The other process of primary production is lithoautotrophy. Lithoautotrophs use reduced chemical compounds such as hydrogen gas, hydrogen sulfide, methane, or ferrous ion to fix carbon and participate in primary production. Lithoautotrophic organisms are prokaryotic and are represented by members of both the bacterial and archaeal domains.  Lithoautotrophy is the only form of primary production possible in ecosystems without light such as ground-water ecosystems, hydrothermal vent ecosystems, soil ecosystems, and cave ecosystems.