Optimization of the growth and marennine production by the diatom Haslea ostrearia in photobioreactor

The benthic diatom Haslea ostrearia which has the capacity to excrete a blue pigment called marennine remains a challenging organism to culture in a photobioreactor (PBR). This study investigates the interest of culture in conventional mixed PBR over immobilized-cell protocols which proved successful but have rather low extracellular marennine (EMn) due to a limitation in mass or light transfer in the biofilm. In contrary, culture in mixed PBR has been proven to overcome culture mass transfer limitation, and to provide a high level of control over light access to cells, enabling the application of systematic optimization of photosynthesis-related kinetics. Increasing the dissolved inorganic carbon concentration up to values of around 5?10 mM was found to increase both growth kinetics and EMn production. Growth medium enrichment with silica (Si) was however found to be challenging due to chemical precipitation. A fed-batch strategy on Si-P supply was implemented, leading to an increase in biomass productivity (51.8 ? 2.3 mgX L-1 d-1). Although EMn was produced continuously, the nutrient-limited conditions led to higher productivity with 9.2 ? 1.7 mgEMn m- 2 d-1, 37.9 ? 0.7 mgEMn m- 2 d-1 and 54.5 ? 1.8 mgEMn m- 2 d-1 for phosphorus, silicon and nitrogen deprivation respectively. The effect of light availability was also investigated, as represented by the Mean Rate of Photon Absorption (MRPA). A direct relation was shown for both biomass and EMn production kinetics. However, maximal biomass and EMn productivities were found at different MRPA values, respectively 1.7 ? 0.1 gx m- 2 d-1 at 12.3 ?molh? gx- 1 s-1 and 11.0 ? 0.5 mgEMn m- 2 d-1 at 8.4 ?molh? gx- 1 s- 1. Finally, a continuous EMn production was obtained in optimal conditions, leading to a productivity of 4.5 ? 0.16 mgEMn L-1 d-1, enabling validation of the conventional mixed PBR for H. ostrearia culture and continuous EMn production.