Genome complexity in a lean, mean photosynthetic machine
- Canadian Institute for Advanced Research, Program in Evolutionary Biology, and Department of Biochemistry and Molecular Biology, Dalhousie University, Sir Charles Tupper Medical Building, 5850 College Street, Halifax, NS, Canada B3H 1X5
Photosynthetic organisms come in all shapes and sizes. From a human perspective, the trees and plants of dry land are the most conspicuous examples, but the next time you admire a colorful tulip or marvel at the girth of a giant Sequoia, consider the following: Approximately half of the oxygen we breathe is generated by single-celled photosynthesizers, phytoplankton, adrift in the world’s oceans, invisible to the naked eye and unfathomably large in number, quietly harnessing solar energy, fixing carbon dioxide, and producing oxygen by the bucket-load. In this issue of PNAS, Derelle et al. (1) present the complete genome sequence of the smallest of the small eukaryotic (nucleus-containing) phytoplankton, Ostreococcus tauri. This organism is best known for its diminutive cell size, about that of a typical bacterium. Its genome is equally remarkable for its small size and extreme compactness. However, the O. tauri genome is also unexpectedly complex and provides a fascinating glimpse into the genetic makeup and metabolic potential of the smallest known eukaryote at the base of the marine food chain.
Oxygenic photosynthesis first evolved in the ancestors of modern-day cyanobacteria. In terms of sheer numbers, these organisms dominate the ocean (2), but from the perspective of primary productivity, eukaryotic algae are considered more significant. Marine diatoms, for example, produce up to 40% of the organic carbon generated in the ocean each year (3) and represent just one of the abundant and well studied algal lineages in the sea. Least understood of all eukaryotic phytoplankton are those with a diameter of <2–3 μm, the so-called “picoeukaryotes.” The first descriptions of bacterial-sized eukaryotes date back more than 40 years (e.g., ref. 4), but it is only with the application of flow cytometry (2) and molecular approaches (5) to the study of marine microbes that we have begun …
*E-mail: jmarchib{at}dal.ca
