Studies on the suitability of alginate-entrapped Chlamydomonas reinhardtii cells for sustaining nitrate consumption processes

Some aspects of the suitability of alginate beads entrapping Chlamydomonas reinhardtii cells for nitrate consumption from nitrate-containing waters were studied and discussed. Among 14 different metal cations tested as gel bead stabilizing agents, only calcium and barium formed beads showing nitrate-consuming activity. Pure calcium alginate cell entrapment resulted in the most suitable method for active cell immobilization compared to alginate-composite-gel beads based on poly-vinylcaprolactam (PVCL) and poly-vinylpyrrolidone (PVP). To perform a continuous nitrate consumption process, calcium alginate-entrapped cells were first grown in a 2.5 l airlift-loop reactor. A cell loading of about 150 μg Chl. g−1 gel was achieved. Afterwards, five days nitrate consumption processes were performed and three different dilution rates were applied: (i) D<μ; (ii) D=μ; (iii) D>μ, where μ is the specific growth rate (h−1). The maximum consumption rates calculated for each dilution rate were: (i) 3.8, (ii) 6.4 and (iii) 7.2 mg nitrate mg−1 Chl. h−1. For low dilution rates (D<μ) some nitrite (<300 μM) was excreted into the culture medium. However, this concentration of nitrite was not high enough to inhibit nitrate consumption.