Optimization of continuous astaxanthin production by Haematococcus pluvialis in nitrogen-limited photobioreactor

The industrial production of astaxanthin from Haematococcus pluvialis is mainly operated following a two-stage cultivation strategy in photobioreactors (PBRs) operating in batch mode. This study provides a strategy for optimizing continuous astaxanthin production in a one-stage approach, by optimizing both astaxanthin accumulation and biomass productivity of H. pluvialis under nitrogen-limited condition. To achieve the good balance in culture conditions to maintain growth under nitrogen-limited conditions, while triggering significant astaxanthin accumulation in reddish vegetative cells, the role of light absorption, as represented by the mean rate of photon absorption (MRPA), and nitrate concentration was especially investigated. For that purpose, H. pluvialis cultures were grown in a flat-panel PBR with constant dilution rate D of 0.015 h-1 , different photon flux densities PFDs ranging from 75 to 750 mu mol h nu center dot m- 2 center dot s- 1 and different nitrogen concentration levels in the feeding medium [NO 3- ] of 1, 3 and 8.8 mM. A parabolic relationship between MRPA and astaxanthin production rate was obtained. This indicated that astaxanthin synthesis was limited by the MRPA, opening further optimization by adjusting incident light intensity, nitrogen concentration in the feeding medium or culture dilution rate. After optimization of operating conditions, we demonstrated that a large quantity of astaxanthin can be produced in continuous mode, following then a one-stage strategy which may be advantageous for industrial use. A maximum astaxanthin productivity of 1.27 +/- 0.03 g center dot m- 2 center dot d-1 with an astaxanthin accumulation of 3.9 +/- 0.2 % DW was reached for the culture featuring MRPA of 9000 mu mol h nu center dot kg x- 1 center dot s- 1 . The process was also proved reversible, allowing to tailor the biomass composition and physiological state (from green to red cells enriched in astaxanthin) by adjusting operating conditions, opening perspectives from an optimized coupling with downstream processing steps.