Bacteria's move from sea to land may have occurred much later than thought

Igor Jouline, UT-Oak Ridge National Laboratory joint faculty professor of microbiology and researcher at ORNL's Joint Institute for Computational Sciences, performed a genome sequence analysis of the soil bacteria Azospirillum, a species' whose forebearers made the sea-to-land move. The analysis indicates the shift may have occurred only 400 million years ago, rather than approximately two billion years earlier, as originally thought.

Published in the journal PLoS Genetics, Jouline calculated the timing of the sea-land transition through studies of genome sequences of two species of Azospirillum, a terrestrial genus with almost exclusively aquatic relatives.

Jouline conducted his research with Kristin Wuichet and Leonid Sukharnikov of the Department of Microbiology, Gladys Alexandre of Department of Biochemistry, Cellular, and Molecular Biology, and Kirill Borziak, a graduate student in the ORNL-UT Genome Science and Technology program.

"In the absence of fossil records for bacteria, it is hard to estimate when and how bacteria transitioned from sea to land," said Jouline. "Using genome sequencing and analysis of bacteria of the genus Azospirillum, which colonizes roots of important cereals and grasses, we show that these organisms transitioned from aquatic environments to land approximately at the same time that plants appeared on land -- 400 million years ago."

Jouline said the Azospirillum lineage the team studied has obtained nearly half of its genome from terrestrial organisms, which suggests the much later water-land transition, which coincides with the first appearance of plants on land.

The study is of interest to researchers beyond its evolutionary significance. Azospirillum is currently used as a biofertilizer for grasses and some other plants. Commercial fertilizers containing the bacteria are available world wide.

"Because these bacteria colonize roots of grasses and improve their growth and development, they might be important for bioenergy research," Jouline said.

"Switchgrass is one of the most important potential sources of bioethanol. In this study, we have shown that genomes of Azospirillum contain as many cellulolytic enzymes as those from known effective cellulose degrading bacteria," he said. "We have also demonstrated experimentally that azospirilla do degrade cellulose, especially the strain that can penetrate grass roots."

The team also included Greg Hurst of the ORNL Chemical Sciences Division; research groups from Pasteur Institute in Paris, France, universities of Lyon and Toulouse in France, University of Sydney in Australia, and the National University of Mexico; and Florence Wisniewski-Dye from the University of Lyon.

The research was supported with funding from the National Science Foundation and the Department of Energy's Office of Science.