermophilic ammonia-oxidizing archaea are known to perform the first step of nitrification, the identity of heat-tolerant nitrite oxidizers is still a challenging issue in microbial ecology. In a long-term cultivation approach, we inoculated mineral media containing ammonium and nitrite as substrates with biofilms and sediments of two hot springs in Yellowstone National Park (YNP, USA), which were incubated at 68°C. The nitrifying consortia obtained consisted mostly of novel Chloroflexi as revealed by metagenomic sequencing. Among these, two deeply branching novel Chloroflexi species were identified as putative nitrite-oxidizing bacteria (NOB) by the finding of cytoplasmic and periplasmic NXR. Stoichiometric oxidation of nitrite to nitrate was stimulated by organic matter, but also occurred under lithoautotrophic conditions. The most abundant NOB candidate formed miniaturized cells and was heat-resistant, but only grew in co-culture with heterotrophic bacteria. One of the accompanying bacteria (Thermomicrobium sp.) also contained an NXR of the Nitrobacter-type, but no nitrite oxidation was observed. This detection of novel thermophilic NOB exemplifies our still incomplete knowledge of nitrification, and indicates that nitrite oxidation might be an ancient and wide-spread form or energy conservation.Yellowstone National Park, Permit: YELL-2007-SCI-5698
