Under what environmental condition would we expect euglena to ingest food rather than produce food via photosynthesis?

When acting as a heterotroph (animal), the Euglena surrounds a particle of food and consumes it by phagocytosis. When acting as an autotroph, the Euglena uses chloroplasts to produce sugars by photosynthesis.

The number and shape of chloroplasts within Euglena varies greatly. Euglena are able to move through aquatic environments by using a large flagellum for locomotion. To detect light, the cell has an eyespot, a primitive organelle that filters sunlight into light-detecting, photo-sensitive structures at the base of the flagellum; allowing only certain wavelengths of light to hit it.

This photo-sensitive area detects the light that is able to be transmitted through the eyespot. When such light is detected, the Euglena may shift its position to get better photosynthesis.

The motility of Euglena also allows hunting. Most Euglena are considered mixotrophs: autotrophs in sunlight and heterotrophs in the dark. Euglena do not have plant cell walls, but have a pellicle instead. The pellicle is made of protein bands that spiral down the length of the Euglena and lie beneath the plasma membrane.[2]

Euglena can survive in fresh and salt water. In low moisture conditions, Euglena forms a protective wall around itself and lies dormant as a spore until environmental conditions improve. Euglena can also survive in the dark by storing paramylon granules inside the chloroplast.