Trends in Mo-, V- and Fe-only-based Biological Nitrogen Fixation across a Temperature Gradient in the Alphaproteobacterium Rhodopseudomonas palustris

Abstract:

Biological nitrogen fixation is the primary natural source of nitrogen for the biosphere. It is catalyzed by the enzyme nitrogenase, of which there are three known isoforms. The complementary V- and Fe-only nitrogenase isoforms are generally considered less efficient than the canonical Mo-nitrogenase. This has led to questions about the evolutionary benefits of conserving the complementary isoforms and the conditions under which they are active in the nitrogen biogeochemical cycle. Most studies of nitrogenase physiology have occurred at 30ºC, and recent observations that, for the filamentous cyanobacterium Anabaena variabilis, growth rates based on the Mo- and V-nitrogenase isoforms converge at colder temperatures may help shed some light on this mystery (Darnajoux et al. in preparation). Here, we grew Mo-, V- and Fe-only nitrogenase utilizing strains of the anaerobic photoheterotroph Rhodopseudomonas palustris across a temperature gradient from 10 to 30°C. Absolute growth rates decrease with temperature, and under 14°C, growth rates based on Mo- and V-nitrogenase isoforms are indistinguishable. Carbon substrate modulates the temperature effect, with no growth on acetate at the coldest temperatures for any of the strains and with faster relative V-nitrogenase based growth at 20°C on acetate compared to succinate. These relative growth rate trends are not driven by changes in the H₂:N₂ ratio, a measure of reductant use efficiency for nitrogenase. Our results suggest that the temperature dependency observed for A. variabilis is a general property of the nitrogenase isoforms and demonstrates the importance of metabolism, particularly carbon substrate, as a control of nitrogenase isoform function. These results provide a mechanistic basis to predict how Mo-, V- and Fe-only nitrogenase contributions to terrestrial nitrogen fixation are likely to shift in future climates.