Microbial communities from Arctic marine sediments respond slowly to methane addition during ex situ enrichments

Abstract

Anaerobic methanotrophic archaea (ANME) consume methane in marine sediments, limiting its release to the water column, but their responses to changes in methane and sulfate supplies remain poorly constrained. To address how methane exposure may affect microbial communities and methane-and sulfur-cycling gene abundances in Arctic marine sediments, we collected sediments from offshore Svalbard that represent geochemical horizons where anaerobic methanotrophy is expected to be active, previously active, and long-inactive based on reaction-transport biogeochemical modelling of porewater sulfate profiles. Sediment slurries were incubated at in situ temperature and pressure with different added methane concentrations. Sediments from an active area of seepage began to reduce sulfate in a methane-dependent manner within months, preceding increased relative abundances of anaerobic methanotrophs ANME-1 within communities. In previously active and long-inactive sediments, sulfurcycling Deltaproteobacteria became more dominant after 30 days, though these communities showed no evidence of methanotrophy after nearly 8 months of enrichment. Overall, enrichment conditions, but not methane, broadly altered microbial community structure across different enrichment times and sediment types. These results suggest that active ANME populations may require years to develop, and consequently microbial community composition may affect methanotrophic responses to potential largescale seafloor methane releases in ways that provide insight for future modelling studies.

Methane transport and sources in an Arctic deep-water cold seep offshore NW Svalbard (Vestnesa Ridge, 79°N)

“…With our model we show that at site PC017 it requires a fast-moving aqueous fluid with dissolved methane to explain the observed pore water profiles. Given the constant δ 13 C-DIC profiles we suggest that cores PC007, PC009, and PC008 (Yao et al, 2019) (2020) documented the presence of gas hydrate as shallow as 45 cmbsf from Vestnesa Ridge, and also at Storfjordrenna south of Svalbard, gas hydrate was observed at 35 cmbsf (Klasek et al, 2020). The shallow SMT at our investigated sites, but the absence of gas hydrate in all of our cores seems to support the relatively lower concentration of dissolved methane derived by our model.…”