AUTHOR=Hetz Stefanie A. , Horn Marcus A. 

TITLE=Burkholderiaceae Are Key Acetate Assimilators During Complete Denitrification in Acidic Cryoturbated Peat Circles of the Arctic Tundra

JOURNAL=Frontiers in Microbiology

VOLUME=12

YEAR=2021

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2021.628269

DOI=10.3389/fmicb.2021.628269

ISSN=1664-302X

ABSTRACT=<p>Cryoturbated peat circles (pH 4) in the Eastern European Tundra harbor up to 2 mM pore water nitrate and emit the greenhouse gas N<sub>2</sub>O like heavily fertilized agricultural soils in temperate regions. The main process yielding N<sub>2</sub>O under oxygen limited conditions is denitrification, which is the sequential reduction of nitrate/nitrite to N<sub>2</sub>O and/or N<sub>2</sub>. N<sub>2</sub>O reduction to N<sub>2</sub> is impaired by pH &lt; 6 in classical model denitrifiers and many environments. Key microbes of peat circles are important but largely unknown catalysts for <italic>C</italic>- and <italic>N</italic>-cycling associated N<sub>2</sub>O fluxes. Thus, we hypothesized that the peat circle community includes hitherto unknown taxa and is essentially unable to efficiently perform complete denitrification, i.e., reduce N<sub>2</sub>O, due to a low <italic>in situ</italic> pH. 16S rRNA analysis indicated a diverse active community primarily composed of the bacterial class-level taxa Alphaproteobacteria, Acidimicrobiia, Acidobacteria, Verrucomicrobiae, and Bacteroidia, as well as archaeal Nitrososphaeria. Euryarchaeota were not detected. <sup>13</sup>C<sub>2</sub>- and <sup>12</sup>C<sub>2</sub>-acetate supplemented anoxic microcosms with endogenous nitrate and acetylene at an <italic>in situ</italic> near pH of 4 were used to assess acetate dependent carbon flow, denitrification and N<sub>2</sub>O production. Initial nitrate and acetate were consumed within 6 and 11 days, respectively, and primarily converted to CO<sub>2</sub> and N<sub>2</sub>, suggesting complete acetate fueled denitrification at acidic pH. Stable isotope probing coupled to 16S rRNA analysis via Illumina MiSeq amplicon sequencing identified acetate consuming key players of the family <italic>Burkholderiaceae</italic> during complete denitrification correlating with <italic>Rhodanobacter</italic> spp. The archaeal community consisted primarily of ammonia-oxidizing Archaea of Nitrososphaeraceae, and was stable during the incubation. The collective data indicate that peat circles (i) host acid-tolerant denitrifiers capable of complete denitrification at pH 4–5.5, (ii) other parameters like carbon availability rather than pH are possible reasons for high N<sub>2</sub>O emissions <italic>in situ</italic>, and (iii) <italic>Burkholderiaceae</italic> are responsive key acetate assimilators co-occurring with <italic>Rhodanobacter</italic> sp. during denitrification, suggesting both organisms being associated with acid-tolerant denitrification in peat circles.</p>