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

Abstract

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/m 2 ⋅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 μmol hν /kg X A 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.

Producing Energy-Rich Microalgae Biomass for Liquid Biofuels: Influence of Strain Selection and Culture Conditions

“…However, a higher limitation, from 29 to 13% NO 3 , reduced the biomass concentration by 36%. The biomass trend is very similar to that noted with P. kessleri, as reported by Kandilian et al [35]. The most notable difference between the two strains is related to the level of stress needed to accumulate energy-storage molecules.…”

“…Bold Basal Medium (8.02 mM) was used for the cultivation of P. kessleri (the composition of the medium can be found in Kandilian et al [35]). Artificial Sea Water (ASW) enriched with CONWAY solution, as in Berges et al [46], was used for the cultivation of N. gaditana in this work.…”

“…The limitation levels were 200%, 56%, 46%, 29% and 13% NO 3 (21.2, 5.94, 4.88, 3.07 and 1.38 mM, respectively) and 0% NO 3 (0 mM) for nitrogen depletion. These values were chosen for the purposes of comparison with the work of Taleb et al [30] and Kandilian et al [35] on P. kessleri. The 200% NO 3 value was used to set the non-limited (replete) physiological state.…”

“…The response of N. gaditana to four consecutive levels of nitrogen limitation in continuous mode (56%, 46%, 29% and 13% NO 3 ) was investigated. The results were compared to those obtained for P. kessleri under the same conditions as used by Kandilian et al [35]. The PBR was operated in continuous mode until steady-state was achieved, to allow the microalgae to adapt to each NO 3 − concentration (around 20 days between each steady state).…”

“…In the mature stage, it can reach 5-10 µm in size [31,33,45]. In this work, the experiments on P. kessleri UTEX2229 (Culture Collection of Algae, University of Texas, USA) by Taleb et al [30] and Kandilian et al [35] have been used for comparison with N. gaditana.…”