Bioenergetic Modeling of Methanogens in Europa's Subsurface Ocean Environment

The ultimate goal for many is to find life elsewhere in the universe, whether it be in our own Solar System or further, but current technological, physical, and/or other limitations prevent a definitive answer. Furthermore, the surface and subsurface oceans on the icy moons of our Solar System as well as on exoplanets, such as Hycean worlds, are manifestly of great astrobiological interest. Modeling these environments and the growth of putative organisms within them can aid in this grand endeavor of understanding and identifying other habitable and inhabited worlds. Simulating the interactions of these organisms with each other and with the available environmental nutrients and substrates, as well as the accessible energy sources and sinks, is crucial for not only determining the habitability potential of such environments but also developing a theoretical framework for later use during comparisons with direct observation and data collection. To elaborate on this theme further, ascertaining putative properties of ecosystems from a bioenergetic standpoint is valuable for the following two reasons: (1) interpretation and analysis of data from future missions, such as Europa Clipper and JUICE, and (2) theoretical predictions of what to expect in these ecosystems, thus potentially aiding in selecting the design and functionality of future missions and instruments. In this study, modeling is achieved through use of the python code package NutMEG (Nutrients, Maintenance, Energy and Growth),  in conjunction with The Geochemist's Workbench (referred to as GWB), with the chief objective to simulate hydrogenotrophic methanogens in the ocean environment of Europa, which may be more acidic relative to Earth (among other properties). The initial theoretical composition of Europa's ocean was formed through a literature search of various other models and laboratory experiments. This composition was then used as an input for GWB, where the activities of CO2 and H2O were determined for a range of pH values from 4 to 7, in half-pH increments, and a temperature range of 0 to 200 degrees Celsius, in 10 degree increments. These activities, along with the theoretical composition of Europa's ocean and the chosen temperature and pH ranges, were then used as inputs to NutMEG where the metabolic and environmental chemical reactions were simulated to determine bioenergetic habitability of Europa's subsurface ocean. High and low salinity scenarios were also tested to determine the power supply available and whether the power available would meet various habitability criteria, including exponential growth of methanogens. The results presented show that the theoretically available maintenance power and specific combinations of lower ocean pH (roughly from 4 to 5.5) and higher temperature meet the criteria for methanogens to survive in a relatively habitable environment. Lower pH and higher temperatures also allow for a lower salinity environment to meet the same habitability criteria. This work will also be expanded to Hycean worlds (which are thought to host global oceans under a thick Hydrogen, and sometimes Helium, atmosphere) and potentially to the early Earth as well, specifically the Hadean-Archean Earth.