<i>Azotobacter vinelandii</i> N ₂ fixation increases in co-culture with the PGPR <i>Bacillus subtilis</i> in a nitrogen concentration-dependent manner

Reducing inputs of chemical N fertilizers is essential to develop a more sustainable agriculture. The stimulation of biological nitrogen fixation by N2 fixers in multispecies cultures, here the plant growth-promoting rhizobacteria <i>Azotobacter vinelandii</i> and <i>Bacillus subtilis</i>, opens opportunities for the formulation of biofertilizers consortia. While most research on N2 fixation historically focussed on the exponential growth phase of microorganisms, we observed that <i>Bacillus subtilis</i> stimulated <i>Azotobacter vinelandii</i> N2 fixation mostly during the stationary phase. This result highlights that more research on the factors controlling N2 fixation repression during the stationary growth phase, especially bacteria-bacteria interactions, is eagerly needed.