Dolichospermum Facilitates Microcystis Growth Under N₂ Fixation

During phytoplankton blooms, high biomass accumulation depletes nitrogen (N) and phosphorus (P). Despite low nutrient concentrations, it is not uncommon the presence of multiple species, an unexpected scenario considering the importance of competitive interactions. The co-occurrence of freshwater cyanobacteria species N₂ and non N₂-fixers poses a possible scenario for facilitative interaction. We investigate whether, under N deprivation and limitation, the N₂-fixing Dolichospermum circinale could benefit the non-N₂-fixing Microcystis aeruginosa. We used mono- and mixed batch cultures and compared the performance of each species without the addition of nitrate (0N) and with 5 mg N-NO₃ ^- L^-1 (5N). We found that M. aeruginosa benefited from the presence of D. circinale in 0N and reached a biovolume at least six times greater than in monoculture. Approaches such as the relative density of species and the Lotka-Volterra competition model confirmed the positive interaction in 0N. During the experiment, total N and NH₄ ⁺ concentrations increased and the increase in NH₄ ⁺ was associated with the biomass of D. circinale, suggesting the release of fixed N₂ as NH₄ ⁺. Our results strengthen the importance of positive interactions between cyanobacterial populations during nutrient limitation and expand the ecological understanding of the microbial dynamics in freshwater ecosystems.