Purple sulfur bacteria fix N(2) via molybdenum-nitrogenase in a low molybdenum Proterozoic ocean analogue

Biological N(2) fixation was key to the expansion of life on early Earth. The N(2)-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(2) 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(2) fixation rates, to our knowledge providing the first direct evidence for PSB in situ N(2) fixation. Surprisingly, no alternative nitrogenases are detectable, and N(2) 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(2) fixation in the Proterozoic ocean.