Combined application of exogenous phytohormones and blue light illumination to the marine diatom Phaeodactylum tricornutum

Diatoms constitute a diverse group of photosynthetic microorganisms with great potential for biotechnological applications. Compounds which they can synthesise, such as ω3-fatty acids or carotenoid pigments, can find uses in the pharmacology, nutraceutical or cosmetic sectors. However, a major challenge to date has been to stimulate the synthesis of valuable metabolites without hampering cell growth. In this study, the model microalgal species Phaeodactylum tricornutum (marine diatom) was subjected to blue light illumination and phytohormonal supplementation (gibberellic acid, GA3, and abscisic acid, ABA) as single factors or in combination. P. tricornutum was grown under a batch culture regime for 30 days and treatments were tested at two different stages (day 0 and day 20). The highest yield of dry biomass was obtained when combining blue light and GA3 from day 0 (25 % higher than control) while no relevant variation occurred when applying blue light from day 20. When applied from day 0, blue light led to a reduction in the fucoxanthin and PUFA content of the biomass. Single dose supplementations of the phytohormones seemed less dependent on the time of application. When applied without blue light, GA3 (2 mg/l) increased the fucoxanthin content and yield by 25 and 30 %, respectively. However, the co-supplementation of GA3 and ABA (2 mg/l) was more effective than single applications, significantly boosting fucoxanthin yield up to 1.3 mg/l and EPA proportions by 17 %, compared to the control set. These increments were also associated with enhanced antioxidant activity (60 % increment). These results highlight the importance of optimising the timing of application of treatments to avoid deleterious effects on pigment and PUFA accumulation in cells. Switching to blue light illumination and applying GA3 and ABA later in the growth of the diatom P. tricornutum constitutes a promising strategy to improve its yields in high-value metabolites prior to biomass harvest.