Oxygen Balanced Mixotrophy under Day–Night Cycles

Abstract

Using sunlight to fuel photosynthesis exposes microalgae to day–night cycles. Under day–night cycles microalgae tend to synchronize their metabolism by optimizing light utilization during daytime. During night storage compounds are consumed, leading to biomass losses and demand of O2. We investigated “oxygen balanced” mixotrophy under 14:10 day/night cycles. In this mixotrophic setup, photosynthetic O2 production was balanced by respiratory oxygen consumption and CO2 required for photosynthesis was provided by aerobic conversion of acetic acid. This strategy allowed operation of the reactor without any gas–liquid exchange during daytime. Under these conditions Chlorella sorokiniana SAG 211/8K converted 96% of the substrate into biomass. Mixotrophic cultivation did not affect the photosystem II maximum quantum yield (Fv/Fm) or pigment contents of the microalgal cells. Mixotrophic biomass contained 50% w/w of protein and 7.3 mg g–1 of lutein. Acetic acid feeding was discontinued at night and aeration initiated. Respiration was monitored by online off-gas analysis and O2 consumption and CO2 production rates were determined. Biomass night losses were around 7% on carbon basis with no significant difference between mixotrophic and photoautotrophic cultures. Over 24 h, the mixotrophic culture required 61 times less gaseous substrate and its biomass productivity was doubled compared to the photoautotrophic counterpart.