Consortia of the oleaginous yeast Lipomyces starkeyi and the microalga Chlamydomonas reinhardtii for microbial oil production
Résumé
Renewable energy is necessary to reduce our dependence on fossil fuels and can help to substantially lower the emission of the greenhouse gas carbon dioxide. A growing effort is being devoted to the use of non-food feedstocks to obtain microbial Single-Cell Oils (SCO) suitable for the production of biodiesel, bioplastics and other products [1]. Lignocellulosic biomass is the most abundant residue of agricultural crops. Various pretreatment methods to hydrolyze the complex fibers produce organic molecules such as glucose, xylose and acetate, which can be used as substrates for oleaginous microorganisms to produce SCO. Yeasts and microalgae have been most often used for the industrial production of SCO [2]. The yeast Lipomyces starkeyi is very efficient in accumulating high levels of lipids [3]. In aerobic conditions it exhibits high growth rates, involving high levels of O2 consumption and CO2 emission. When cultivated in consortium, microalgae can provide O2 to the yeast and consume its CO2 if light is present. This virtuous gas exchange cycle is projected to maximize the yield of biomass and oil production. Questions regarding a yeast-alga system relate to the proper medium and conditions to support growth of the consortium, the growth rates and yields of biomass and lipids, and the extent to which the system is symbiotic/synergistic.
In this work, we have developed a system using L. starkeyi and the green alga Chlamydomonas reinhardtii. The yeast can use the sugars found in lignocellulosic hydrolysate, whereas the alga can use only acetate. Due to this specific complementarity, competition for carbon is very low in these consortia, which may imply higher carbon conversion efficiency. To test the consortium in clearly defined conditions, a synthetic medium was developed that integrates necessary elements of known culture media for both organisms, with the use of pH control. Growth series were done in batch, under constant light, agitation and temperature, in closed bottles to monitor gas exchange. In this model system, symbiotic growth was observed of the consortium with synergistic effects on biomass yield. Similar results were obtained in a system using the hydrolysate of acid-treated wheat straw, except for an increase in lag phase due the presence of phenols. Growth even in nitrogen-flushed bottles confirmed the interdependence of the microalga and the oleaginous yeast. The performance of the consortium under different conditions is discussed in terms of growth rate, substrate assimilation, gas exchange, biomass production and lipid content of the biomass.