Toward the sustainable cultivation of microalgae to produce renewable biofuels and fine chemicals

Microalgae are considered as renewable feedstock for the production of next-generation biofuels. However to achieve the financial sustainability to the algal biofuel production, it will be necessary to integrate it with the processing of high-value products in the biorefinery concept. Carotenoids have been proposed as added-value compounds that could contribute to make microalgal biofuel production economically feasible. Therefore, the viability and sustainability of extracting carotenoids before the hydrothermal treatment of the remaining biomass to produce syngas were investigated. On the other hand, harvesting microalgae cells remains a technical challenge and typically contributes to 20-30% of biomass production costs and represents more than 50% of the total cost of algal biofuels. The potential of co-culture of filamentous fungal species with microalgae in a lichenization process as a strategy to reduce the cost and energy consumption of harvesting and of the whole process seems promising and was thus investigated. In submerged cultures, filamentous microorganisms actually aggregate and grow as loosely packed pellets or compact granules. Microalgae cells were immobilized in these pellets and easily removed as an aggregate with the fungal cells. Pellet formation is strain specific and highly dependent on operational conditions during cultivation. This study especially was focused on lichen pellet formation during the co-culture of Chlorella sorokiniana and of an unidentified filamentous fungus, which was eventually characterized. While algae growth was optimal between pH 6 and 10, the highest pellet formation was observed in the pH range of 4-7, thus requiring a strict control of pH during the whole cultivation. The effect of such a co-cultivation on the production of added-value chemicals (carotenoids) and on the biofuel potential of the remaining biomass is under evaluation, and will be compared to results obtained with cultivation of microalgae only.