Green alga can lose their photosynthetic capability but are sometimes able to adapt to a completely heterotrophic lifestyle and evolve into non-photosynthetic species. Several heterotrophic species exist that are still clearly related to contemporary green algae, the best known case being the colorless Polytomella species and the green alga Chlamydomonas reinhardtii. Striking similarities exist between these Chlorophyte algae, especially in their mitochondria, that clearly show their common ancestry. Major differences reflect the loss of photosynthesis and relate mostly to a highly versatile heterotrophic energy metabolism in the colorless species. The absence of a photosynthetic apparatus in Polytomella greatly simplifies its morphological and metabolic framework and allowed easy isolation and study of mitochondria, which facilitated the study of green Chlorophyte mitochondria. More recently, due to its unparalleled heterotrophic capacities, Polytomella has been used as model for the study of heterotrophic metabolism of volatile fatty acids in the context of environmental biorefinery applications.