Growth of the green microalga Chlorella vulgaris outside a cultivation medium

Microalgae can be used for various applications, such as food, cosmetics or biofuel production. Usually they are cultivated in bioreactors, which makes harvesting and processing strenuous. Growing them adhering on a surface, i.e. as a biofilm, can reduce these problems. On the other hand, introducing substrates into bioreactors may cause additional problems regarding the light intensity on the microalgae. The next logical step would therefore be to let the microalgae grow on substrates outside bioreactors which are regularly wetted, but not constantly submerged in the medium, to improve light intensity and make harvesting easier. Here we report on experiments with the green microalgae Chlorella vulgaris grown on different textile substrates at the air. A polyester (PES) nonwoven as well as a Tencel (cellulose) plush knitted fabric showed the best water retention properties and were thus used for the final tests. C. vulgaris started to grow on these textiles in suspension, before the substrates were transferred to cultivation in the air with regular wetting and defined light. The algae biofilms, measured by areal density, were constantly increasing during 4 weeks of cultivation, showing more algae on the Tencel fabric and similar oxygen production rates of the algae on both substrates. The experiments confirm the possibility to let algae grow outside cultivation medium.