Biogeochemical cycling in iron-rich Lake Matano, Indonesia: An early ocean analogue

Lake Matano of Indonesia, the 8th deepest lake on Earth, is an Fe-rich (ferruginous) end-member aquatic ecosystem and is the best known modern analogue for the ecology of Earth's earliest oceans. A culmination of geological, climatic and biological processes has resulted in an Fe-centric aquatic ecosystem in which anaerobic microbial communities are largely sustained by cycling Fe and CH4. Fe-rich soils of the catchment basin supply an abundance of Fe (hydr)oxides to Lake Matano's sediments. Limited vertical mixing in Lake Matano has generated a persistent, 100 m deep pycnocline which is sustained by minimal seasonal temperature fluctuations and steep basin morphology. This pycnocline separates an oxic surface layer from Fe(II) and CR4-rich bottom waters.;Lake Matano's physical structure, abundance of Fe and dearth of sulfate mirror the stratified ferruginous conditions thought to prevail in Earth's early oceans. The finding that anoxygenic phototrophs proliferate under ferruginous conditions supports models of early ocean ecology in which anoxygenic phototrophs dominate primary production. An active methane cycle in Lake Matano reveals that methanogenesis and methanotrophy could have been important components of the earth's early marine C cycle.;Ferruginous conditions in Lake Matano limit dissolved P concentrations in the oxic surface layer where they are apt to restrict primary production. Chlorobiaceae, which populate the deep ferruginous chemocline comprise an important component of Lake Matano's phototrophic community. Given the absence of sulfide, the metabolisms of these novel, deep-water, anoxygenic phototrophs are likely driven by Fe(II) oxidation. Methanogenesis dominates organic matter degradation despite Lake Matano's thermodynamic propensity for Fe reduction. Biogenic methane accumulates in the anoxic bottom water and supports the activity of aerobic and anaerobic methanotrophs. Cr concentrations in Lake Matano are relatively high and reflect the geology of the catchment basin. Removal of Cr from the surface waters is driven by the reduction of Cr(VI) by Fe(II) supplied from the anoxic bottom water. The ensuing downward Cr flux maintains dissolved Cr(VI) concentrations in the surface waters below international guidelines.