Gas from bacteria that's not methane Is gas produced by bacteria always mainly methane? Or, are there bacteria out there that produce some biogas composed mainly of hydrogen, natural gas, propane, butane?

Microbes can produce several gasses other than methane.

* All microbes produce $CO_2$ through the oxidation of reduced carbon



Additionally some metabolic pathways produce other gases.

* Photosynthetic microbes produce $O_2$

* Sulfur reducing bacteria can produce $H_2S$ (as @ohcanada points out)

* Denitrifying bacteria produce $NO$, $N_2O$, and $N_2$

* Fermenters produce $H_2$ (as @mart indicates)




There may be others that I am not thinking of but these are some of the major players...