2024MNRAS.531..468E

In order to assess observational evidence for potential atmospheric biosignatures on exoplanets, it will be essential to test whether spectral fingerprints from multiple gases can be explained by abiotic or biotic-only processes. Here, we develop and apply a coupled 1D atmosphere-ocean-ecosystem model to understand how primitive biospheres, which exploit abiotic sources of H$_2$, CO, and O$_2$, could influence the atmospheric composition of rocky terrestrial exoplanets. We apply this to the Earth at 3.8 Ga and to TRAPPIST-1e. We focus on metabolisms that evolved before the evolution of oxygenic photosynthesis, which consume H$_2$ and CO and produce potentially detectable levels of CH$_4$. O$_2$-consuming metabolisms are also considered for TRAPPIST-1e, as abiotic O$_2$ production is predicted on M-dwarf orbiting planets. We show that these biospheres can lead to high levels of surface O$_2$ (approximately 1-5 per cent) as a result of CO consumption, which could allow high O$_2$ scenarios, by removing the main loss mechanisms of atomic oxygen. Increasing stratospheric temperatures, which increases atmospheric OH can reduce the likelihood of such a state forming. O$_2$-consuming metabolisms could also lower O$_2$ levels to around 10 ppm and support a productive biosphere at low reductant inputs. Using predicted transmission spectral features from CH$_4$, CO, O$_2$/O$_3$, and CO$_2$ across the hypothesis space for tectonic reductant input, we show that biotically produced CH$_4$ may only be detectable at high reductant inputs. CO is also likely to be a dominant feature in transmission spectra for planets orbiting M-dwarfs, which could reduce the confidence in any potential biosignature observations linked to these biospheres.