What are some possible atmospheric bio-signatures for a planet with anaerobic life? or even microorganisms before oxygen builds up in the atmosphere? Would there be a way of telling if methane, etc. is possibly not from abiotic causes?
Anaerobic life has much less impact on the atmosphere than aerobic life. Anaerobic life could exist in the deep sea and have no gas exchange with the atmosphere at all.
Methane would get everyone excited. Biotic methane might be expected to have a different isotope mix compared with abiotic methane. But it is hard enough to distinguish isotope mixes when you have a rover on a planet.
More complex compounds would be very exciting, but a very sensitive detector would be needed to find the very low concentrations required.
It may be that the marker of abiotic life is the detection of anomalously low CO2 levels. Most abiotic atmospheres have very high levels of Carbon dioxide, but life will progressively reduce this level. A low level of carbon dioxide, in an atmosphere containing water, especially if coupled with methane would look bery interesting.