Helarchaeota and Co-occurring Sulfate-Reducing Bacteria in Subseafloor Sediments from the Costa Rica Margin

Abstract

Deep sediments host many archaeal lineages, including those of the Asgard superphylum that may depend on/require syntrophic partnerships. Our knowledge about sedimentary archaeal diversity and their metabolic pathways and syntrophic partners is still very limited. We present here new genomes of Helarchaeota and co-occurring sulfate-reducing bacteria (SRB) recovered from organic-rich sediments off Costa Rica Margin. Our molecular analyses revealed three new metagenome-assembled genomes (MAGs) affiliating with Helarchaeota, each of which has three variants of the methyl-CoM reductase-like (MCR-like) complex that may enable them to oxidize short-chain alkanes anaerobically. These Helarchaeota have no multi-heme cytochromes (MHCs) but have Group 3b and Group 3c [NiFe] hydrogenases, and formate dehydrogenase, and therefore could transfer the reducing equivalents generated from alkane oxidation to external partners via the transfer of these substances. We also recovered five MAGs of SRB affiliated with the class of Desulfobacteria, two of which showed relative abundances (represented by genome coverages) positively correlated with those of the three Helarchaeota. Genome analysis suggested that these SRB bacteria have the capacity of H2 and formate utilizations and may facilitate electron transfers from other organisms by means of these reduced substances. Our findings suggest that Helarchaeota may metabolize synergistically with SRB in marine anoxic sediments, and exert an important influence on the carbon cycle by mitigating the hydrocarbon emission from sediments to the overlying ocean.