New twist on life's power source

Stanford, CA — A startling discovery by scientists at the Carnegie Institution puts a new twist on photosynthesis, arguably the most important biological process on Earth. Photosynthesis by plants, algae, and some bacteria supports nearly all living things by producing food from sunlight, and in the process these organisms release oxygen and absorb carbon dioxide. But two studies by Arthur Grossman and colleagues*+ reported in Biochimica et Biophysica Acta and Limnology and Oceanography suggest that certain marine microorganisms have evolved a way to break the rules—they get a significant proportion of their energy without a net release of oxygen or uptake of carbon dioxide. This discovery impacts not only scientists’ basic understanding of photosynthesis, but importantly, it may also impact how microorganisms in the oceans affect rising levels of atmospheric carbon dioxide.

Grossman’s team investigated photosynthesis in a marine Synechococcus, a form of photosynthetic bacteria called cyanobacteria (formerly blue-green algae). These single-celled organisms dominate phytoplankton populations over much of the world’s oceans and are important contributors to global primary productivity. Grossman and his colleagues wanted to understand how Synechococcus could thrive in the iron-poor waters that cover large areas of the ocean, since certain activities of normal photosynthesis require high levels of iron. While others had suggested a potential role of oxygen as accepting electrons from the photosynthetic apparatus in place of carbon dioxide, the studies by Grossman’s group show that this activity is significant in the oligotrophic (nutrient-poor) oceans, which cover about half the ocean’s area.

“It seems that Synechococcus in the oligotrophic oceans has solved the iron problem, at least in part, by short-circuiting the standard photosynthetic process,” says Grossman. “Much of the time this organism bypasses stages in photosynthesis that require the most iron. As it turns out, these are also the stages in which carbon dioxide is taken from the atmosphere.”