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Light-regulated movement patterns of hydrogen-producing green algae could improve photobioreactor design

Researchers from the University of Bayreuth and the Max Planck Institute for Dynamics and Self-Organization in Göttingen have investigated the movement patterns of unicellular, hydrogen-producing green algae under different light intensities. Their findings will contribute to optimizing the use of these microorganisms in biotechnological applications, such as the production of renewable energy sources.

The team has its results in PNAS.

Photosynthetic microorganisms use light as an energy source and function as living miniature factories that produce oxygen and store carbon while converting sunlight into chemical energy. In addition to their ecological role as carbon sinks, these microorganisms are also crucial for sustainable technological applications in climate-neutral industries.

They are used, for instance, in so-called photobioreactors to selectively produce chemical compounds for renewable biofuels and hydrogen for fuel cells. However, environmental conditions influence the motility of individual microorganisms, which in turn affects the movement of the entire community. A better understanding of these movement patterns will help improve the design of efficient photobioreactors.

Photosynthetic microorganisms such as the unicellular green alga Chlamydomonas are true masters of adaptation. In the absence of light and oxygen, the metabolism of this alga switches to a kind of energy-saving mode, in which molecular hydrogen is produced as a by-product.

The intensity of light directly regulates the motility of individual cells: the stronger the light intensity, the faster the cells move in water; the weaker the light, the slower they swim. This reveals a direct correlation between light intensity and swimming speed.

Until now, however, the impact of individual movement on the collective swimming behavior of an entire population of unicellular organisms had not been known.

Researchers led by Prof. Dr. Oliver Bäumchen, chair of Experimental Â鶹ÒùÔºics V at the University of Bayreuth, have examined the movement of individual Chlamydomonas cells and their effect on a so-called Chlamydomonas suspension—a microbial community that swims in an aqueous environment.

Their study has shown that Chlamydomonas cells are more likely to be found near the water surface under high light intensity rather than at lower depths.

"This behavior is due to the microorganisms' tendency to move against gravity. In a natural body of water, this provides an evolutionary advantage, as unicellular organisms at the surface have better access to light than those at greater depths," explains Bäumchen.

Furthermore, the research team discovered that as photosynthetic activity and, consequently, the motility of individual cells decrease, directed currents emerge within 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 deficiency and could potentially help the microbial collective to expand its exploration of its natural habitat in search of better conditions," says Bäumchen.