Gaia theory posits that “living and nonliving parts of the earth are viewed as a complex interacting system that can be thought of as a single organism.” This predicts that organisms will change their environment to improve its habitability to life. Darwinian selection says that organisms will adapt to their environment.
We only have one planet to observe Gaia on. Mars and Venus do not seem to have successfully carried out Gaia feedback mechanisms. Other extraterrestrial environments cannot yet be studied with enough detail to find life yet alone examine how life is altering it’s environment.
However there exists sealed off environments on earth where extremophile organisms have lived cut off from the general biosphere. If Darwinian selection is correct these organisms will adapt to their environment. This can be seen in cave animals losing the use of their eyes or in the case of bacteria such as Snotties that live on sulfur compounds. Another example of extremophiles found in gold mines that use radioactivity as an energy source
Do these organisms alter their environment is a way that is beneficial for life? Note that is beneficial for their life, they are adapted for living in these extreme conditions. Have these cave bacteria altered the cave environment in a way that aids their existence? Particularly you are looking for a symbiotic relationship between two forms of bacteria whose ratio alters in the environment depending on whether the dominant bactria’s byproducts are reaching toxic levels. The two forms of bacteria would be expected to be adapted to live ideally at one extreme of the conditions present in the cave but whose dominance would move the cave environment to the other extreme of conditions where the other form is better adapted.
These bacteria would have the roles of white and black Daisies in a simple daisy world model.
If Gaia theory is accurate you would expect to find that in any cut off cave biosystem (at least) two types of bacteria will coexist whose ratio will depend primarily on the environmental factors that the other type of bacteria has created. If such symbiotic cave biosystems are found then Gaia theory has another piece of supporting evidence.