Metagenomic analysis of a meromictic humic lake revealed microbial community patterns structured by light, redox gradients and cryptic interactions

Humic lakes and ponds are net sources of greenhouse gases and hotspots in the global carbon cycle. Microbial communities in the darkly stained humic water, especially in the O₂-depleted compartments are under-studied. Here, we performed metagenomic analyses on samples from a meromictic humic lake collected at 12 depths ranging 0 to 17 m and recovered 157 metagenome-assembled genomes (MAGs). MAG abundance profiles revealed that cyanobacteria peaked within the first meter, together with aerobic photoheterotrophs that rely on bacteriochlorophylls or rhodopsins. Anaerobic anoxygenic phototrophic Chlorobium with genes for sulfur (S) and Fe(II) oxidation peaked below the oxycline at the 4 m depth. Coincidently, MAGs for Fe(III) reducers (Geothrix and Rhodoferax) and sulfate reducers (Desulfatirhabdiaceae) also peaked at 4 m, suggesting cryptic Fe and S cycles facilitated by the Chlorobium-Fe(III) reducer and Chlorobium-sulfate reducer couples, respectively. Similarly, an Fe cycling couple peaked at 7.5 m, including a Geobacteraceae Fe(III) reducer and Gallionella which can use oxidized nitrogen compounds to oxidize Fe(II). The most abundant methanotroph (Methylomonadaceae) capable of denitrification peaked at 7.5 m and largely sustained its abundance below 10 m, where methanogens (Methanoregula) started to emerge, suggesting cryptic methane cycling. In addition, putative extracellular electron transfer (EET) genes were frequently detected in MAGs from anoxic samples, suggesting the importance of EET in humic-rich anoxic water. Overall, our analyses revealed that humic water microbial community is structured by light, redox gradients and inter-species syntrophic interactions. Such interactions facilitate cryptic redox cycling and impact the overall carbon metabolism by recharging the electron accepting capability of humic water.

Primary Presenter: Shaomei He, University of Wisconsin-Madison (she@wisc.edu)

Authors:

Shaomei He, University of Wisconsin-Madison  (she@wisc.edu)

Katherine McMahon, University of Wisconsin-Madison  (trina.mcmahon@wisc.edu)