Dear Cheng,

You can simply observe methanogens using fluorescence microscopy at wavelengths of 420-430 nm. The coenzymes F420 and F430 contained by most methanogenic species are autofluorescent, they absorb light at these wavelengths and become visible in bright green-blue color. I can not give you any concrete technical details because I am not a specialist in microscopy, but I easily and quickly observed coccus methanogens in sewage sludge at these wavelengths (of course after a minimal preliminary study). I hope you get a more explicit answer from more experienced researchers. Regards.