  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
H2, CO and O2, 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 H2 and CO and produce potentially detectable levels of CH4.
O2-consuming metabolisms are also considered for TRAPPIST-1e, as abiotic O2
production is predicted on M-dwarf orbiting planets. We show that these
biospheres can lead to high levels of surface O2 (approximately 1-5 %) as a
result of \ch{CO} consumption, which could allow high O2 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. O2-consuming metabolisms could also lower O2 levels to around
10 ppm and support a productive biosphere at low reductant inputs. Using
predicted transmission spectral features from CH4, CO, O2/O3 and CO2 across the
hypothesis space for tectonic reductant input, we show that biotically-produced
CH4 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.
