Purple sulfur bacteria fix N2 via molybdenum-nitrogenase

Biological N2 fixation was key to the expansion of life on early Earth. The N2-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 N2 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 N2fixation rates, to our knowledge providing the first direct evidence for PSB in situ N2 fixation. Surprisingly, no alternative nitrogenases are detectable, and N2 fixation is exclusively catalyzed by molybdenum-nitrogenase.

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Fig:  Lake Cadagno chemocline nutrient, δ15N and turbidity depth profile, bulk CO2 and N2 fixation rates, and PSB abundance: a Molybdenum (light blue) and sulfide (yellow) depth profiles. bAmmonium (dark blue) and δ15N values of bulk biomass (red circles, duplicate measurements at depths 13.7 m and 14 m, single measurement at 15.5 m). c Bulk N2 fixation rates (green) and bulk CO2 fixation rates (blue, determined in the same samples as N2 fixation rates). All rate measurements were performed in biological triplicates (except 15.5 m where one replicate was lost), individual replicates are shown as small circles, and the average rate is shown in large circles with the respective standard deviation. N2 fixation rates at 15.5 m depth were below the detection limit. d Abundances of key PSB populations (stacked bars, cell numbers based on FISH counts) and turbidity (gray line, a measure of total cell abundance). The location of the chemocline, as determined from the conductivity profile, is indicated by gray shading (a–d).

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 N2 fixation in the Proterozoic ocean.

Philippi, M., Kitzinger, K., Berg, J.S. et al. Purple sulfur bacteria fix N2via molybdenum-nitrogenase in a low molybdenum Proterozoic ocean analogue. Nat Commun 12, 4774 (2021). https://doi.org/10.1038/s41467-021-25000-z

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