Anaerobic oxidation of methane in sediments of a nitrate-rich, oligo-mesotrophic boreal lake

Abstract

The importance of nitrate in promoting anaerobic oxidation of methane (AOM) in the sediments of boreal lakes is currently unknown. Here we investigated the extent to which sediment AOM is linked to nitrate reduction in a nitrate-rich, oligo-mesotrophic, boreal lake (Lake Paajarvi, Finland). AOM potential of sediment slurries, collected from three profundal stations of the study lake, was measured at varying nitrate concentrations using 13C-labelling. This was coupled with analysis of vertical profiles of the sediment and porewater geochemistry and the microbial communities (16S rRNA gene and shotgun metagenomic sequencing). Sediment AOM potential was enhanced by nitrate at a shallow station with high contents of labile phytoplankton-derived organic matter (Station 1), but not at the other two stations. AOM was also much higher at Station 1 (2.0-6.8 nmol C cm-3 d-1) than at the other stations (0-0.3 nmol C cm-3 d-1). Accordingly, methanotrophic archaea (Candidatus Methanoperedens sp.) and bacteria (Methylococcales) had the highest relative abundance at Station 1. Furthermore, geochemical profiles indicated that AOM was potentially coupled with reduction of nitrate, iron, or sulfate at all stations. We conclude that AOM linked to nitrate reduction takes place in boreal lake sediments. However, our data cannot resolve whether nitrate affects the process directly, via enhancing nitrate reduction-mediated AOM, or indirectly, via enhancing AOM mediated by reduction of other electron acceptors.

Environmental predictors of electroactive bacterioplankton in small boreal lakes

“…Methylococcales and Methylophilaceae oxiEET genes were typically Cyc2 and/or the MHC MtoA. MtoA was nearly always accompanied by the outer membrane porin MtrB—which is not distinguished from MtoB in FEET (Garber et al ., 2020), consistent with Methylococcales observed in Rissanen et al (2021). Because MtoA, though likely used for oxiEET, has been observed to operate in redEET (Liu et al ., 2012), one might suspect they can couple redEET to methane oxidation like in Yang et al (2020).…”

“…Only eight out of 5569 MAGs analyzed here were predicted to encode machinery for methanogenesis, among only 21 archaeal MAGs that passed the quality cutoffs. This, along with dissolved methane measurements in similar systems (Rissanen et al, 2021;van Grinsven et al, 2021), suggests that the majority of methanogenesis is occurring in sediments rather than the water column. The observed correlation between iron and methane, both of which are negatively correlated with dissolved oxygen (DO) (Fig 2B ), is unsurprising because methanogenesis occurs typically when available iron and other electron acceptors are in the reduced state and Fe(II) is more soluble than Fe(III).…”

Enigmatic persistence of aerobic methanotrophs in oxygen-limiting freshwater habitats

“…Rissanen et al . [ 79 ] detected stimulation of CH 4 oxidation by nitrate in boreal lake sediment containing multiple methanotrophs (namely gammaproteobacterial MOB, methanotrophic archaea (“Ca. Methanoperedens”), and “Ca.…”