Black Sea and diazotrophs, toward an improvement of modeling the nitrogen cycle

Abstract

The nitrogen cycle in oxygen depleted environment is a major concern in our world as soon as the oxygen minimum zones are spreading due to global warming and climate changes. The Black Sea basin has a constant stratification in which the conditions are anoxic under 100m all the yearlong. This feature creates an outdoor laboratory for scientists which facilitates the studying of anoxia and where typical events of the world ocean like ventilation, denitrification or nitrogen fixation can be more easily investigated. Thus, we use a 3D model of the Black Sea to investigate the nitrogen cycle, where we have added di-nitrogen fixers (diazotrophs). Previously di-nitrogen was considered to be lost in the water column because it was considered to be inert regarding biology. During this work, we have investigated di-nitrogen fixers and we have found that a non-negligible amount of nitrogen is recycled in the water column which enhance the accuracy of the nitrogen cycle. Indeed, the production of di nitrogen in the water column was found to be constantly inferior to the di-nitrogen fixation by diazotrophs, which shows their importance. However, we have to investigate more efficiently our knowledge of diazotrophs as soon as their physiology is only poorly understood.