Genomic Clues To The Evolution Of Photosynthesis

The bacterium, Chlorobium tepidum, was originally isolated from a hot spring in New Zealand. It is a member of the green-sulfur bacterial group, so known because of the microbes' color and their dependence on sulfur compounds to carry out photosynthesis. Biologists say green-sulfur bacteria are important because they perform photosynthesis in a different way from that of other bacteria and that of plants.

For example, instead of the choloroplasts found in plants, green-sulfur bacteria have organelles called chlorosomes that help generate energy through an electron-transport chain in the microbe's cytoplasmic membrane. Inside the chlorosomes, the chlorophyll and carotenoid molecules that capture light differ from the molecules that other species use to perform photosynthesis. Also, green-sulfur bacteria carry out photosynthesis in the absence of oxygen and do not produce oxygen as a byproduct as plants do.

"Because of their unusual mechanisms of harvesting and using the energy of light, the green-sulfur bacteria are important to understanding the evolution and the mechanisms of both photosynthesis and cellular energy metabolism," said Jonathan A. Eisen, an evolutionary biologist at The Institute for Genomic Research (TIGR) in Rockville, Maryland. "The ability to carry out photosynthesis in the absence of oxygen is particularly important to evolutionary studies since it is believed that the early atmosphere of Earth had little oxygen. That is why some scientists have suggested the green-sulfur bacteria were the first photosynthetic organisms."

The sequenced genome of C. tepidum, published in the Proceedings of the National Academy of Sciences, represents the first time that a microbe in the green-sulfur group has been fully sequenced. It is also the first time that a bacteria that is both photosynthetic and anaerobic has been sequenced. The sequencing of the C. tepidum genome was funded by a grant from the U.S. Department of Energy

TIGR President Claire M. Fraser said the project is part of a wider effort at TIGR to study organisms that play important roles in global energy and nutrient cycles. "We chose C. tepidum because it is a model system for studies of the green-sulfur bacteria and anaerobic photosynthesis," she said. "Also, scientists have developed a suite of genetic methods to help study this important microbe."

Green-sulfur bacteria such as C. tepidum are widely distributed in aquatic environments where light reaches anoxic (low-oxygen) layers of water containing reduced sulfur compounds. When TIGR researchers analyzed the microbe's single circular chromosome, they identified numerous genes that may play novel roles in photosynthesis or other processes that make use of the energy of light.

Using a method of comparing complete genomes known as phylogenomic analysis, the scientists found strong similarities between the metabolic processes of the green-sulfur bacteria and processes in many species of Archaea, the organisms that represent the third domain of life. This analysis also revealed the likely duplication of genes that are involved in the pathways for phytosynthesis and in the metabolism of sulfur and nitrogen. "These duplication events may help explain why this microbe is able to use lower levels of light to carry out photosynthesis than other species," said Eisen, the first author of the PNAS paper.

Another reason why biologists study green-sulfur bacteria is that their mechanism of capturing carbon dioxide differs from that of plants and other bacteria. The green-sulfur bacteria use an unusual chemical cycle – called the reductive tricarboxylic acid (TCA) cycle – that differs from the Calvin Cycle that is used by higher plants. The TCA cycle uses electrons derived from hydrogen or reduced sulfur compounds to fix carbon dioxide; in contrast, the Calvin Cycle requires oxygen. In fact, the reductive TCA cycle was first discovered in C. tepidum.

The authors of the paper write that "further genome analysis and experimental work should help provide insights into the evolution of photosynthesis and other pathways of energy metabolism." Scientists have developed methods to genetically manipulate C. tepidum, allowing experimental testing of hypotheses generated from the analysis of the microbe's genome. Also, the completion of a genome sequence makes experimental studies easier and less expensive.

Said Eisen: "We hope that this genome sequence will serve as a launching pad for future studies of the evolution and mechanisms of photosynthesis and the biology of this important group of organisms."

The Institute for Genomic Research (TIGR) in Rockville, Maryland, is widely regarded as the world's leading research center for microbial genomics. Founded in 1992, TIGR is a not-for-profit research institute whose primary research interests are in structural, functional and comparative analysis of genomes and gene products from a wide variety of organisms, including viruses, eubacteria, archaea, and eukaryotes.