Researchers from the University of Bayreuth and the Max Planck Institute for Dynamics and Self-Organization in Göttingen have investigated the movement patterns of single-celled hydrogen-producing green algae under different light intensities. The results will help to improve the use of these microorganisms in biotechnological applications such as the production of renewable energy carriers.
Photosynthetic microorganisms use light as an energy source and are living miniature factories that produce oxygen and store carbon while converting sunlight into chemical energy. Besides their ecological importance as carbon sinks, these microorganisms also play a major role in sustainable technological applications for a climate-neutral industry: They are used, for example, in so-called photobioreactors to produce targeted chemical compounds for renewable biofuels and hydrogen for fuel cells. Environmental conditions affect the motility of individual microorganisms, which in turn impacts the movement of the entire community of single-celled organisms. A better understanding of these movement patterns thus contributes to the improved design of efficient photobioreactors.
Photosynthetic microorganisms like the single-celled green alga Chlamydomonas are true masters of adaptation. In the absence of light and oxygen, the alga's metabolism switches to a kind of energy-saving mode, producing molecular hydrogen as a byproduct. Light intensity controls the individual motility of each cell: the stronger the light intensity, the faster the individual cells move in the water; the less light, the slower they move. Thus, a direct correlation exists between light intensity and swimming speed. However, the effects of this individual movement on the collective swimming motion of the entire population of single-celled organisms were previously unknown. Therefore, researchers led by Prof. Dr. Oliver Bäumchen from the Chair of Experimental Physics V at the University of Bayreuth investigated the movement of individual Chlamydomonas cells and their effect on a so-called Chlamydomonas suspension – a microbial community floating in an aqueous environment.
Their study showed that Chlamydomonas cells tend to be found near the water's surface rather than deeper down under high light intensity. "This behavior is due to the microorganisms' tendency to move against gravity. In a natural body of water, this offers an evolutionary advantage, as the single-celled organisms have a better chance of receiving sufficient light at the surface than at greater depths," Bäumchen explained.
The research team also discovered that as photosynthetic activity and thus the motility of individual cells decrease, directional currents emerge throughout the entire cell community. This collective motility manifests as a regular three-dimensional flow pattern in which flow rates and cell distributions are directly controlled by light intensity. "These currents arise under the extremely unfavorable conditions of simultaneous light and oxygen deprivation and could therefore potentially help the microbial collective expand its exploration of their natural habitat in search of better conditions," says Bäumchen.
original publication
Metabolic activity controls the emergence of coherent flows in microbial suspensions. Alexandros A. Fragkopoulos, Florian Böhme, Nicole Drewes, Oliver Bäumchen. PNAS (2025)
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