The Ultimate Minimalist
In the vast, sunlit upper layer of the world's oceans lives the most successful organism on the planet, a bacterium called Candidatus Pelagibacter ubique. It is the first cultured representative of a group of bacteria known as the SAR11 clade, which accounts for about one in every three microbial cells at the ocean's surface. During the summer in temperate oceans, P. ubique and its relatives can make up nearly half of all cells in the water. Their total global population is estimated at around 2 x 10^28 cells—two followed by twenty-eight zeroes.
This organism's dominance results of its radically simplified design. Its discovery traces back to 1990, when scientist Stephen Giovannoni's lab at Oregon State University first detected its ribosomal RNA genes in water from the Sargasso Sea. For twelve years, it defied all attempts at cultivation. Researchers finally succeeded in 2002 by growing it in extremely low-nutrient natural seawater, revealing a creature molded by evolution for extreme efficiency. The crescent-shaped cells are tiny, measuring just 0.37–0.89 micrometers in length. This small size optimizes their surface-area-to-volume ratio for absorbing scarce nutrients from the surrounding water.
A Streamlined Genome
The core of P. ubique's success lies in its genome. With a length of just 1,308,759 base pairs and 1,354 protein-coding genes, it is the smallest genome of any free-living organism known. The bacterium's genetic code is "streamlined," a process of shedding any non-essential genetic material. The genome contains no junk DNA, no duplicate genes, no viral insertions, and the shortest-known distances between genes.
This genetic minimalism comes with trade-offs. The organism has lost the genes for producing certain essential compounds. It is a conditional auxotroph for glycine and requires an external source of pyruvate or its precursors to grow. It also cannot make an essential component of Vitamin B1 on its own and must absorb it from the water, where it is released by other plankton. This dependency works in the ocean, where dissolved organic matter provides these necessary building blocks. The bacterium saves enormous energy by not having to replicate genes for pathways whose end products are readily available. It also saves nitrogen—a limited resource in the ocean—by favoring A and T base pairs in its DNA, which contain less nitrogen than G and C pairs.
Its metabolic role is significant. P. ubique is a primary consumer of dissolved organic carbon, a massive reservoir of energy at the base of the marine food web. It also possesses proteorhodopsin, a pigment that allows it to use sunlight to create ATP, supplementing its energy budget without the complexity of photosynthesis. By efficiently recycling carbon and nutrients, this tiny cell shapes the chemistry of the entire ocean.