Euglena gracilis, photosynthetic protozoa, adapt to both aerobic and anaerobic culture conditions. When cells are exposed to anaerobic conditions, storage polysaccharide paramylon is degraded rapidly, and wax ester is produced. This metabolic pathway has been called wax ester “fermentation” for a long time. But recent studies showed fatty acid synthesis during anaerobic wax ester production is coupled with the anaerobic respiratory chain. It was the first finding for all organisms that couples long-chain fatty acid synthesis and anaerobic respiratory chain.