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

Abstract

Cryoturbated peat circles (pH 4) in the Eastern European Tundra harbor up to 2 mM pore water nitrate and emit the greenhouse gas N2O like heavily fertilized agricultural soils in temperate regions. The main process yielding N2O under oxygen limited conditions is denitrification, which is the sequential reduction of nitrate/nitrite to N2O and/or N2. N2O reduction to N2 is impaired by pH < 6 in classical model denitrifiers and many environments. Key microbes of peat circles are important but largely unknown catalysts for C- and N-cycling associated N2O 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 N2O, due to a low in situ 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. 13C2- and 12C2-acetate supplemented anoxic microcosms with endogenous nitrate and acetylene at an in situ near pH of 4 were used to assess acetate dependent carbon flow, denitrification and N2O production. Initial nitrate and acetate were consumed within 6 and 11 days, respectively, and primarily converted to CO2 and N2, 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 Burkholderiaceae during complete denitrification correlating with Rhodanobacter 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 N2O emissions in situ, and (iii) Burkholderiaceae are responsive key acetate assimilators co-occurring with Rhodanobacter sp. during denitrification, suggesting both organisms being associated with acid-tolerant denitrification in peat circles.

Emissions of atmospherically reactive gases nitrous acid and nitric oxide from Arctic permafrost peatlands

“…Although the consumption of atmospheric N 2 O usually occurs in water-logged soils (Hallin et al 2018), this process has also been found in dry soils under oxic conditions (Wu et al 2013, Siljanen et al 2020), including high-arctic soils (Brummell et al 2014, Wagner et al 2019. Furthermore, the nosZ gene encoding the N 2 O reductase enzyme, which carries out N 2 O reduction to N 2 , has been detected in the surface layer of uplifted permafrost peatlands, similar to those studied here (Palmer andHorn 2012, Hetz andHorn 2021). Usually, N 2 O reduction occurs when the energetically more favorable electron acceptor NO 3 − is not available.…”

Glycerol-driven denitratation: process kinetics, microbial ecology, and operational controls

“…4b). Other studies identified both taxa as widely present in heterotrophic denitrification systems [53][54][55] with certain members able to express complete denitrification pathways using myriad carbon sources. 54,56,57 Their combined increase in relative abundance as influent COD : NO 3 − -N approached the theoretical requirement for complete denitrification (5.9 : 1; see ESI †) coincided with increased fully-denitrifying performance (Fig.…”

Sources of nitrous oxide and fate of mineral nitrogen in sub-Arctic permafrost peat soils

“…Few, highly specialized taxa, mostly belonging to the family of Burkholderiaceae (co-occurring with Rhodanobacter sp. ), seem to be responsible for most of the denitrification occurring in these acidic soils using acetate as their energy source (Hetz & Horn, 2021). Despite the clear dominance of denitrification, the relative contribution of total 25 nitrification to the N2O emissions from the BP surfaces (~20%) was still significant and could be particularly important during drier summers (low soil water content) and at the end of the growing season when the N2O emissions are generally lower, as shown here and also in Gil et al (2017).…”

“…This corroborates that moisture is a key N2O emission driver for the prevalence 30 of nitrification or denitrification to occur. Nitrifier denitrification can be, however, ruled out as a possible source of N2O in these soils since we know now that in Seida peat ammonia oxidizing archaea (AOA) are responsible for ammonia oxidation and ammonia oxidizing bacteria (AOB) are lacking there (Siljanen et al, 2019;Hetz & Horn, 2021). AOA are not capable for denitrification (anaerobic) in contrast to AOB.…”