Life forms that use carbon dioxide as the sole or principal carbon source for growth are described as autotrophs, and the use of carbon dioxide is called autotrophic carbon dioxide fixation.  In the reticulo-rumen the autotrophs are the methanogens and the homoacetogens and both of these groups of microorganisms use the acetyl CoA pathway of CO2 fixation. These, and other autotrophs that live in the dark, (and who do not carry out photosynthesis) may also be called chemoautotrophs just to make that point (organisms that carry out photosynthesis are phototrophs and most are autotrophic, hence the descriptor photoautotrohic). The term chemolithoautotrophy can also be applied to the methanogens and the homoacetogens. The "litho" component of the name conveys the idea that they derive reducing equivalents (for reductive biosynthesis and for ATP generation) from the oxidation of hydrogen which is an inorganic compound.