The photosynthetic efficiency of Chlorella biomass growth with reference to solar energy utilisation

The photosynthetic efficiency (PE) of a growing algal culture was determined from the growth yield ( Y ), that is, biomass produced/light absorbed and the calorific value of the biomass ( k ); PE = kY . To obtain the maximum photosynthetic efficiency the algae were grown in light‐limited chemostat cultures in urea‐mineral salts media plus CO 2 and steady‐states were obtained at different specific growth rates. With a given light input the biomass output rate was independent of the specific growth rate up to at least 70% of the maximum specific growth rate. The photosynthetic efficiency was independent of the incident light intensity over the range studied, 5.3–21.3 W m −2 . The light source had a spectral range of 400–700 nm and its mean wavelength was assumed to be 575 nm. The values of the maximum growth yields ( Y G , g dry weight kJ −1 ) were 0.0153 for the Sorokin Chlorella strain 211/8k and 0.0206 for a newly selected mixed culture MA003 which consisted of an alga and three species of heterotrophic bacteria. The maintenance energy ( m ) of the mixed culture MA003 was in the range 0–0.32 kJ g −1 dry weight h −1 and the specific maintenance rate ( mY G ) was in the range 0–0.0066 h −1 . In Chlorella strain 211/8k the maximum PE was 34.7% which corresponds to a quantum demand ( n ) of 6.6 per O 2 molecule evolved. In the mixed culture MA003 the maximum PE was 46.8% with 95% confidence limits, 42.7–51.5. This PE value corresponds to a quantum demand ( n ) of 4.8 per O 2 molecule evolved. These results call in question the current model of photosynthesis which predicts that the maximum PE with absorbed light of mean wavelength 575 nm should not exceed 29% and the minimum quantum demand, n = 8. From our results with culture MA003 it is deduced that the maximum practicable storage of total solar energy by algal biomass growth in vitro is 18%.