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 behaviour of an entire population of unicellular organisms had not been known. This is why researchers led by Prof. Dr Oliver Bäumchen, Chair of Experimental Physics 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 behaviour 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 unfavourable 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.
