It's just a tiny thing -- a single-celled organism visible only under a microscope -- yet it is one of the most successful life forms on the planet.

So say scientists who on Wednesday published the DNA code of an ocean alga called Emiliania huxleyi, whose astonishing adaptability enables it to thrive in waters from the equator to the sub-Arctic.

Known under the more useful moniker of "Ehux," the alga has a thin, hard, chalky shell of calcium carbonate.

Condensed piles of billions of long-dead Ehuxes comprise, for instance, the White Cliffs of Dover.

In the ocean, "blooms" of Ehux algae can cover thousands of square kilometres (square miles), and their milky reflected light can be seen from space.

Less visibly, the micro-alga also has an essential place in the ecosystem and the complex equation of climate change, which explains the bid to sequence its genome.

As a phytoplankton, Ehux is a basic link in the ocean's food chain. It also absorbs lots of carbon dioxide (CO2) at the ocean surface, helping to attenuate the greenhouse-gas problem.

Unveiled in Nature, the genome project turned out to be something of a nightmare, the researchers admit.

The Ehux genome was originally thought be only about 30 million bases, or "rungs" in the DNA ladder.

In the end, it turned out to be a whopping 141 million bases, with at least 30,000 genes -- a third more than the gene tally among humans, although our overall genome is many times bigger.

The investigation took more than 10 years to complete.

"Because of the size and inherent complexities, the genome became known as The Beast," said Betsy Read, a professor of biology at California State University, who initiated the scheme in 2002 and was eventually supported by 74 other researchers from a dozen countries.

What made Ehux such a challenge was remarkably high genetic diversity within the species, enabling it to thrive in seas that can be cold or warm, rich or low in nitrogen, iron and phosphorus and dim or bright in sunlight.

It was only by sequencing not one but 13 strains of Ehux that the team was able to get a complete picture -- the first algal pan-genome.

Comparing and contrasting the 13 strains shows that the alga has a core genome, which accounts for about three-quarters of its DNA, says the paper.

The rest comprises different gene sets that help a specific strain meet challenges of the local environment.

By way of comparison, humans share around 99 percent of their DNA.

Finding out how Ehux works could one day aid medical research and better understand the impact on this vital organism from greenhouse gases.

Identifying the genes and proteins that help it make its tough little shell could lead to new composite materials for bone replacement.

"The genome, so to say, is the 'hard drive' of an organism," said Klaus Valentin of Germany's Alfred Wegener Institute for Polar and Marine Research.

"All properties are encoded there -- how it looks, how it can adapt, how it competes with others. If we know the data on this hard drive, we can learn a great deal about what this organism can do and how it reacts to changes as a result of climate change, for example."