Combined inorganic nitrogen sources influence the release of extracellular compounds that drive mutualistic interactions in microalgal‒bacterial co-cultures

We investigated the role of extracellular metabolites released during mutualistic interactions in co-cultures of a microalga, Tetradesmus obliquus IS2 or Coelastrella sp. IS3, and a bacterium, Variovorax paradoxus IS1, grown with varying levels of NO3–N and NH4–N. Both NO3–N and NH4–N were added to modified Bold’s basal medium at 16:0, 12:4, 8:8; 4:12 and 0:16 molar ratios by keeping a final N:P ratio of 16:1. Monocultures of microalgae grown with nitrate alone showed enhanced growth (> twofold) than ammonium, while the bacterial strain cultured with ammonium alone exhibited a > 1.3-fold increase in growth than nitrate. Co-culturing performed higher growth at combined nitrate and ammonium supply as compared to the single cultures. The same ratio of nitrate and ammonium resulted in superior growth of microalgae (> 1.7-fold) and the bacterium (> 4.1-fold) as compared to the monocultures. Uptake of NO3–N, NH4–N and PO4–P by monocultures or co-cultures depended on the ratio of two inorganic nitrogen sources used. The composition of organic acids, amino acids and simple sugars in exudates from monocultures varied with the ratios of nitrate and ammonium in the medium. Thus, the present novel study demonstrates that the release of exudates is affected both qualitatively and quantitatively during mutualistic interactions in microalgal‒bacterial co-cultures under the impact of inorganic nitrogen sources. Our results suggest that the variables such as inorganic nitrogen sources and extracellular metabolites released need to be considered while using co-cultures for effective bioremediation of wastewaters.