Overall assessment of Monodus subterraneus cultivation and EPA production in outdoor helical and bubble column reactors

The outdoor production of Monodus subterraneus was studied in bubble column and helical reactors, mainly analysing the influence of dilution rate, air flow rate and solar irradiance on growth rate and biochemical composition. Photoinhibition and photo-oxidation phenomena were also analysed. The cultures were stressed at high solar irradiance and dissolved oxygen concentrations. A clear relationship between stress of the cultures and the fluorescence from PSII measurements was observed, the Fv/Fm ratio being lower in the helical reactor than in the bubble column. Growth rate and biomass productivity were both a function of the average irradiance and the Fv/Fm ratio; maximum values of 0.040 h(-1) and 0.54 g L-1 d(-1) were measured. The influence of photoinhibition and average irradiance was modelled, the model also fitting the experimental data reported by another author. The chlorophyll content hyperbolically decreased, whereas the carotenoid content decreased linearly with the average irradiance. The higher the dilution rate the higher the protein and carbohydrate content of the biomass, and the lower the lipid content. The eicosapentaenoic acid (EPA) content ranged from 2.3 to 3.2% d.wt, the higher the dilution rate, the lower EPA content, although the higher the EPA proportion. Maximum EPA productivity was only 9 mg L-1 d(-1), due to the stress to which the cultures were exposed.