Euglenoids show mixotrophy because they
Euglenoids exemplify mixotrophy by combining phototrophic and heterotrophic nutrition modes within single cell demonstrating metabolic versatility. In illumination they utilize chloroplasts for oxygenic photosynthesis producing paramylon, while in darkness they lose chlorophyll and switch to absorbing dissolved organics or engulfing bacteria via cytostome. Pellicle rather than rigid cell wall permits flexibility for phagocytosis. This plasticity arises from secondary endosymbiotic acquisition of green algal plastid retaining photosynthetic genes while preserving ancestral phagotrophic machiner
Ref: Campbell Biology 12th ed., Chapter 28: Mixotrophy Euglenoids; NCBI Bookshelf, Protist Metabolism - Euglena Physiology