DOE Human Genome Program Contractor-Grantee Workshop VIII
February 27-March 2, 2000  Santa Fe, NM

125. Prochlorococcus: The Smallest and Most Abundant Photosynthetic Microbe in the Oceans

Sallie W. Chisholm, Gabrielle Rocap, and Lisa Moore

Departments of Civil and Environmental Engineering and Biology; Massachusetts Institute of Technology; 15 Vassar St. 48-425; Cambridge, MA 02139

chisholm@mit.edu

http://web.mit.edu/chisholm/www/

Prochlorococcus is a unicellular cyanobacterium that dominates the temperate and tropical oceans. It lacks phycobilisomes that are characteristic of cyanobacteria, and contains chlorophyll b as its major accessory pigment. This enables it to absorb blue light efficiently at the low-light intensities and blue wavelengths characteristic of the deep euphotic zone. It contributes 30-80% of the total photosynthesis in the oligotrophic oceans, and thus plays a significant role in the global carbon cycle and the Earth's climate. Description of the complete genome of this microbe will greatly advance our understanding of the regulation of these globally important processes.

To this end, colleagues at the DOE Joint Genome Institute (J. Lamerdin et al.) have been working on the complete genome sequence of Prochlorococcus marinus (MED4). The work has progressed rapidly, and the sequence is almost complete. Prochlorococcus is an ideal candidate for complete genome sequencing because (1) it is the smallest known phototroph with a relatively small genome (1.8 Mb), (2) it is widespread and abundant and is easily identified and enumerated in situ using flow cytometry, (3) its unique photosynthetic pigment (divinyl chlorophyll) makes its contribution to total photosynthetic biomass in the oceans easily assessed, and (4) we have an extensive culture collection of isolates from different oceans and environments.

Moreover, we have recently demonstrated that at least two ecotypes of Prochlorococcus coexist in the oceans that are distinguished by their photophysiology and molecular phylogeny. One is capable of growth at irradiances where the other is not. Ultimately, a comparison of the complete genomes of these two ecotypes would provide valuable insights into the regulation of this type of microdiversity in marine microbial systems. In addition, the use of microarray technology for the analysis of gene expression patterns will give us unprecedented insights into how these microbes cope with the dilute environment of the oligotrophic oceans.