Résumé
Recently, several studies showed the feasibility of coupling hydrogen production by dark fermentation and subsequent lipid accumulation by heterotrophic microalgae growth [1]. Dark fermentation by-products (i.e. volatile fatty acids such as acetate and butyrate) can indeed be used as low cost substrates, instead of glucose, to sustain microalgal heterotrophic growth. Nevertheless, fermentative components in effluents, such as proteins, vitamins and invasive anaerobic bacteria, could impact microalgae growth. The objective of this study is to characterize the growth of one lipid-producing microalgae, Chlorella sorokiniana, on raw effluents. For this purpose, Chlorella sorokiniana (CCAP 211/8K) was cultivated under complete darkness until exhaustion of carbon sources. Raw effluents, containing fermentation bacteria, sterilized effluents and synthetic effluents were tested. Microorganisms’ growth was monitored by microscopic observations and flux cytometer analyses. A model predicting heterotrophic growth and validated on synthetic effluents was used to represent both biomass growth and substrate uptakes [2]. The carbon compound composition of the raw fermentative effluent was solely acetate and butyrate. During the first three days of experiments, acetate was completely uptaken by microalgae whatever the type of effluents. Bacteria growth did not occur during that period. Butyrate was not degraded. When microalgae were grown on synthetic effluent, butyrate was completely exhausted after 20 days of cultivation. After three days of cultivation, the microalgal biomass, grown on raw effluents, decreased rapidly and flocculation of microalgae was observed. Investigation on the reason of such flocculation phenomenon is undergoing. Butyrate was not degraded after 20 days of cultivation on sterilized effluent. Bacteria began to grow on the unsterilized effluent after 4-5 days of aerobic cultivation. Butyrate uptake occurred in this medium after 5 days and was completely exhausted after 10 days of cultivation. Full characterization of the ecosystem is undergoing. This study showed that Chlorella sorokiniana can grow and assimilate acetate even though the effluents were not sterilized. Bacteria growth began after a lag period of 4-5days that is sufficient for the growth of the microalgae. A metabolic shift, from anaerobic to aerobic conditions, could explain this observation. Thanks to this lag period, microalgae were able to grow on acetate. This result shows the feasibility of directly coupling dark fermentation with microalgal heterotrophy, with or without effluent sterilization. Further investigation is required to determine what kind of effluent’s compounds was responsible for microalgae flocculation. A full characterization of bacteria population is undergoing and will give new insights to optimize the coupling of dark fermentation with microalgal heterotrophy.