As a committee member on various advisory boards for NASA, he was preoccupied, in his tinkery sort of way, with the problem of how one could objectively determine if life existed on Mars. What was obvious to him, after measuring reactive gases of biological origin such as ammonia, methane, and nitrogen and sulfur oxides, was that the air was loaded with exudates of life. Yet, as every chemistry student knows, all of these reduced organic compounds react readily and easily with oxygen and, from the point of view of chemistry alone, they should not be present for long in air samples. He suggested looking for reactive mixtures, or at least changes in gas concentrations, in the atmospheres of Mars and other planets, to see if any explanation beyond chemistry and physics would be required for understanding them. As we talked about the biological effects of life on a planetary scale we made each other aware of the fact that all live beings produce and remove gas.
Microbes produce and remove a great range of other gases as well.
Yes, we concluded with enthusiasm, the atmosphere is the circulatory system of the biosphere. Lovelock complained that no one seemed to understand what he was saying. When we first recorded our preliminary Gaia conversation, Stewart Wilson failed to engage one of the two recording buttons. We opted for the latter. The consequence of our decision was striking.