Molybdenum threshold for ecosystem scale alternative vanadium nitrogenase activity in boreal forests

Abstract

Biological nitrogen fixation (BNF) by microorganisms associated with cryptogamic covers, such as cyanolichens and bryophytes, is a primary source of fixed nitrogen in pristine, high-latitude ecosystems. On land, low molybdenum (Mo) availability has been shown to limit BNF by the most common form of nitrogenase (Nase), which requires Mo in its active site. Vanadium (V) and iron-only Nases have been suggested as viable alternatives to countering Mo limitation of BNF; however, field data supporting this long-standing hypothesis have been lacking. Here, we elucidate the contribution of vanadium nitrogenase (V-Nase) to BNF by cyanolichens across a 600-km latitudinal transect in eastern boreal forests of North America. Widespread V-Nase activity was detected (∼15–50% of total BNF rates), with most of the activity found in the northern part of the transect. We observed a 3-fold increase of V-Nase contribution during the 20-wk growing season. By including the contribution of V-Nase to BNF, estimates of new N input by cyanolichens increase by up to 30%. We find that variability in V-based BNF is strongly related to Mo availability, and we identify a Mo threshold of ∼250 ng·glichen−1 for the onset of V-based BNF. Our results provide compelling ecosystem-scale evidence for the use of the V-Nase as a surrogate enzyme that contributes to BNF when Mo is limiting. Given widespread findings of terrestrial Mo limitation, including the carbon-rich circumboreal belt where global change is most rapid, additional consideration of V-based BNF is required in experimental and modeling studies of terrestrial biogeochemistry.

Carbon substrate re‐orders relative growth of a bacterium using Mo‐, V‐, or Fe‐nitrogenase for nitrogen fixation

“…The Mo-Nase is considered the most efficient isoform and the primary form responsible for N 2 fixation in the environment. However, alternative V-and Fe-Nase genes are widespread (Betancourt et al, 2008;McRose et al, 2017) and active (Hodkinson et al, 2014;Zhang et al, 2016;Darnajoux et al, 2017Darnajoux et al, , 2019McRose et al, 2017) in diverse taxa and environments, prompting questions on their roles in ecosystem nitrogen and trace metal cycling. To better understand the persistence of Nase diversity and the controls on environmental N 2 fixation, it is necessary to study how alternative N 2 fixation affects the growth physiology of diazotrophs (Glazer et al, 2015).…”

Large Hydrogen Isotope Fractionation Distinguishes Nitrogenase-Derived Methane from Other Methane Sources

“…The controls on alternative nitrogenase activity are not fully understood (e.g., references 25 , 27 , 89 , and 98 ), although new tools ( 27 , 34 ) are rapidly advancing our understanding of their distribution. It is now well established that alternative nitrogenases are favored under conditions of low Mo availability ( 28 , 90 ), although their activity has been observed in some sedimentary environments that appeared to be Mo-replete as well ( 26 , 27 ). Aerobic soils, cyanolichens, mosses and other biocrusts, lake and marine waters ( 8 , 91 ), or sediment systems with high sulfate concentrations, where sulfate reducers generally outcompete methanogens for substrates ( 3 , 92 ), are possible targets to test where and when alternative nitrogenases are active using methane stable isotopes ( 10 ).…”

“…The vanadium (V-) and Fe-only nitrogenases produce the most byproduct methane of the nitrogenase isoforms (10). They are found in both the bacterial and archaeal domains and are present in diverse environments (26)(27)(28)(29)(30)(31). In addition, certain artificial mutations near the active site of the molybdenum (Mo)-nitrogenase enable this more common isoform to produce methane (32,33).…”

Broad‐scale distribution of diazotrophic communities is driven more by aridity index and temperature than by soil properties across various forests

“…It is known that V can substitute for Mo in an alternative nitrogenase, which has been found in diverse diazotroph species including free‐living soil bacteria, cyanobacteria and cyanolichens (Bellenger et al, 2020). Alternative N 2 fixation by V‐based nitrogenase was found to be an important ecological feature of BNF and contributed 15−50% of the total BNF rates in boreal forests (Darnajoux et al, 2019). The importance of V nitrogenase and its potential role in diazotrophic biogeography need further evaluation.…”