EN-Virtual Seminar

Speaker: Dr. Jean-Philippe Bellenger, Université de Sherbrooke​

Abstract: Nitrogen is, with phosphorus, the primary limiting nutrient on earth. Biological nitrogen fixation (BNF) is an important source of new reactive N in unmanaged ecosystems; contributes up to 50% of new N in high-latitude ecosystems. Factors controlling BNF thus critically affect ecosystems function and their response to global climate change. The role of environmental factors such as atmospheric N deposition, temperature, humidity, on BNF is well known and have been the subject of countless studies. Here, I will summarize findings that revealed that trace metal availability also plays a critical role.

Three isoforms of the enzyme nitrogenase have been described; the canonical molybdenum (Mo) nitrogenase which requires Mo in its active site and two alternative nitrogenases, the vanadium and iron-only nitrogenases. For almost a century, BNF has been considered to primarily rely on the Mo-based isoform. Over the last 20 years we revealed that (i) the low bioavailability of Mo on land limits BNF in many ecosystems from the tropical forest to the arctic tundra, (ii) alternative nitrogenase are more common than previously believed, (iii) the use of alternative nitrogenases to cope with Mo limitation is widespread, and (iv) alternative nitrogenases contribute up to 50% of BNF in boreal ecosystems.

These results reveal a strong link between the biogeochemical cycle of macro- and micronutrients in terrestrial ecosystems, question the reliability of current BNF estimates, which have likely been largely underestimated in many ecosystems and call for additional consideration of alternative nitrogenases in experimental and modelling studies of terrestrial biogeochemistry.