Creating an Improved Diatoxanthin Production Line by Knocking Out CpSRP54 in the zep3 Background in the Marine Diatom Phaeodactylum tricornutum.

Diatoxanthin is a photoprotective carotenoid found in a few groups of microalgae displaying in vitro anti-inflammatory and anti-cancer properties, making it a promising candidate for nutraceutical, pharmaceutical, and cosmetic applications. However, large-scale production is currently nonexistent because of two major challenges: Instability during microalgae harvesting, where diatoxanthin is rapidly converted back to its inactive precursor diadinoxanthin under non-stressful light conditions, and dependence on prolonged exposure to high-intensity light, which is costly and technically challenging during indoor high-cell-density cultivation. The first limitation was previously addressed by knocking out zeaxanthin epoxidase 3 (ZEP3) in the marine diatom Phaeodactylum tricornutum, resulting in a mutant that stabilized diatoxanthin under non-stressful light conditions. Here, we report an improved diatoxanthin production line where both of the described challenges have been overcome. This was achieved by creating P. tricornutum mutants where the phenotype of the zep3 mutant was combined with the light-sensitive phenotype of the chloroplast signal recognition particle 54 (cpsrp54) mutant. Growth rates were maintained at wild-type levels at light intensities ≤ 150 µmol photons m-2 s-1 in the zep3cpsrp54 mutants, but prolonged medium light exposure resulted in a 1.5- and 7-fold increase in diatoxanthin concentration compared with zep3 and wild-type, respectively. When returned to low light, the zep3cpsrp54 cultures retained ~80% of their accumulated diatoxanthin. The improved production lines allow for diatoxanthin accumulation without the use of high-intensity light and with limited loss of diatoxanthin when returned to non-stressful light conditions.