Effect of light absorption rate and nitrate concentration on TAG accumulation and productivity of Parachlorella kessleri cultures grown in chemostat mode

This study aims to elucidate the role of light absorption rate and nitrate concentration on triacylglycerol (TAG) productivity and cellular TAG accumulation in nitrogen limited Parachlorella kessleri cultures grown in chemostat mode. In batch operated nitrogen starved cultures the cells accumulate large amounts of TAGs at the expense of biomass growth leading to severe decrease in cell division rate prompting eventual cell death. In PBRs operated in continuous mode cell multiplication can persist in nitrogen limited conditions with simultaneous TAG accumulation. P. kessleri cultures were grown in a flat-plate PBR with constant dilution rate of 0.01 1/h and feed-medium nitrate concentration ranging from 1 to 16 mM. The steady-state biomass, pigment, carbohydrate, protein, total lipid, and TAG concentrations as well as the spectral absorption and scattering cross-sections of the microalgae were measured for each culture. These were used to estimate the specific mean rate of photon absorption (MRPA) in the PBR. Maximum areal TAG productivity of 2.6 g/m2⋅day was obtained for the culture grown with nitrate concentration equal to 3.65 mM and a mean rate of photon absorption equal to 17,700 μmolhν/kgXA⋅s. TAG productivity and cellular TAG accumulation varied with the nitrate concentration in the feed medium and MRPA. Finally, our results show that TAG productivity can be further increased by optimizing the mean rate of photon absorption through adjusting culture dilution rate and feed-medium nitrate concentration.