Scientists Unravel Ancient Evolutionary History Of Photosynthesis

In a paper in the Sept. 8 issue of the journal Science, a team of biologists lead by Carl E. Bauer, Clyde Culbertson Professor of Biology at Indiana University, reports the results of their study of the evolution of photosynthesis.

In photosynthesis, green plants use the energy of sunlight to convert carbon dioxide into hydrocarbons that promote plant growth while generating oxygen from water and releasing the oxygen to the atmosphere. All animals including humans depend either directly or indirectly on this source of food and oxygen. Consequently, photosynthesis is considered the most important chemical process on Earth.

Scientists agree that photosynthesis originated in bacteria, with some bacteria containing photosystems that release oxygen in a way very similar to that found in green plants today, and some other bacteria containing simplified photosystems that do not release oxygen. What was not clear is which species of bacteria contains the most ancient photosystem and how photosynthesis in green plants evolved from photosynthesis in bacteria.

By generating a large new molecular data set, Bauer's group, which includes IU postdoctoral fellow Jin Xiong and IU doctoral student William Fischer, has determined that non-oxygen-producing bacterial species such as the purple and green bacteria are the most ancient photosynthetic bacteria. Another group of non-oxygen-producing bacteria known as heliobacteria evolved later.

The scientists also found that heliobacteria are the most closely related to the common ancestor of the oxygen-producing photosynthetic cyanobacteria. Cyanobacteria eventually gave rise to chloroplasts in algae and green plants, and chloroplasts are the small bodies in plant cells that carry out photosynthesis today.

The Bauer group's results reverse the conventional thinking on the evolution of photosynthetic bacteria, in which purple bacteria were considered one of the last bacterial species to evolve, not one of the first.

The Bauer group's work also reinforces recent fundamental changes in molecular genetics that show bacteria evolved in a complex manner that resembles a tangled briar patch, with branches going every which way from a number of stems, instead of a traditional evolutionary tree that shows all species neatly branching out from a single stem that represents their common ancestor, usually regarded as the universal ancestral cell. This change in perspective is necessary because gene-swapping was common among ancient bacteria early in evolution.

The work of Bauer's group demonstrates that scientists must study the evolution of individual metabolic processes such as photosynthesis rather than the evolution of entire bacterial organisms.

Bauer's Web site is at http://sunflower.bio.indiana.edu/~cbauer/bauerlab/.