Phototrophic biofilms – the bio-factories of the future?

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An article by Dr Dorina Strieth and Professor Roland Ulber, Chair of Bioprocess Engineering, TU Kaiserslautern

Can the greenhouse gas CO2 be used to make high-quality and resource-efficient food products, cosmetics and medical drugs? Biofilms comprising microalgae, cyanobacteria and fungi can bring us closer to achieving this goal. "Microalgae" is an umbrella term referring to all microscopically-small microorganisms that use light energy to convert CO2 into oxygen and sugar, i.e. that perform photosynthesis. It is currently estimated, for example, that microalgae are already able to bind roughly 30 % of atmospheric CO2 through photosynthesis. Put simply, they bind CO2 and produce food.

Cyanobacteria such as the organism "Spirulina" are already being used as food supplements today and were cultivated by the Azteks hundreds of years ago. They can also be used as independent fertiliser producers. The pigments contained in the cells are used in the food industry as natural colourants (carotenoid (orange), phycocyanin (blue)), and antimicrobial compounds can form the basis for new medical drugs. Terrestrial cyanobacteria grow on surfaces and form what is known as a "phototrophic biofilm". Phototrophic biofilms are a community of different microorganisms that settle on a surface; driven by light energy, they live within a coating of slime they produce themselves. This coating keeps the biofilm together and protects it against various environmental factors, such as drying out or predators.

We tend to have negative associations when it comes to biofilms because they coat the glass in aquariums, for example, or cause unattractive discoloration on the walls of houses. Consequently, biofilms are regarded as damaging, and are often combated. Biofilm researchers see this very differently, for biofilms adapt quickly to new – and above all extreme – conditions, are hardy and difficult to kill. These are precisely the characteristics that are needed for successful cultivation under difficult environmental conditions.

Cultivation of phototrophic biofilms

It is difficult, if not impossible, to cultivate biofilms in submerged reactors (= vessels for the cultivation of microorganisms; submerged = in solution). The "mist reactors" developed at TU Kaiserslautern supply the biofilm with CO2, various trace elements and water in the form of a nutrient mist. Thus the natural conditions of a desert (very little mist) and of a rain forest (lots of mist) can be reproduced in the reactor. The cultivation of phototrophic biofilms showed that biomass production in these systems is much higher than in conventional reactors. Furthermore, less water, nutrients and energy are needed than in submerged reactors, as these ingredients are sprayed into the reactor in specific quantities as a mist.

The use of phototrophic biofilms in biotechnology could therefore be an innovative way of meeting the growing demand for natural products, while at the same time reducing the use of resources and binding CO2 through photosynthesis. It is possible that humans in the future will eat various products, derived from phototrophic biofilms, that can be produced regionally, ecologically and sustainably in biofilm reactors.