Characterization of a Chlamydomonas reinhardtii mutant strain with improved biomass production under low light and mixotrophic conditions

Biophysical and biochemical characteristics of a spontaneous “mutant” strain (IM) of Chlamydomonas reinhardtii were quantified and compared with its progenitor (KO), a “knock-out” mutant with defects in phototaxis, and to its wild-type (WT); defects were shown to be preserved in the IM mutant. Growth curves showed that IM cultivated under mixotrophic conditions (TAP medium) and low light (10 and 20 μmol photons m− 2 s− 1), had 5-27% higher dry cell weight than WT and KO. This advantage was most likely attributable to increased acetate metabolism because it was not observed under purely photoautotrophic conditions using high salt minimal medium. Further characterization of these strains grown under mixotrophic conditions revealed several other unique features for the KO and IM mutant strains. Specifically, the IM and KO cells, grown under 60 μmol photons m− 2 s− 1, showed higher rates of net oxygen evolution and respiration than the WT cells. Further, the slow (minute range) SM rise phase of chlorophyll a fluorescence transient was much reduced in IM cells, which has been ascribed to a regulatory event, labeled as “state 2 to state 1 transition”. Additionally, modulated fluorescence measurements showed that, when the IM strain is grown under low light, non-photochemical quenching of excited chlorophyll rises faster and recovers faster than in the other strains. Finally, compared to the WT, IM cells had a higher amount of metabolites related to carbon metabolism and protection against oxidative stress. These results suggest that the IM strain of C. reinhardtii has unique features that may be advantageous for improving algal biofuel production under mixotrophic conditions, such as algae cultivated in conjunction with wastewater treatment.