Nitrogen uptake and associated pH changes influence growth and biomass composition in microalgae from a polluted river

The uptake of different nitrogen species (NO₃⁻, NH₄⁺) by microalgae affects their growth rates, biomass yields, composition, by-product production, and nutrient use efficiency. These effects vary among species and strains. This study examines the growth of two microalgal species with different nitrogen sources to understand these effects. It analyzes microalgal culture parameters in relation to nitrogen uptake and resulting pH variations. Furthermore, simple alternative techniques for estimating biomass growth based on nitrogen consumption and/or acid/base titrations are proposed. Two strains, CI (Parachlorella kessleri) and DIII (Desmodesmus communis) were isolated from a polluted river. The pH decrease, resulting from NH₄⁺ uptake, led to growth inhibition in both species. However, at neutral or alkaline pH, growth was unimpaired. CI results showed increased extracellular polymeric substances production at controlled pH (≥ 5) with NO₃⁻ or NH₄⁺. Under N-starvation, both species exhibited a C/N ratio increase. DIII presented the highest C/N values at neutral pH, regardless the nitrogen sources, accumulating N-free substances. CI cultivated at free-pH showed a higher C/N ratio in alkaline cultures (NO₃⁻ sources) compared to acidic cultures (NH₄⁺ sources) due to EPS production. These variations in biomass composition and cellular agglomeration indicate that biomass estimation techniques may not be directly comparable across different growth phases. The findings highlight the impact of nitrogen source on microalgal growth and biomass composition, underscoring the need for tailored cultivation strategies to optimize productivity and resource efficiency, while proposing simple growth estimation methods based on nitrogen consumption and OH⁻ and/or H⁺ variation in the culture medium.