Coming out of the cold: Vanadium is an important player in N-fixation in warm tropical environments.

Nitrogen (N)-fixation is a crucial source of reactive N in terrestrial environments. The nitrogenase enzyme (Nase), responsible for this conversion, has three isoforms: molybdenum (Mo)-, vanadium (V)-, and iron (Fe)-Nases. The Mo-Nase is found in all N-fixers, but the distribution pattern and drivers of V- and Fe-Nases remain poorly understood. Carbon and micronutrients (Mo and V) might be key contributors to N-fixation by affecting the metabolic demand for N and participating in Nase synthesis, respectively. Here, we investigated the presence of V-Nase genes in a tropical cyanolichen and explored the impact of micronutrient supply and elevated carbon dioxide (CO2) concentrations on N-fixation. The experiment revealed a significant influence of V and CO2 concentrations on heterocyst investment. V-Nase genes were also detected in tropical lichen specimens, suggesting that tropical species might have both Mo- and V-Nases. Our study suggests that V-Nase is prevalent beyond cold environments, by reporting its presence in the warm tropics and demonstrating a significant response of N-fixing cells to V addition. We emphasize the importance of further research on N-fixation in the tropics, particularly on the role of different Nase isoforms.