Publications

4368

Abstract We investigated microbial methane oxidation in the water column of two connected but hydrodynamically contrasting basins of Lake Lugano, Switzerland. Both basins accumulate large amounts of methane in the water column below their chemoclines, but methane oxidation efficiently prevents methane from reaching surface waters. Here we show that in the meromictic North Basin water column, a substantial fraction of methane was eliminated through anaerobic methane oxidation (AOM) coupled to nitrite reduction by Candidatus Methylomirabilis. Incubations with 14CH4 and concentrated biomass from this basin showed enhanced AOM rates with nitrate (+62%) and nitrite (+43%). In the more dynamic South Basin, however, aerobic methanotrophs prevailed, Ca. Methylomirabilis was absent in the anoxic water column, and no evidence was found for nitrite-dependent AOM. Here, the duration of seasonal stratification and anoxia seems to be too short, relative to the slow growth rate of Ca. Methylomirabilis, to allow for the establishment of anaerobic methanotrophs, in spite of favorable hydrochemical conditions. Using 16 S rRNA gene sequence data covering nearly ten years of community dynamics, we show that Ca. Methylomirabilis was a permanent element of the pelagic methane filter in the North Basin, which proliferated during periods of stable water column conditions and became the dominant methanotroph in the system. Conversely, more dynamic water column conditions led to a decline of Ca. Methylomirabilis and induced blooms of the faster-growing aerobic methanotrophs Methylobacter and Crenothrix. Our data highlight that physical (mixing) processes and ecosystem stability are key drivers controlling the community composition of aerobic and anaerobic methanotrophs.

Abstract The ammonia monooxygenase (AMO) is a key enzyme in ammonia‐oxidizing archaea, which are abundant and ubiquitous in soil environments. The AMO belongs to the copper‐containing membrane monooxygenase (CuMMO) enzyme superfamily, which also contains particulate methane monooxygenase (pMMO). Enzymes in the CuMMO superfamily are promiscuous, which results in co‐oxidation of alternative substrates. The phylogenetic and structural similarity between the pMMO and the archaeal AMO is well‐established, but there is surprisingly little information on the influence of methane and methanol on the archaeal AMO and terrestrial nitrification. The aim of this study was to examine the effects of methane and methanol on the soil ammonia‐oxidizing archaeon ‘ Candidatus Nitrosocosmicus franklandus C13’. We demonstrate that both methane and methanol are competitive inhibitors of the archaeal AMO. The inhibition constants ( K i ) for methane and methanol were 2.2 and 20 μM, respectively, concentrations which are environmentally relevant and orders of magnitude lower than those previously reported for ammonia‐oxidizing bacteria. Furthermore, we demonstrate that a specific suite of proteins is upregulated and downregulated in ‘ Ca. Nitrosocosmicus franklandus C13’ in the presence of methane or methanol, which provides a foundation for future studies into metabolism of one‐carbon (C1) compounds in ammonia‐oxidizing archaea.

The discovery of the new halophilic archaeon, Halorutilus salinus , representing a novel order, family, genus, and species within the class Halobacteria and phylum Euryarchaeota clearly enables insights into the microbial dark matter, expanding the current taxonomical knowledge of this group of archaea. The in-depth comparative genomic analysis performed on this new taxon revealed one of the first known examples of an Halobacteria representative coding the archaeal RuBisCO gene and with a streamlined genome, being ecologically successful in nature and explaining its previous non-isolation.