Cultivation of Arthrospira (spirulina) platensis (Cyanophyceae) by fed-batch addition of ammonium chloride at exponentially increasing feeding rates

Arthrospira (Spirulina) platensis (Nordstedt) Gomont was cultivated under light-limited conditions in 5-L open tanks by daily supplying NH4Cl as nitrogen source. Exponentially increasing feeding rates were adopted to prevent ammonia toxicity. The total feeding time (T) was varied between 12 and 20 days, and the starting (m(0)) and total (m(T)) quantities of the nitrogen source per unit reactor volume were varied in the ranges 0.19-1.7 mM and 2.3-23.1 mM, respectively. This intermittent addition of the nitrogen source prevented ammonia from reaching inhibitory levels and ensured final cell concentrations (X-m) and cell productivities (P-x) comparable with those of batch runs with KNO3. Moreover, the lower nitrogen addition due to the use of NH4Cl rather than KNO3 allowed for higher nitrogen-to-cell conversions (Y-x/n). These results were evaluated using three-factor, five-level, central composite experimental planning, combined with the response surface methodology, selecting T, m(0), and m(T) as the independent variables and X-m, P-x, and Y-x/n as the response variables. This approach allowed us to identify, through the simultaneous optimization of the variables, T=16 days, m(0)=1.7 mM, and m(T)=21.5 mM as the best conditions for A. platensis cultivation at 72 mumol photons.m(-2).s(-1). Under these conditions, a maximum cell concentration of 1239 mg .L-1 was obtained, which is a value comparable with that obtained using KNO3 as nitrogen source and nearly coincident with the theoretical one estimated by the response surface methodology.