EAGER: Meta-gut Dynamics Influence Aquatic Ecosystem Processes

When animals defecate or die, they transfer a portion of their gut microbes to the environment, together with the organic matter, nutrients, and metabolic byproducts that comprise their feces or carcasses. Microbial decomposition of these inputs can change the characteristics of an ecosystem. In this EAGER project, the proposed research will test whether this coupled gut-environment system, the 'meta-gut', allows microbial communities from the gut of hippopotami to persist external to the animal and have a greater influence on ecosystem function.

Hippos are the focus of this study by this team that specialized in the ecosystem ecology of the Mara River in Kenya, which has thousands of hippos and is the only permanent source of water for wildlife in the world-famous Serengeti National Park. Although environmental microbial communities are traditionally considered important drivers of ecosystem function, demonstration that animal gut microbes can significantly influence ecosystem function directly would represent a paradigm shift in our understanding of the influence of animals on ecosystem processes.

This research will increase our understanding of the influence of organic matter loading in aquatic ecosystems, as well as microbial drivers of organic matter decomposition and nitrogen cycling under low oxygen conditions, which are increasingly common in some aquatic ecosystems. This project will train a postdoc and 2-3 undergraduate students in each year of the grant and seek to broaden the participation of underrepresented students in science.

Products from this research will be used to develop a module for zoos that have hippo facilities to educate the general public about the important role these animals play in structuring river ecosystems.

The proposed study will use an experimental approach based on observations from field research on hippos in the Mara River. Hippos aggregate in slow-moving pools along the river where they defecate large quantities of organic material that alter pool biogeochemistry. Previous research has shown that high subsidy pools, with large hippo inputs and low flushing rates, can become anoxic.

As a result, these pools develop microbial communities that closely resemble the hippo gut, suggesting hippo gut microbes may persist in the external environment. Researchers will conduct an experiment in replicated stream mesocosms using coupled, high-frequency sampling of microbial communities and biogeochemical responses to test whether hippo gut microbes can persist and function in the external environment, and whether they influence carbon and nutrient cycling.

Hippo feces from a captive animal facility will be added to recirculating streams across a gradient from fresh feces with active gut microbes to sterilized feces with no gut microbes. Presence and activity of microbes from the hippo gut will be measured using metagenomics, metatranscriptomics, and metabolomics, as the mesocosms transition from aerobic to anaerobic metabolism.

These measurements will be coupled with detailed measurements on aquatic biogeochemistry, carbon cycling, and nitrogen metabolism. A stochastic population model will simulate how microbes from the hippo gut survive and influence key ecosystem processes in the pools. In addition to training the postdoc and undergraduate students, findings from this research will be shared with resource managers and stakeholders in Kenya, broadening the understanding of the ecological role of hippos and their influences on water resources in the region.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.