Biophysics of microalgae flotation – FLOTALG

First, analysis of the wall of the green microalga Chlorella vulgaris by XPS and liquid chromatography showed the presence of chitin polymer on the cell surface. On this basis, given the ability of polysaccharides to form homotypic interactions, chitosan was identified as a molecule that could enable cell adhesion. To confirm this, we carried out atomic force microscopy (AFM) experiments to measure interactions between C. vulgaris cells and chitosan at the molecular level. The results showed that chitosan interacts with the cell wall via specific pH-dependent biological interactions, confirming its potential to make bubbles adhesive to cells. To use it to modify the bubble surface, we added a hydrophobic function to chitosan and measured the interactions between the functionalized bubbles and the cells, using a method coupling AFM and microfluidics developed in our team. These results showed that amphiphilic chitosan functionalized on the surface of the bubbles adheres to the cells with a force 10 times greater than that obtained for chitosan. Finally, flotation experiments carried out on microalgae suspensions confirm this result, showing that harvesting efficiency with functionalized bubbles increases significantly (+40%) compared with the use of bare bubbles.

Main results of the project

- New knowledge on the composition and architecture of the cell wall of C. vulgaris, a green microalgae species widely used for biofuel production (Demir-Yilmaz et al., Algal Research, 2023)

- Understanding of the molecular mechanisms involved in the interactions between chitosan and the cell wall of C. vulgaris using atomic force microscopy experiments (Demir et al., ACS Applied BioMaterials, 2023)

- Development of a new method to probe the interactions between micro-sized bubbles and cells at the molecular level based on FluidFM technology, a combination of AFM and microfluidics (Demir et al., Journal of Colloid and Interface Science, 2021)

- Development of an original flotation process with chitosan-functionalized bubbles that is efficient to harvest C. vulgaris cells (Demir-Yilmaz et al., Chmical Engineering Journal, 2023)

This project has demonstrated that bubbles used in flotation processes can be functionalized, enabling better attachment to cells and thus better separation. The next step will be to functionalize the bubbles with a molecule that will allow specific interaction with a cell type, allowing them to be sorted and separated from a complex medium containing different cell types, such as mixed cultures or consortium cultures.

1. Demir-Yilmaz I, Schiavone M, Esvan J, Guiraud P, Formosa-Dague C† (2023) Combining AFM, XPS and chemical hydrolysis to understand the complexity and dynamics of C. vulgaris cell wall composition and architecture, Algal Res, 72, 103102. †corresponding author

2. Demir-Yilmaz I, Ftouhi MS, Balayssac S, Guiraud P, Coudret C, Formosa-Dague C† (2023) Bubble functionalization in flotation process improve microalgae harvesting, Chem Eng J, 452, 139349. †corresponding author

3. Demir-Yilmaz I, Guiraud P, Formosa-Dague C† (2021) The contribution of Atomic Force Microscopy (AFM) in microalgae studies: a review, Algal Res, 60:102506. †corresponding author

4. Demir I, Lüchtefeld I, Lemen C, Dague E, Guiraud P, Zambelli T, Formosa-Dague C† (2021) Probing the interactions between air bubbles and (bio)-interfaces at the molecular scale using FluidFM technology, J Colloid Interface Sci, 604:785-797. †corresponding author

5. Laifa R, Morchain J, Barna L, Guiraud P (2021) A numerical framework to predict the performances of a tubular photobioreactor from operating and sunlight conditions, Algal Res, 60:102550.

6. Demir I, Blockx J, Dague E, Guiraud P, Thielemans W, Muylaert K, Formosa-Dague C† (2020) Nanoscale evidences unravel microalgae flocculation mechanism induced by chitosan, ACS Appl Bio Mater, 3(12):8446-8459. †corresponding author

7. Demir I, Besson A, Guiraud P, Formosa-Dague C† (2020) Towards a better understanding of microalgae natural flocculation mechanisms to enhance flotation harvesting efficiency, Water Sci Technol, 82(6):1009-1024. †corresponding author

8. Vergnes JB, Gernigon V, Guiraud P, Formosa-Dague C† (2019) Bicarbonate concentration induces production of exopolysaccharides by Arthrospira platensis that mediate bio-flocculation and enhance flotation harvesting efficiency, ACS Sustain Chem Eng, 7(16):13796-13804. †corresponding author

In the context of climate change and increasing energy needs of the world population, the global interest for sustainable sources to produce energy is growing. One promising resource for biofuel production is microalgae, although their industrial use is limited by the lack of efficient harvesting techniques. Assisted flotation represents a promising harvesting technique that consists in air dispersed into microbubbles rising through a microalgae suspension. As a result, microalgae cells get attached to gaz-liquid interfaces and are carried out and accumulated on the surface, without being damaged. Flotation is thus a relatively rapid operation that needs low space, has moderate operational costs, and that could thus overcome the bottleneck of feasible microalgal biofuel production. However, the efficiency of this method is limited by the fact that the interaction between the bubbles and the cells is generally repulsive, due to the negative surface charge of the cells and the bubbles in water, and the low hydrophobicity of the algal cells. The goal of this project is to improve the efficiency of flotation, in order to better exploit the potential of the microalgal bioressource. Fundamental knowledge at the molecular and cellular scales will be acquired on the cell wall of microalgae and on the molecular mechanisms underlying its adhesion to gaz/liquid interfaces, using advanced force spectroscopy techniques such as optical tweezers and FluidFM technology. These data will then be further used to identify adhesive components promoting cell aggregation at the cells interface, and functionalize them at the surface of bubbles, thus improving flotation efficiency without altering the cells. Finally the overall evaluation of the efficiency of the functionalized bubbles for microalgae flotation will be evaluated and compared to other harvesting techniques. The results obtained in this project will allow to generate fundamental knowledge on the cell wall of microalgae and on the molecular mechanisms underlying their adhesion to gaz-liquid interfaces. These are not the only benefits of this project, as it will also provide a new technological solution to measure the interactions between fluid and biological interfaces, as well as a way to increase the efficiency of flotation process. Therefore, understanding the biophysics of microalgae flotation will open up new strategies to transform the microalgal biomass into 3rd generation biofuels.