Shear stress tolerance and biochemical characterization of Phaeodactylum tricornutum in quasi steady-state continuous culture in outdoor photobioreactors.

Abstract A bubble column and two airlift photobioreactors (a draft-tube sparged vessel and a split-cylinder device) of the same general design (0.19 m column diameter, 2 m tall, 0.06 m3 working volume) were evaluated for outdoor continuous culture of the microalga Phaeodactylum tricornutum at a dilution rate of 0.03 h−1. At a daily averaged irradiance (photosynthetically active) value of 900 μE m−2 s−1, all bioreactors attained a quasi steady-state biomass concentration of ∼1 kg m−3 and a biomass productivity of ∼0.3 kg m−3 per day when the aeration velocity was 0.01 m s−1. The microalgal cells were susceptible to aeration-associated hydrodynamic stress if the superficial aeration velocity exceeded 0.01 m s−1. Supplementing the culture medium with 0.02% or more carboxymethyl cellulose (CMC), allowed stable culture under conditions that had previously damaged the cells. The average elemental composition of the biomass was: 49.2% C, 6.3% H, 0.8% N, and 1.3% S. The chlorophylls, carotenoids, and pigments content of the biomass changed with irradiance within a given day. Low irradiance favored accumulation of the light capture pigments. Increasing daily irradiance led to accumulation of carbohydrates. Some of the carbohydrate accumulated during the day was consumed at night and partly converted to proteins. Eicosapentaenoic acid (EPA, 20:5n3) constituted between 27 and 30% of the total fatty acids present, or 2.6–3.1% of the dry biomass. The other main fatty acids present were palmetic acid (16:0), palmoleic acid (16:1n7), and myristic acid (14:0). On average, these three fatty acids constituted 16.9% (16:0), 14.0% (16:1n7) and 9.4% (14:0) of the total fatty acids present.