Increased consumer awareness for healthier and more sustainable products has driven the search for naturally sourced compounds as substitutes for chemically synthesized counterparts. Research on pigments of natural origin, such as carotenoids, particularly lutein, has been increasing for over three decades. Lutein is recognized for its antioxidant and photoprotective activity. Its ability to cross the blood-brain barrier allows it to act at the eye and brain level and has been linked to benefits for vision, cognitive function and other conditions. While marigold flower is positioned as the only crop from which lutein is extracted from and commercialized, microalgae are proposed as an alternative with several advantages over this terrestrial crop. The main barrier to scaling up lutein production from microalgae to the commercial level is the low productivity compared to the high costs. This review explores strategies to enhance lutein production in microalgae by emphasizing the overall productivity over lutein content alone. Evaluation of how culture parameters, such as light quality, nitrogen sufficiency, temperature and even stress factors, affect lutein content and biomass development in batch phototrophic cultures was performed. Overall, the total lutein production remains low under this metabolic regime due to the low biomass productivity of photosynthetic batch cultures. For this reason, we describe findings on microalgal cultures grown under different metabolic regimes and culture protocols (fed-batch, pulse-feed, semi-batch, semi-continuous, continuous). After a careful literature examination, two-step heterotrophic or mixotrophic cultivation strategies are suggested to surpass the lutein productivity achieved in single-step photosynthetic cultures. Furthermore, this review highlights the urgent need to develop technical feasibility studies at a pilot scale for these cultivation strategies, which will strengthen the necessary techno-economic analyses to drive their commercial production.

It is often read that industrial microalgal biotechnology could contribute to carbon capture through photosynthesis. While technically accurate, this claim is rarely supported by sound figures nor put in regard to the carbon emissions associated with said processes. In this view, this work provides a quantitative assessment of the extent microalgal processes compensation for their carbon dioxide emissions. To do so, microalgae were cultivated under photolimited conditions. Their growth dynamic and photosynthetic apparatus status were monitored by daily cell density measurement and fluorescence assays. Ultimate analyses were used to determine microalgal carbon content. Simultaneously, the power consumption of the process was recorded, and the associated carbon dioxide emissions were computed using European electrical production carbon intensity. All in all, the recorded values confirmed microalgae growth under good physiological conditions and allowed computing the carbon capture rate, the energy storing rate, and the carbon dioxide emissions of the process. The process captured 0.72 ± 0.19 gCO₂/day while emitting 182 gCO₂/day, on average (over 15 days). The photoconversion efficiency was 4.34 ± 0.68%. Even if it were highly optimized (red/blue LED instead of white, for example), the process could only capture 1.02 ± 0.40% of its emissions. From these figures, the claim stating that a biotechnological microalgal production process could partly compensate for its emission seems rather bold. Authors should, therefore, emphasize other ecosystemic benefits of microalgal cultivation, such as phosphorous intake. Finally, we were also able to evaluate Chlorella vulgaris light and dark respiration (0.0388 ± 0.00179 day⁻¹ and 7.42 × 10⁻³ ± 3.33 × 10⁻³ day⁻¹), which could help to assess carbon emission by biomass respiratory activity.

Phytohormones have the potential to improve microalgae culture productivity. Still, not every phytohormone is water-soluble, and most have to be dissolved in organic solvents before being delivered to the cells. Unfortunately, most studies neglect or do not report the effect of the solvent. This is a problem as the solvent itself can have an effect, and not accounting for it would lead to confounding and potentially erroneous interpretations. Consequently, this work evaluates the effects of classical solvents (methanol, ethanol, DMSO, and hexane) in the context of phytohormone delivery. Thus, Chlorella vulgaris was exposed to different dosages of those solvents. The cultures were monitored for three days using optical density to evaluate growth, the OJIP test to assess photosynthetic apparatus status, and flow cytometry to measure viability. Individual outputs and integrated biomarkers were used to analyze the results. Overall, ethanol triggers cell flocculation for the tested concentrations (as low as 0.1%vol) and lysis above 0.5%vol. Methanol also led to flocculation above 0.5%vol, but started to impair viability from 0.1%vol on. DMSO and hexane appear as viable solvents, with no deviation from control for 0.25 and 0.1%vol dosage. Their dosages can be increased to 1%vol while still claiming admissibly perturbed cultures, as absolute values remain in line with control, but dispersion increases. Finally, a validation run was carried out. DMSO-delivered salicylic acid was used to modify Chlorella vulgaris lutein production. Results confirmed the efficiency of salicylic acid and the safety of DMSO at the chosen dosage.