CO2 Conversion Into Thermoplastic Starch By The Microalga Chlorella vulgaris

Keywords: Microalgae, Chlorella vulgaris, Thermoplastic starch, Solar energy, Bioplastic, Carbon dioxide

Chlorella vulgaris as a photosynthetic vector for converting solar energy and CO₂ into starch

Microalgae are unicellular organisms capable of rapidly adapting their metabolism to environmental conditions, which allows them to achieve both high area productivities and starch contents under natural light. While conventional photosynthetic crops such as maize and rice reach maximum starch productivities of 10 t/ha/year, this metabolic flexibility enables potential higher area productivities.

With Chlorella vulgaris, a green microalga species commonly used as a food supplement, we notably obtained a productivity of 0.43 g/L/day in total sugars (mainly starch). Based on extrapolation assumptions (150 production days per year, solar resources from Southern France, etc.), this leads to an annual area sugar productivity of over 20 t/ha/year. Beyond this potential gain in productivity, microalgae offer the advantage of not mobilizing arable land for the manufacture of thermoplastic starch.

Producing, extracting, and transforming Chlorella vulgaris starch into bioplastic

We optimized the production of Chlorella vulgaris starch under natural light at the pilot scale (180 L). During cultivation at low concentration (<1 gDM/L) over three days of nitrogen stress, approximately 3% of incident solar energy (including infrared) is converted into sugars, primarily in the form of starch.

To manufacture a bioplastic, the cells are then lysed at high pressure to separate the denser starch granules from other cellular components (mainly proteins and lipids). After rinsing and drying, the granules are formulated with glycerol (a plasticizer) in a twin-screw extruder at 120-140°C. The resulting microalgae-based TPS was compared via tensile tests to a commercial TPS and a reference TPS made from potato starch.

Structural characterizations of Chlorella vulgaris starch granules reveal a structure close to that of cereal starch (Type A with an inter-lamellar distance of 9.5 nm for amylopectin, and an amylose content of around 10-15%). However, these granules have a small average diameter (1.5 µm). This latter property, which allows for excellent dispersion, is rather unique compared to conventional starch sources.

In our trials, the TPS produced from chlorella is found less stiff than the two selected reference materials, while having a higher elongation at break. These characteristics are attributed to imperfect starch purification, a classic bias for a process carried out at a small scale. To reach a mass market such as bioplastics, biorefinery and co-valorization schemes must still be developed, as is the case for conventional starch sources. This small-grain starch could first find outlets as a texturing agent in cosmetics or nutraceuticals. Finally, transferring the process to a marine strain using open culture systems is a promising way to reduce its environmental impact and production costs.

Scientific references to go further

1. Microalgae, sunlight, and starch: low cell concentration is optimal for outdoor production under nutrient stress. – A. Six, P. Chambonnière, P. Alvarez, S. Fon-Sing, C. Lancelon-Pin, J.-L. Putaux, J.-F. Sassi, Y. Li-Beisson, G. Fleury. Bioresource Technology, 2026, 443, 133840. https://doi.org/10.1016/j.biortech.2025.133840

2. From raw microalgae to bioplastics: Conversion of Chlorella vulgaris starch granules into thermoplastic starch. – A. Six, D. Dauvillée, C. Lancelon-Pin, A. Dimitriades-Lemaire, A. Compadre, C. Dubreuil, P. Alvarez, J.-F. Sassi, Y. Li-Beisson, J.-L. Putaux, N. Le Moigne, G. Fleury. Carbohydrate Polymers 2024, 122342. https://doi.org/10.1016/j.carbpol.2024.122342

3. Red light induces starch accumulation in Chlorella vulgaris without affecting photosynthesis efficiency, unlike abiotic stress. – A. Six, A. Dimitriades-Lemaire, C. Lancelon-Pin, J.-L. Putaux, D. Dauvillée, D. Petroutsos, P. Alvarez Diaz, J.-F. Sassi, Y. Li-Beisson, G. Fleury. Algal Research 2024, 80, 103515. https://doi.org/10.1016/j.algal.2024.103515

Academic Partners

• Aix Marseille Univ., CEA, CNRS, DRF/BIAM/EBMP, Institute of Bioscience and Biotechnology of Aix Marseille, UMR72565, CEA Cadarache, Saint Paul-Lez-Durance 13108, France

• IMT Mines Alès, Polymers Composites and Hybrids (PCH), Alès, France

• Univ. Grenoble Alpes, CNRS, Grenoble F-38000 CERMAV, France

• Univ. Lille, CNRS, UMR 8576 - UGSF - Unité de Glycobiologie Structurale et Fonctionnelle, F-59000 Lille, France

• Uppsala Univ., Dep. Of Organismal Biology, Petroutsos lab, SE-751 05 Uppsala, Sweden