Dark grown chlorella kessleri fed corn, sorghum and lignocellulosic hydrosylates for algal biodiesel production
Biodiesel production by microalgae is typically driven by photosynthetic light harvesting. Production in open ponds is hampered by high water requirements and contamination. Photobioreactor are highly engineered systems with high capital costs. Both approaches are limited by a requirement for high surface area-to-volume ratios that promote light penetration. The current study avoids these issues by growing algae heterotrophically. If algae are grown in the dark, existing bioethanol facilities may be used to co-produce, or readily be changed to produce, biodiesel. Here we fed Chlorella kessleri grown in the dark with the same corn and sorghum hydrosylates used for bioethanol production. Chlorella cultures also were grown heterotrophically in the dark on pure sugars (fructose, glucose, sucrose and a mixture of three sugars) in shake-flasks or continuously sparged and stirred bioreactors. The rate of growth in heterotrophic cultures was comparable to light-grown autotrophic cultures and mixotrophic cultures supplemented with exogenously added sugars in the light. Strong heterotrophic growth was observed for Chlorella maintained heterotrophically on corn mash, sweet sorghum juice, and sorghum mash prepared by hydrolysis of bulk grains. When these bioethanol feedstocks were added to a final concentration of 1% sugar, growth rates were comparable to pure sugar substrates. Biodiesel production was low in these experiments since no attempt was made to promote nitrogen starvation. The current study suggests that algal biodiesel production may be possible during heterotrophic growth in the dark on sugar feedstocks already in use by the biofuel industry. This provides new opportunities for flexible production of renewable liquid fuels using existing infrastructure.