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

Area of Science:

• Ecology
• Biogeochemistry
• Microbiology

Background:

• Biological nitrogen fixation (BNF) is crucial for nitrogen input in high-latitude ecosystems.
• Molybdenum (Mo) availability often limits BNF due to nitrogenase (Nase) enzyme requirements.
• Vanadium (V) and iron-only Nases are proposed alternatives to Mo-limited BNF, but field evidence is scarce.

Purpose of the Study:

• To investigate the contribution of vanadium nitrogenase (V-Nase) to BNF in cyanolichens across a latitudinal gradient.
• To provide ecosystem-scale data on the role of V-Nase in boreal forest nitrogen cycling.
• To determine the relationship between V-based BNF and Mo availability.

Main Methods:

• Field study across a 600-km latitudinal transect in eastern boreal forests.
• Quantification of V-Nase activity in cyanolichens.
• Analysis of V-Nase contribution to total BNF rates over a 20-week growing season.
• Assessment of Mo availability in lichen samples.

Main Results:

• Widespread V-Nase activity was detected, contributing 15-50% to total BNF rates.
• V-Nase activity was more prevalent in the northern part of the transect.
• V-Nase contribution increased by 3-fold during the growing season.
• BNF estimates increased by up to 30% when V-Nase was included.
• A Mo threshold of ~250 ng·g lichen⁻¹ was identified for the onset of V-based BNF.

Conclusions:

• V-Nase is a significant contributor to BNF in cyanolichens, particularly under Mo-limiting conditions.
• This study provides the first compelling ecosystem-scale evidence for V-Nase functioning as a surrogate enzyme.
• The findings necessitate the inclusion of V-based BNF in biogeochemical studies, especially in rapidly changing circumboreal regions.