Purple sulfur bacteria fix N via molybdenum-nitrogenase in a low molybdenum Proterozoic ocean analogue.

Biological N fixation was key to the expansion of life on early Earth. The N-fixing microorganisms and the nitrogenase type used in the Proterozoic are unknown, although it has been proposed that the canonical molybdenum-nitrogenase was not used due to low molybdenum availability. We investigate N fixation in Lake Cadagno, an analogue system to the sulfidic Proterozoic continental margins, using a combination of biogeochemical, molecular and single cell techniques. In Lake Cadagno, purple sulfur bacteria (PSB) are responsible for high N fixation rates, to our knowledge providing the first direct evidence for PSB in situ N fixation. Surprisingly, no alternative nitrogenases are detectable, and N fixation is exclusively catalyzed by molybdenum-nitrogenase. Our results show that molybdenum-nitrogenase is functional at low molybdenum conditions in situ and that in contrast to previous beliefs, PSB may have driven N fixation in the Proterozoic ocean.