Photosynthesis in Eukaryotes

The photosynthesizing ability of eukaryotes was made possible by one or more endosymbiotic associations between heterotrophic eukaryotes and photosynthetic prokaryotes (or their descendents). Several primary symbioses occurred between eukaryotes and blue green algae. In one of the lineages, the photosynthetic organism lost much of its genetic independence and became functionally and genetically integrated as plastids – chloroplasts within the host cell. At least two types of protists – chloroarachniophytes and cryptomonads –acquired 'plastids' by forming symbioses with eukaryotic algae. Such acquisitions are referred to as secondary symbioses.

Autotrophic prokaryotes derive all their carbon from inorganic sources, and photoautotrophs utilize light energy (anaerobic Chromatium movie and aerobic Cyanobacteria). Some prokaryotes are heterotrophic, utilizing organic substance as a source of carbon, and the purple and green photosynthetic bacteria obtain energy from light (Rhodospirillum rubrum movie).

Photosynthetic bacteria include: Cyanobacteria, green filamentous, green sulfur, prochlorophytes, purple sulfur, and purple nonsulfur bacteria.

Oxygenic photosynthesis, utilizing H2O as electron donor : Cyanobacteria, prochlorophytes. The Cyanobacteria, of course, are considered both the stimulus for and the endosymbionts in chloroplasts.

Nonoxygenic photosynthesis utilizing S– or So or H2 as electron donor:green filamentous, green sulfur, purple sulfur, and purple nonsulfur