Stephen Giovannoni, Ph.D
Oregon State University, Department of Microbiology
The Evolution of Small Bacterial Genomes in the Ultra-Oligotrophic Ocean
Wednesday- May 14, 2008
Pacific Forum – 3:00 p.m.
The heterotrophic alphaproteobacterial clade SAR11 accounts for approximately 25% of microbial plankton cells in the ocean surface layer, and may exceed 50% of the cells in temperate ocean gyres during the summer, making it a key to understanding the marine carbon cycle. Evidence for the diversification of SAR11 ecotypes has emerged from Fluorescence in Situ Hybridization (FISH) and terminal restriction fragment length polymorphism (tRFLP) data from the Bermuda Atlantic Time series Study (BATS). This data supports the view that at least three SAR11 ecotypes, each with a different seasonal and spatial distribution pattern, occupy the surface layer in the Western Sargasso Sea. Complete genome sequences from three candidatus Pelagibacter ubique strains provided insight into natural variation in SAR11 genomes, supporting the hypothesis that genome streamlining has been an important factor in SAR11 evolution. Recently we found that SAR11 cells are deficient in assimilatory sulphate reduction genes, making them dependent on exogenous sources of reduced sulphur, such as 3-dimethylsulphoniopropionate (DMSP) or methionine, for growth. Further support for the genome streamlining hypothesis came from the discovery that the OM43 clade of betaproteobacteria have genomes that are even smaller than the SAR11 genome. These organisms are obligate methylotrophs that are common in coastal ecosystems. We hypothesize that unusual nutrient requirements associated with genome streamlining and genome reduction may explain why some important bacterioplankton clades continue to elude cultivation.