Cryptic sulfur cycling sustains lithotrophy in the deep biosphere of ferruginous Lake Towuti
Lake Towuti, Sulawesi, Indonesia is an ancient tectonic lake, exhibiting iron-rich, sulfate-poor anoxic deep waters, conditions under which sediments accumulated under variable redox fluctuations. Such ferruginous settings make Lake Towuti an ideal study site to evaluate how a cryptic sulfur cycle could possibly operate under a scarcity of sulfate and abundance of iron minerals, alike Earth's primitive oceans. Here, we integrate downcore profiles for pore water geochemistry, reactive iron mineralogy, and bulk sediment elemental composition with microbial cell counts, sulfate reduction rates, 16S rRNA genes and metagenomes to resolve microbial sulfur transformations down to 15 m below lake floor (mblf). Scarce sulfate was rapidly consumed below the sediment-water interface, sulfate reduction rates dropped consequently within the upper mblf, while pore water ferrous iron increased to its highest concentration at 3 mblf. Any sulfide produced microbially precipitated as reduced inorganic sulfur in the sediment, apparently forming authigenic millerite (NiS). Cell densities decreased sharply with depth, equally tracking pore water depletion in electron acceptors. From 3 to 10 mblf, low but sustained sulfate reduction rates were observed with intermittent presence of nitrate in pore water and increased goethite in the sediment, both potential oxidants of sulfur intermediates. A subsequent re-increase in pore water sulfate occurred in parallel with syntrophic fermentation, which we inferred from dynamic pore water profiles of volatile fatty acids. The taxonomic diversity of microbial populations directly reflected this geochemical evolution as it shifted from a bacterial assemblage near the surface to selective Bathyarchaeia clearly prevailing down to 15 mblf. The corresponding metagenome-assembled genomes predicted metabolic potential for complete sulfate reduction (aprAB, dsrAB) in Thermodesulfovibrionia, whereas Desulfobacterota (incl. Geobacterales, Desulfuromonadales, Syntrophales) and Aminicenantia exhibited versatility in reducing iron, nitrate (narG, napA), nitrite (nirS, nrfA) and sulfate (dsrAB, asrA). By contrast, Bathyarchaeia were predicted to disproportionate sulfur to polysulfides and reduce ferredoxin via electron bifurcation (hyd I-II, sudA, dsrC, dsrE) to fuel a Wood-Ljungdahl pathway, thereby defining homoacetogenesis as the terminal electron sink in the deep ferruginous subsurface. Together, these mineralogical, geochemical, and metagenomic features provided evidence for a spatially confined but active cryptic sulfur cycle tightly coupling reduction of mineral ferric iron and intermittent pore water nitrate to syntrophic and lithotrophic (homo)acetogenesis.