Novel microbial guilds implicated in N2O reduction

N₂O is a long-recognized greenhouse gas (GHG) with potential in global warming and ozone depletion. Terrestrial ecosystems are a major source of N₂O due to imbalanced N₂O production and consumption. Soil pH is a chief modulating factor controlling net N₂O emissions, and N₂O consumption has been considered negligible under acidic conditions (pH <6). In this dissertation, we obtained solids-free cultures reducing N₂O at pH 4.5. Furthermore, a co-culture (designated culture EV) comprising two interacting bacterial population was acquired via consecutive transfer in mineral salt medium. Integrated phenotypic, metagenomic and metabolomic analysis dictated that the Serratia population excreted certain amino acid to support Desulfosporosinus population growth. Characterization of co-culture EV demonstrated that organisms synthesizing functional NosZ exist in acidic soils. To further close the knowledge gap of low pH N₂O reduction, we conducted enrichment experiments on two contrasting soils representing natural and agricultural soils, respectively. With varying combination of carbon sources and H₂, N₂O reduction activity was observed in a total of six solids-free cultures at pH 4.5. Comparative growth experiments documented that N₂O was essential for N₂O-reducing organisms’ growth. Reconstruction of the communities suggested the cultures were highly enriched following consecutive transfer efforts. Surprisingly, N₂O-reducing organisms recovered from the cultures were all identified as clade II lineage or a novel clade lineage. These results together suggested that clade II and the putative novel clade N₂O-reducing organisms are closely associated with low pH N₂O reduction. The observation of low pH N₂O reduction in current research was inconsistent to field observation, where the abiotic-biotic interaction occurs frequently. To simulate potential abiotic-biotic interaction of low pH N₂O reduction, we tested the impact of geochemistry factors on N₂O reduction by culture EV. The impeded N₂O reduction of co-culture EV in the presence of trace amount (as low as 0.01 mM for nitrate and 0.001 mM for nitrite) of nitrogen oxyanions was unexpected, which may explain the weak N₂O reduction activity under field conditions. These observations indicated N₂O reduction in acidic soils may be limited by interacting abiotic factors, instead of pH itself.