Only a few amino acid changes from mesophile proteins seem necessary in some cases to allow high temperature stability. Also more salt bridges in proteins. But at very high temperatures, membranes function well. But many microbes grow anaerobically some or all of the time. Anaerobic habitats more common than expected.
Facultative anaerobes consume oxygen, create anaerobic microenvironment fit for obligate anaerobes.
In general, wherever organic matter accumulates, microbes will use up oxygen faster than it can be replaced, create anaerobic environment. Esp. true under water, since oxygen is poorly soluble in water. But oxygen can easily generate very toxic byproducts , strong oxidizing agents that react indiscriminately with any organic molecules, including DNA, proteins, etc. Superoxide dismutase and catalase are two crucial enzymes. Effects of solutes and water activity on growth Cells require certain amount of free water to be able to carry out metabolism.