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

Abstract

Microalgae exhibit diverse responses to various nitrogen forms in natural environments, affecting growth rates, biomass yields, composition, by-product production, and nutrient utilization efficiency, which vary among species and strains. In the present study, the growth of two microalgae species was investigated with different nitrogen sources to study their effect on these parameters. In particular, the evolution of several parameters associated with microalgal cultures was examined in relation to the nitrogen source and the pH of the culture medium. Furthermore, simple alternative techniques for estimating biomass growth in microalgal cultures based on nitrogen consumption and/or acid/base titrations are proposed. CI (Parachlorella kessleri) and DIII (Desmodesmus communis) were isolated from the Reconquista River, a polluted watercourse in Buenos Aires, Argentina. CI showed higher productivity, faster biomass growth, and increased extracellular polymeric substances (EPS) production, which promoted cell agglomeration, when cultured in culture medium with NO3- as the nitrogen source, compared to cultures with NH4+ as the nitrogen source. On the other hand, DIII grew only when cultured in medium with NO3-, reaching a maximum growth rate comparable to that of CI in culture medium with NO3-. Growth inhibition in cultures containing NH4+ was attributed to pH decrease associated with NH4+ uptake rather than NH4+ effects. Both species exhibited an increase in the C/N ratio. Under N starvation DIII accumulates N-free substances like lipids and starch in both culture conditions, at controlled pH (≈7). In contrast, the results for CI showed a more pronounced increase in the C/N ratio when cultured in medium containing NO3-, which was attributed to the formation of EPS. These variations in the composition of the microalgae biomass and the occurrence of cellular agglomeration throughout a culture suggest that different biomass estimation techniques may not be directly comparable across different phases of a culture. In this sense, in the present work it was demonstrated that for certain conditions, it is possible to accurately estimate growth from the consumption of nitrogen, and from the variation in the concentration of OH- and/or H+ of the culture medium.