The green microalga, Haematococcus pluvialis, is currently cultivated for natural astaxanthin in suspended systems. Immobilised cultivation in a twin-layer (TL) porous substrate bioreactor is a potential revolution in microalgal biotechnology worldwide. For the first time in Vietnam, small-scale (0.05 m2) and large-scale (2 m2) biofilm-based photobioreactor systems arranged at an angle of 150 were successfully designed, assembled, and operated; the temperature, humidity, air, and light conditions for H. pluvialiscultivation were successfully controlled. Studies were conducted of both systems to determine theoptimal storage time of algae after harvest from suspension before inoculation into the TL system, carbon dioxide supply method, light intensity, and initial cell density. In the 0.05 m2 and 2 m2 systems, dry biomass productivity reached 12 g m-2 d-1 (3% astaxanthin content in the dry biomass) and 11.25 g m-2 d-1 (2.8% astaxanthin) after 10 days of cultivation. The 2 m2biofilm-based photobioreactor system provides many advantages in scaling up astaxanthin production from H. pluvialis.

The microalga Haematococcus pluvialis is mainly cultivated in suspended systems for astaxanthin production. Immobilized cultivation on a Twin-Layer porous substrate photobioreactor (TL-PSBR) has recently shown promise as an alternative approach. In Vietnam, a TL-PSBR was constructed as a low-angle (15 °) horizontal system to study the cultivation of H. pluvialis for astaxanthin production. In this study, the biomass and astaxanthin productivities and astaxanthin content in the dry biomass were determined using different initial biomass (inoculum) densities (from 2.5 to 10 g dry weight m−2), different storage times of the initial biomass at 4 °C (24, 72, 120 and 168 h) and different light intensities (300–1000 µmol photons m−2 s−1). The optimal initial biomass density at light intensities between 400–600 µmol photons−2 s−1 was 5–7.5 g m−2. Algae stored for 24 h after harvest from suspension for immobilization on the TL-PSBR yielded the highest biomass and astaxanthin productivities, 8.7 g m−2 d−1 and 170 mg m−2 d−1, respectively; longer storage periods decreased productivity. Biomass and astaxanthin productivities were largely independent of light intensity between 300–1000 µmol photons m−2 s−1 but the efficiency of light use per mole photons was highest between 300–500 µmol photons m−2 s−1. The astaxanthin content in the dry biomass varied between 2–3% (w/w). Efficient supply of CO2 to the culture medium remains a task for future improvements of angled TL-PSBRs.

In the production of astaxanthin from Haematococcus pluvialis, the process of growing algal biomass in the vegetative green stage is an indispensable step in both suspended and immobilized cultivations. The green algal biomass is usually cultured in a suspension under a low light intensity. However, for astaxanthin accumulation, the microalgae need to be centrifuged and transferred to a new medium or culture system, a significant difficulty when upscaling astaxanthin production. In this research, a small-scale angled twin-layer porous substrate photobioreactor (TL-PSBR) was used to cultivate green stage biomass of H. pluvialis. Under low light intensities of 20–80 µmol photons m−2·s−1, algae in the biofilm consisted exclusively of non-motile vegetative cells (green palmella cells) after ten days of culturing. The optimal initial biomass density was 6.5 g·m−2, and the dry biomass productivity at a light intensity of 80 µmol photons m−2·s−1 was 6.5 g·m−2·d−1. The green stage biomass of H. pluvialis created in this small-scale angled TL-PSBR can be easily harvested and directly used as the source of material for the inoculation of a pilot-scale TL-PSBR for the production of astaxanthin.

The production of natural astaxanthin is usually accomplished by suspended cultivation of the microalgae Haematococcus pluvialis. In this study, for the purpose of cost reduction, H. pluvialis is grown in pilot scale angled twin-layer porous substrate photobioreactors with light energy from red/blue LEDs that can produce red light, blue light, or a combination of blue-red light. The total dry biomass of the microalgae reached a maximum of 40.74 g.m-2under blue-red LEDs. The early initiation of blue-red LED illumination (on day 2) after algae immobilization in the biofilm resulted in the highest accumulation of astaxanthin in the dry biomass, which reached a maximum of 1.3% (w/w) after 10 d of culture.

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