Microalgae screening for heterotrophic and mixotrophic growth on butyrate

Volatile fatty acids (VFAs) produced by fermentative bacteria can be used instead of high value carbohydrates like glucose to reduce the costs associated to mixotrophic microalgal cultivation. This process is however limited by a poor assimilation of butyrate and a low tolerance of microalgae to high VFA concentrations. In this study, 10 microalgae strains, isolated from local natural environments or selected from culture collections were screened for their ability to grow on either acetate or butyrate. The non-photosynthetic Chlorophyte Polytomella sp. was found to be the fastest growing strain on both VFAs, exhibiting a growth rate of 4.0 d(-1) on acetate and 2.5 d(-1) on butyrate. The tolerance of the different strains to concentrated VFA was further assessed using increasing acid concentrations (1.5-60 g.L-1 for acetate and 1-40 g.L-1 for butyrate). Polytomella sp. appeared as the most resistant, being able to grow up to 38 g.L-1 on acetic acid and to 18 g.L-1 on butyric acid with no inhibitory effect. Finally, biomass yield and productivity, as well as lipid and carbohydrate yields were assessed on a synthetic VFAs mixture (0.8 g.L-1 acetate and 1.2 g.L-1 butyrate) for the most promising strains. Polytomella sp. and Euglena gracilis exhibited the highest carbohydrate yield (respectively 0.65 and 0.58 g.g(-1)) while lipid yield was maximum for Chlorella sorokiniana (0.42 g.g(-1)). This work provides new insights in the trophic metabolism of microalgae of highly divergent lineages using VFAs as substrates.