A bacterial trap for the search for metabolic virtuosos

Scientists have known for some time that bacteria can move around in aqueous solutions thanks to very fine cilia on their surface. Until now, however, experts have assumed that the microbes are blind to complex polymers. And that they only orientate themselves towards highly diffusible substances such as simple sugars that are very easily metabolised or eaten up.

Conventional wisdom refuted

But now the findings of Roman Stocker's research team at the Department of Civil, Environmental and Geomatic Engineering (D-BAUG) at ETH Zurich have disproved the conventional wisdom: using a microfluidic chip co-developed with their collaborators at UTS Sydney, consisting of a credit card-​​sized plastic plate with small chambers inside, the researchers have shown during field work in the Norwegian Raunefjord that bacterial communities follow the concentration gradient of laminarin and other complex polysaccharides.

Laminarin is found in numerous species of microscopic brown algae and other members of marine phytoplankton. Laminarin contains up to a quarter of the carbon that is bound by photosynthesis in the oceans. "Laminarin is therefore one of the most important food sources for marine bacteria," says Estelle Clerc, postdoctoral fellow in Stocker's research group and first author of the study recently published in external page Nature communications.

Well-developed sensorium

The fact that marine microbes can actively swim towards complex molecules to break them down has not yet been taken into account in models of global carbon fluxes. Their new results might therefore play a role in the future calculation of climate scenarios, says Clerc. But in addition to that, the evidence that microbes have a better developed sensorium than previously assumed gave Clerc the next idea: "Perhaps bacteria also recognise other complex and poorly degradable substances."

To test the idea, the researchers simply had to equip their instrument with such substances – and then release it in the water at various locations (such as in Lake Zurich or in the basin of a wastewater treatment plant). Her initial, still preliminary and unpublished results show that there are indeed bacterial communities in the environment that are attracted to microplastics or pesticide residues, for example.