Manipulation of trophic capacities in Haematococcus pluvialis enables low-light mediated growth on glucose and astaxanthin formation in the dark

Haematococcus pluvialis is an industrial green microalga, which presents the best photosynthetic source of the keto-carotenoid astaxanthin due to its high content, reaching over 4% of dry weight. Astaxanthin production by Haematococcus is currently achieved by a two-stage photoautotrophic process, which involves the transition of green biomass to nitrogen starvation under high light to induce astaxanthin accumulation. This production mode is associated with high costs, due to relatively low biomass productivities and other factors. The heterotrophic mode of cultivation, based on organic carbon assimilation under controlled conditions, may offer a solution to the disadvantages of phototrophic cultivation. We created transgenic strains of H. pluvialis by transformation of the hexose uptake protein (HUP1) gene from the green microalga Parachlorella kessleri into the nuclear genome. This manipulation resulted in the ability of modified H. pluvialis strains to proliferate in the dark and under very low illumination on nutrient media supplemented with glucose. Furthermore, modified strains were able to grow and produce astaxanthin under specific conditions. All tested transformants were susceptible to the toxic glucose analogue 2‑deoxyglucose, supporting that the foreign glucose carrier was functional. Here, we report the initial characterization of the transformants cultivated on solid and in liquid media, and address the implications of manipulating trophic regimes in H. pluvialis for biotechnological applications.