  The search for life beyond the Solar System is a major activity in exoplanet
science. However, even if an Earth-like planet were to be found, it is unlikely
to be at a similar stage of evolution as the modern Earth. It is therefore of
interest to investigate the sensitivity of biomarker signals for life as we
know it for an Earth-like planet but at earlier stages of evolution. Here, we
assess biomarkers i.e. species almost exclusively associated with life, in
present-day and in 10% present atmospheric level oxygen atmospheres
corresponding to the Earth's Proterozoic period. We investigate the impact of
proposed enhanced microbial emissions of the biomarker nitrous oxide, which
photolyses to form nitrogen oxides which can destroy the biomarker ozone. A
major result of our work is regardless of the microbial activity producing
nitrous oxide in the early anoxic ocean, a certain minimum ozone column can be
expected to persist in Proterozoic-type atmospheres due to a stabilising
feedback loop between ozone, nitrous oxide and the ultraviolet radiation field.
Atmospheric nitrous oxide columns were enhanced by a factor of 51 for the
Proterozoic "Canfield ocean" scenario with 100 times increased nitrous oxide
surface emissions. In such a scenario nitrous oxide displays prominent spectral
features, so may be more important as a biomarker than previously considered in
such cases. The run with "Canfield ocean" nitrous oxide emissions enhanced by a
factor of 100 also featured additional surface warming of 3.5K. Our results
suggest that the Proterozoic ozone layer mostly survives the changes in
composition which implies that it is indeed a good atmospheric biomarker.
