Biogeochemistry of an Iron‐ and Manganese‐Rich Stratified Lake: Tasik Biru, Malaysia, as a Modern Model Habitat for the Ancient Ocean

Tasik Biru is a ~70 m‐deep tropical lake in Malaysia, originating from a water‐filled open pit mine. We investigated the biogeochemistry and microbial community of the lake as a modern model habitat to the stratified ancient ocean. We found that a sharp redoxcline exists at around 50 m depth, related to the decrease of O 2 and pH (7.2–6.8) going down into the monimolimnion. Despite being relatively sulfate‐rich (~320 μM), only a slight decrease of sulfate (to ~240 μM) was observed coupled with an increase of dissolved sulfide to 4 μM, attributed to microbial sulfate reduction in the monimolimnion. Comparatively, dissolved Fe and total Mn rose to ~50 μM in the anoxic layer with an unusual 1:1 concentration ratio. Other nutrients (PO 4 3− , Si) and trace metal(loid)s (As, Mo, Sb, Co, U, and V) depth profiles increased or decreased across the chemocline, indicating controls via cycling of redox‐sensitive elements. Microbial community analysis based on 16S rRNA amplicon sequencing reflects various metabolisms, from aerobic metabolisms in the mixolimnion to putative nitrite‐dependent methane oxidation (e.g., by Methylomirabilis ) at the chemocline, to sulfate reduction, methanogenesis, and fermentation in the monimolimnion. Tasik Biru is not in steady‐state, and its anoxic water is predicted to shift from being Fe/Mn‐rich to sulfide‐rich, perhaps lending it as a model habitat to investigate biogeochemical changes from the metal‐rich Archean to the Proterozoic oceans with expanding zones of sulfide‐rich margins. An overview of the current biogeochemical cycles in the lake is presented, and open questions regarding partial sulfate consumption, methane, and Mn cycling and mineralogical distribution are highlighted to guide future studies.