The heterotrophic species of Euglena can either absorb organic compounds from the surrounding water or engulf bacteria and protists by phagocytosis within the food vacuoles.
Euglena obtains nourishment from dead and decaying organic matter(saprotrophic nutrition) in the substrate by secreting digestive enzymes in the absence of light.
Since it can show both autotrophic and heterotrophic(saprotrophic) modes of nutrition, they are referred to as mixotrophic.