Key microalgal species are currently being exploited as biomanufacturing platforms using mass cultivation systems. The opportunities to enhance productivity levels or produce non-native compounds are increasing as genetic manipulation and metabolic engineering tools are rapidly advancing. Regardless of the end product, there are both environmental and industrial risks associated to open pond cultivation of mutant microalgal strains. A mutant escape could be detrimental to local biodiversity and increase the risk of algal blooms. Similarly, if the cultivation pond is invaded by a wild-type (WT) microalgae or the mutant reverts to WT phenotypes, productivity could be impacted. To investigate these potential risks, a response surface methodology was applied to determine the competitive outcome of two Chlamydomonas reinhardtii strains, a WT (CC-124) and a high-lipid accumulating mutant (CC-4333), grown in mixotrophic conditions, with differing levels of nitrogen and initial WT to mutant ratios. Results of the growth experiments show that mutant cells have double the exponential growth rate of the WT in monoculture. However, due to a slower transition from lag phase to exponential phase, mutant cells are outcompeted by the WT in every co-culture treatment. This suggests that, under the conditions tested, outdoor cultivation of the C. reinhardtii cell wall-deficient mutant strains does not carry a significant environmental risk to its WT in an escape scenario. Furthermore, lipid results show the mutant strain accumulates over 200% more TAGs per cell, at 50 mg L-1 NH4Cl, compared to the WT, therefore, the fragility of the mutant strain could impact on overall industrial productivity.
Keywords: Chlamydomonas reinhardtii; Competition; Environmental risk; Lipids; Microalgae; Response surface methodology.