Productivity analysis of outdoor chemostat culture in tubular air-lift photobioreactors

Net productivity and biomass night losses in outdoor chemostat cultures of Phaeodactylum tricornutum were analyzed in two tubular airlift photobioreactors at different dilution rates, photobioreactor surface/volume ratios and incident solar irradiance. In addition, an approximate model for the estimation of light profile and average irradiance inside outdoor tubular photobioreactors was proposed. In both photobioreactors, biomass productivity increased with dilution rate and daily incident solar radiation except at the highest incident solar irradiances and dilution rates, when photoinhibition effect was observed in the middle of the day. Variation of estimated average irradiance vs mean incident irradiance showed two effects: first, the outdoor cultures are adapted to average irradiance, and second, simultaneous photolimitation and photoinhibition took place at all assayed culture conditions, the extent of this phenomena being a function of the (incident)(1) irradiance and light regime inside the culture. Productivity ranged between 0.50 and 2.04 g L(-1) d(-1) in the tubular photobioreactor with the lower surface/volume ratio (S/V = 77.5 m(-)1) and between 1.08 and 2.76 g L(-1) d(-1) in the other (S/V = 122.0 m(-1)). The optimum dilution rate was 0.040 h(-1) in both reactors. Night-time biomass losses were a function of the average irradiance inside the culture, being lower in TPB0.03 than TPB0.06, due to a better light regime in the first. In bath photobioreactors, biomass night losses strongly decreased when the photoinhibition effect was pronounced. However, net biomass productivity also decreased due to lower biomass generation during the day. Thus, optimum culture conditions were obtained when photolimitation and photoinhibition were balanced.