In the last decade, a couple of successful industrial heterotrophic microalgal productions have been established. Many microalgae can assimilate organic substances to cover variable part of their carbon and energy requirements. To cover their entire energy requirements and be able to grow in complete darkness,  the organic substances are respired  in mitochondria with oxygen as electron acceptor, a process similar to the respiration in animal cells. Some algae, such as Chlamydomonas (Fig 1), may also use a slightly modified process to respire acetate, the so-called glyoxylate pathway.  In Chlamydomonas the process is regulated so that it takes place only in the dark.

Chlamydomonas is also able to ferment starch, which was produced during the day, into ethanol under anaerobic conditions. So far, fermentation has been demonstrated only in a few microalgae species (Fig. 2). Algae capable of growing in the dark, are called true heterotrophs, while algae that require light but are able to supplement the metabolism with organic substances, are called mixotrophs.  Very few species, however, can grow in darkness in addition being able to grow also as true autotrophs – i.e., in light without organic supplements.

Organic substances that may be respired, include glucose, acetate, glycerol, TCA cycle intermediates (for example citric acid) and a number of amino acids. Only glucose, acetate and glycerol may play a role as substrates in industrial productions.  These substances are small molecules and algae are not normally able to metabolize large molecules such as proteins or even complex particles – but some algae,  most notably in the classes Dinophyceans and  Prymnesiophyceans are able to engulf large molecules and particles in a process called pinocytosis or fagocytosis, depending on the size of particles. 
Some Dinophyceans have lost their ability to form chloroplasts, but are able to retain functional chlorplast from ingested algae!

A good place to look for heterotrophic species is among decaying seaweed where the decomposition processes result in a rich variety of dissolved organic substances.

Figure 1: Chlamydomonas – a genus of algae with many talents !	Figure 2: Mixotrophic growth in a fermentor