Abstract
Dissimilatory sulfate reduction in peatlands is sustained by a cryptic sulfur cycle and effectively competes with methanogenic degradation pathways. In a series of peat soil microcosms incubated over 50 days, we identified bacterial consortia that responded to small, periodic additions of individual fermentation products (formate, acetate, propionate, lactate or butyrate) in the presence or absence of sulfate. Under sulfate supplementation, net sulfate turnover (ST) steadily increased to 16–174 nmol cm–3 per day and almost completely blocked methanogenesis. 16S rRNA gene and cDNA amplicon sequencing identified microorganisms whose increases in ribosome numbers strongly correlated to ST. Natively abundant (⩾0.1% estimated genome abundance) species-level operational taxonomic units (OTUs) showed no significant response to sulfate. In contrast, low-abundance OTUs responded significantly to sulfate in incubations with propionate, lactate and butyrate. These OTUs included members of recognized sulfate-reducing taxa (Desulfosporosinus, Desulfopila, Desulfomonile, Desulfovibrio) and also members of taxa that are either yet unknown sulfate reducers or metabolic interaction partners thereof. Most responsive OTUs markedly increased their ribosome content but only weakly increased in abundance. Responsive Desulfosporosinus OTUs even maintained a constantly low population size throughout 50 days, which suggests a novel strategy of rare biosphere members to display activity. Interestingly, two OTUs of the non-sulfate-reducing genus Telmatospirillum (Alphaproteobacteria) showed strongly contrasting preferences towards sulfate in butyrate-amended microcosms, corroborating that closely related microorganisms are not necessarily ecologically coherent. We show that diverse consortia of low-abundance microorganisms can perform peat soil sulfate reduction, a process that exerts control on methane production in these climate-relevant ecosystems.
Highlights
Peatlands currently store about one-third of all terrestrial carbon (Limpens et al, 2008), which is predicted to be partially released as the greenhouse gases CO2 and CH4 because of climate change (Hodgkins et al, 2014; Schuur et al, 2015)
Fermentation products differ in their impact on ST Anoxic peat microcosms were set up to reflect naturally occurring concentrations of substrates and generally did not exceed 100–200 μM (Supplementary Figure S2a)
Microcosms were periodically amended with small amounts of formate, acetate, propionate, lactate or butyrate—substrates that represent typical organic carbon degradation intermediates in peatlands (Schmalenberger et al, 2007; Küsel et al, 2008; Limpens et al, 2008)
Summary
Peatlands currently store about one-third of all terrestrial carbon (Limpens et al, 2008), which is predicted to be partially released as the greenhouse gases CO2 and CH4 because of climate change (Hodgkins et al, 2014; Schuur et al, 2015). Low-abundance OTUs responded to sulfate remained constant throughout all incubations as stimulation measured by qPCR (Supplementary Figure S6a).
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