Effect of ammonium/nitrate ratio on microalgae continuous cultures: Species-specificity of nutrient uptake and modelling perspectives

Microalgae-based wastewater treatment is promising in view of circularity as they can uptake most of the nitrogen sources usually present in waste streams: ammonium, nitrite and nitrate as well as some organic species. However, regulation pathways are not fully understood yet. This complexity is a challenge for accurate modelling of these processes, necessary for the design and optimization of wastewater treatment systems. It is generally observed in microalgae that the presence of ammonium inhibits the assimilation of other nitrogen compounds. This was confirmed in this work for Synechocystis sp. PCC6803, even in acclimated continuous cultures. A different uptake regulation was instead observed for the species Auxenochlorella protothecoides, able to uptake nitrate even when high concentrations of ammonium are present. From a modelling perspective, the Droop model was implemented to account for the exploitation of both ammonium and nitrate: a switch function was used to describe the ammonium inhibition, but this was applicable for Synechocystis only. For A. protothecoides, a changing nutrient uptake rate is needed, which was found to be function of the dissolved nitrogen concentration. A bioinformatic analysis was performed to infer putative players involved in nitrogen uptake, accounting for the different behavior of the two taxonomic groups.