How Bacteria Battle on Water Planets express
Simple equations that describe fox and rabbit cycles have been turned to bacteria on distant exoplanets, like K2-18 b. These worlds hold deep oceans under thick hydrogen skies. Like duckweed on a pond, light-eating microbes cover the sunlit water surface, stealing light from those below.
Researchers simulated an ocean column with five bacterial species. Without viruses, phototrophs monopolize resources — much like an algal bloom turning a lake green. But on planets locked in endless day (one side always facing the star), there are no day-night cycles. Populations stay stable, never surging, though smaller overall.
And here’s the twist: these bacteria might feed not only on light but also on the hydrogen enveloping the planet, a far more abundant fuel.
Original abstract
as written by the authors arXiv:2603.22491
New observations are opening the possibility of characterising habitable environments in exoplanetary systems, with the recent example of the candidate hycean world K2-18 b. This motivates an exploration of the possible ecological conditions on such planets to better interpret biosignatures as well as understand the nature of potential life. On Earth, the Lotka-Volterra equations have been used to model numerous coupled populations within ecosystems, from interactions between large vertebrates, to systems with multiple microbial species. In this work, we apply the Lotka-Volterra equations to the ecology of habitable exoplanets for the first time, focusing on hycean worlds. We simulate scenarios in a vertical water column with between 1-5 bacterial species that thrive in anoxic environments on Earth, i.e. similar to predicted hycean conditions. We find that a wide range of ecological diversity is possible for microbial populations under hycean conditions. We demonstrate that dominating phototrophic bacteria at the top of a water column out-compete deeper dwelling phototrophic bacteria, analogous to bacterial blooms on Earth. Incorporating microbial viruses (bacteriophages) within our models can cause ecosystem collapse depending on the time of their introduction, and such phage inclusion can be beneficial to ecological diversity. Finally, our work shows that bacterial populations inhabiting tidally locked exoplanets may be more stable due to constant illumination of the ocean, but can have lower peak population densities in such cases when compared to seasonal scenarios. Our work provides an initial step towards understanding the possible ecological diversity on habitable worlds beyond Earth.
Some earthly bacteria thrive on hydrogen, anticipating the atmosphere of distant ocean planets.
In Stanisław Lem’s ‘Solaris’, a sentient planetary ocean defies comprehension, a literary echo of these microbial rivalries.