Ecological modelling of hycean worlds

Abstract

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.

Figures (11)

• Figure 1: A schematic diagram of the hycean Lotka-Volterra model we implement in this work. The schematic shows how various parameters in our model are connected in a vertical, one-dimensional water column. Input parameters include water temperature and light. The water temperature (orange-blue oval) depends on Eq. \ref{['Temperature profile equation']}. The incident light (orange oval) can be modified by clouds (grey diamond) to reduce the relative intensity of surface light, and then the light varies with depth due to attenuation by absorption and scattering in water. Light and temperature set the growth rate for the active population, and light also influences the rate of conversion from the inactive species to the active species. The active bacteria population (green dashed rectangle), the inactive bacteria population (blue dashed rectangle), and the phage population (purple dashed rectangle) depend on various parameters which vary with depth in the water column. Growth processes for bacteria are in dotted rectangles, while death rates are in elongated hexagons (active and inactive species in green and blue, respectively, and phages in pink). Both growth and death rates act to modify the population densities. Arrows show the direction of influence, with double-sided arrows showing feedback between the parameters/processes. The species names are given by grey boxes at the top right of the schematic, with GSB standing for Green Sulfur Bacteria. At the top left of the schematic, other variables which influence our model are shown, and these variables are either turned on or off between sets of simulations, or modified.
• Figure 2: Jerlov water types, from open ocean (I -- III) to coastal waters (1C -- 9C). The figure shows how the downwelling diffuse attenuation coefficient, $k_d$ (m-1) varies with wavelength, for the 10 Jerlov water types jerlov1951opticaljerlov1968opticaljerlov1976marine. The lowest $k_d$ occurs in the visible wavelength region for all water types, with UV wavelengths and the reddest wavelengths having higher values of $k_d$. The figure is reproduced from williamson2023depth, Limnology and Oceanography Letters, under the Creative Commons Attribution (CC BY 4.0) License.
• Figure 3: Spectral enegery distributions of GKM dwarf stars. The top of atmosphere irradiance is shown for several stars, all scaled to the flux that the Earth receives (1360 W m-2), plotted against the wavelength of light in nm. The stars shown are the Sun (G2V), HD 40307 (K2V), GJ 176 (M2.5V), GJ 551 (M5.5V; also known as Proxima Centauri), and TRAPPIST-1 (M8.5V). Their stellar spectral type and peak wavelengths are given in nm. Whilst hycean candidates are only known to exist around M dwarfs, there is no reason to expect why they cannot exist around other spectral types, including G dwarfs. The UV regions (100--400 nm) is shaded in grey.
• Figure 4: Light and temperature vertical profiles in the one-dimensional water column. Left: The attenuation coefficients ($k_d$ in m-1) used to determine the relative light intensity which reaches different depths in the ocean column. Depth is defined as distance below the surface which is at 0 m depth. Middle and Right: Temperature profiles with various surface temperatures ($T_\text{sfc}$), deep ocean temperatures ($T_\infty$), and temperature decay constants ($\kappa_T$), with a mixed layer depth ($d_\text{ML}$) of 15 m. Right: Temperature profiles with various $T_\text{sfc}$, $T_\infty$, and $\kappa_T$, with a mixed layer depth ($d_\text{ML}$) of 50 m.
• Figure 5: Biological population growth rates per day (d) of six different bacterial species. Five of these species are psychrophilic sulphate-reducing bacteria from Arctic sediments data, and the data was sourced from knoblauch1999effect. The species shown are ASv26 (black upward triangles), LSv21 (red stars), PSv29 (purple downward triangles), LSv54 (blue crosses), and LSv514 (turquoise pentagons). Green Sulfur Bacteria grown on different electron donors, sulfide (orange squares) and thiosulfate (grey circles), with the data sourced from overmann1989pelodictyon, are also displayed. The fits to the data use either the Ratkowsky or Square root models for bacteria growth.