Diminutive genomes hint at minimal DNA for photosynthesis.
Researchers have sequenced the first genomes of bacteria that live in the sea. The DNA readouts hint at the essential apparatus for photosynthesis and provide new insights into Earth's carbon cycle.
"We can consider these very close to minimal genomes," says molecular biologist Donald Bryant of Pennsylvania State University. What many of the genes in these token toolkits do is still unknown.
The sequenced microbes belong to a group called cyanobacteria. They are the two most common marine bacteria and account for roughly half of the photosynthesis in the oceans, about one-quarter of the global total. They remove about 10 billion tons of carbon from the air each year.
The genomes will help us understand this process. But to work out the microbes' large-scale environmental effects, we need to know more about the spread of different species and strains throughout the oceans, says Sallie Chisholm of the Massachusetts Institute of Technology, Cambridge, leader of one of the sequencing teams.
Mapping this diversity is also necessary if we are to debate boosting the growth of marine microbes to remove carbon from the atmosphere and slow global warming. "To talk about being able to predict what'll happen if we fertilize the oceans is premature," says Chisholm.
Chisholm's team sequenced Prochlorococcus marinus1, the most abundant photosynthetic cell on Earth: a millilitre of seawater can contain more than a million individuals. It is also the smallest, at less than a millionth of a metre across. Its strongholds are the tropical and temperate oceans; it is less common in polar waters and close to land.
One Mediterranean strain has a genome just 1.7 million DNA letters long, featuring about 1,700 genes. Another, from the murkier Sargasso Sea, has a similar-sized sequence, a separate research team has found2. In contrast, the green alga Chlamydomonas has 50 times more DNA and is 15 times larger.
Prochlorococcus is particularly short on genes that would help it respond to changes in the environment. It probably doesn't need them, as the sea is a stable habitat, and it may not be able to afford to make them, as the ocean is low in nutrients.
"Our picture is of bacteria having complex biology to be flexible, but this shows that you don't need all that stuff to be successful," says Bryant.
Bigger and wider
The other species sequenced, Synechococcus, has a larger genome of about 2.4 million DNA letters and 2,500 genes3. This bacterium is less abundant - at about 10,000 cells per millilitre of seawater - but is more widely distributed than Prochlorococcus.
The genome of Synechococcus shows that it can process a wider range of chemicals than Prochloroccus. Some of its enzymes are adapted to breakdown nickel and cobalt, probably as a way of conserving iron, which is in short supply in the sea, and it can use several different sources of nitrogen.
"It seems to be more of a generalist than Prochlorococcus - it's good in a range of environments," says Brian Palenik of the Scripps Institution of Oceanography, part of the University of California, San Diego, and leader of the Synechococcus sequencing team.
Rocap, G. et al. Genome divergence in two Prochlorococcus ecotypes reflects oceanic niche differentiation. Nature, published online, doi:10.1038/nature01947 (2003).
Dufresne, A. et al. Genome sequence of the cyanobacterium Prochlorococcus marinus SS120, a nearly minimal oxyphototrophic genome. Proceedings of the National Academy of Sciences, published online, doi:10.1073/pnas.1733211100, (2003).
Palenik, B. et al. The genome of a motile marine Synechococcus. Nature, published online, doi:10.1038/nature01943 (2003).