Effects of temperature, irradiance, and pH on the growth and biochemical composition of Haslea ostrearia batch-cultured in an airlift plan-photobioreactor.

Haslea ostrearia is a pennate diatom that produces marennine, a water-soluble blue pigment responsible for the greening phenomenon and the increase of organoleptic quality of oysters. Apart from the oyster industry, there is a growing interest in the mass cultivation of this diatom due to the biological activities of marennine. To gain knowledge about the feasibility to upscale production of this diatom, in particular, in the context of global warming, the effects of different temperatures (20, 25, and 30 °C), irradiances (100, 200, and 300 μmol photons m -2  s -1 ), and pH (7.0, 8.0, and 9.0) on growth and biochemical composition were studied in H. ostrearia cultured in an airlift plan-photobioreactor. The maximum growth rate of H. ostrearia (0.9 ± 0.0 day -1 ) was obtained at 20 °C, 200 μmol photons m -2  s -1 , and pH 7.0, referred to as control conditions. The highest concentration in Chla (2.5 ± 0.1 µg 10 -6 cells) and total fatty acids (71.6 ± 1.4 mg g -1 of dry weight, DW) was observed at 20 °C, 300 μmol photons m -2  s -1 , and pH 7.0. The highest concentration of carotenoids (1.4 ± 0.1 µg 10 -6 cells), Chlc (1.3 ± 0.1 µg 10 -6 cells), and extracellular marennine (33.1 ± 0.2 µg 10 -6 cells) was observed at 30 °C, 200 μmol photons m -2  s -1 , and pH 7.0, and a higher protein content (309.7 ± 24.5 mg g -1 of DW) at 25 °C, 200 μmol photons m -2  s -1 , and pH 7.0. The biomass of H. ostrearia was enriched with C14:0 fatty acid at 30 °C, 200 μmol photons m -2  s -1 , and pH 7.0, and with C16:0 and C16:1n - 7 fatty acids at control conditions. However, DHA C22:6n - 3 (ω-3), C22:0, and C20:0 were only observed at 300 µmol photons m -2  s -1 , 20 °C, and pH 7.0. A high abundance of essential polyunsaturated fatty acids C22:1n - 9 (ω-9) + C20:5n - 3 (EPA) was observed at 100 µmol photons m -2  s -1 , 20 °C, and pH 7.0. It is thus possible to anticipate and tune the production of specific metabolites through the control of growth conditions of the benthic diatom H. ostrearia. KEY POINTS: • Validation of H. ostrearia culture in a new photobioreactor in batch mode • Biochemical composition variation of H. ostrearia in function of growth conditions • Growth inhibition and unbalanced metabolites induced by the treatment conditions.