Abstract
Lake Tanganyika (LT) is the largest tropical freshwater lake, and the largest body of anoxic freshwater on Earth’s surface. LT’s mixed oxygenated surface waters float atop a permanently anoxic layer and host rich animal biodiversity. However, little is known about microorganisms inhabiting LT’s 1470 meter deep water column and their contributions to nutrient cycling, which affect ecosystem-level function and productivity. Here, we applied genome-resolved metagenomics and environmental analyses to link specific taxa to key biogeochemical processes across a vertical depth gradient in LT. We reconstructed 523 unique metagenome-assembled genomes (MAGs) from 34 bacterial and archaeal phyla, including many rarely observed in freshwater lakes. We identified sharp contrasts in community composition and metabolic potential with an abundance of typical freshwater taxa in oxygenated mixed upper layers, and Archaea and uncultured Candidate Phyla in deep anoxic waters. Genomic capacity for nitrogen and sulfur cycling was abundant in MAGs recovered from anoxic waters, highlighting microbial contributions to the productive surface layers via recycling of upwelled nutrients, and greenhouse gases such as nitrous oxide. Overall, our study provides a blueprint for incorporation of aquatic microbial genomics in the representation of tropical freshwater lakes, especially in the context of ongoing climate change, which is predicted to bring increased stratification and anoxia to freshwater lakes.
Highlights
Located in the East African Rift Valley, Lake Tanganyika (LT) holds 16% of the Earth’s freshwater and is the secondlargest lake by volume
Our study provides genomic evidence for the capabilities of Bacteria and Archaea in tropical freshwater biogeochemical cycling and describes links between spatial distribution of organisms and biogeochemical processes
These processes are known to impact critical food webs that are renowned for their high biodiversity and that serve as important protein sources for local human populations, as the shoreline of LT is shared by four African countries
Summary
Located in the East African Rift Valley, Lake Tanganyika (LT) holds 16% of the Earth’s freshwater and is the secondlargest lake by volume. The microbial communities in LT that drive much of the ecosystem-scale productivity remains largely unknown. LT is over ten million years old and oligotrophic, which provides a unique ecosystem to study microbial diversity and function in freshwater lakes, tropical lakes. The comparatively thin, oxygenated surface layer of this ancient, deep lake harbors some of the most spectacular fish species diversity on Earth [4], but surprisingly ~80% of the 1890 km of water is anoxic. This causes a large volume of anoxic and nutrient-rich bottom waters to be thermally isolated from the upper ~70 m of well-lit, nutrient-depleted surface waters. Periodically in response to sustained winds, pulses of phosphorus and nitrogen upwelling from deep waters replenish the oxygenated surface layers and sustain its productivity [5]
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