Rhodopseudomonas palustris is a purple facultatively photosynthetic bacterium that lives in environments like the surface layers of water logged soils that tend to straddle oxic to anoxic transition zones. When Rhodopseudomonas is exposed to atmospheric levels of oxygen, it oxidizes carbon compounds such as acetate for carbon and energy by aerobic respiration. When it is exposed to oxygen depleted (2-6% oxygen) or anaerobic conditions, R. palustris turns a characteristic deep purple color as it synthesizes the light absorbing pigments that it needs to carry out photosynthesis. Rhodopseudomonas generates ATP anaerobically and under low oxygen tensions by anxoygenic photosynthesis, a process that differs from the oxygen-generating photosynthesis of green plants and cyanobacteria. Anoxygenic photosynthesis does not generate oxygen and is not obligatorily linked to carbon dioxide fixation. Rhodopseudomonas prefers to use organic compounds for cell carbon when it is generating ATP by anoxygenic photosynthesis. This growth mode is referred to as photoheterotrophic. Under microaerobic or anaerobic growth conditions R. palustris can use nitrogen gas from the atmosphere as a sole nitrogen source for growth by the process of nitrogen fixation. Rhodopseudomonas generates hydrogen along with ammonium as a product of nitrogen fixation.