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Abstract

The anaerobic oxidation of methane (AOM) mitigates the flux of methane
from marine sediments into the water column. AOM is performed by
anaerobic methanotrophic archaea (ANME) that reverse the methanogenesis
pathway and partner bacteria that utilize the released reducing
equivalents for sulfate reduction. Here, we investigated small-molecule
extracts from sediment-free thermophilic enrichment cultures of ANME-1
and sulfate-reducing bacteria using ultraperformance liquid
chromatography with high-resolution mass spectrometry. During the
analysis, we discovered a novel thioquinoxalinol-containing redox
molecule as a major component of the chemically derivatized
small-molecule pool. This compound contains both a redox active
quinoxaline heterocyclic ring and a thiol group. Additionally, the same
core structure was identified that contains a sulfate ester on the
hydroxyl group, which likely makes the molecule more water soluble.
Hydrated versions of both structures were also observed as major
compounds in the extracts. On the basis of reactions of model compounds
such as quinoxalin-6-ol, the hydrated version appears to be formed from
the addition of water to the dehydropyrazine ring followed by an
oxidation. These thioquinoxalinol compounds, which represent completely
new structures in biochemistry, may be involved in electron transport
processes within and/or between ANME-1 and sulfate-reducing bacteria,
may serve protective roles by reacting with toxic compounds such as
hydrogen sulfide, or may transport sulfate as a sulfate ester into the
sulfate-reducing bacteria.