World-ranging alga tops human DNA size


A single-celled organism visible only under a microscope is one of the most successful life forms on the planet. So say scientists who have 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 E. hux, the alga has a thin, hard, chalky shell of calcium carbonate. Piles of billions of the long-dead algae comprise, for instance, England’s White Cliffs of Dover. In the ocean, “blooms” of E. hux algae can cover thousands of square kilometers, and their milky reflected light can be seen from space.

It 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, E. hux is a basic link in the ocean’s food chain. It also absorbs lots of carbon dioxide at the ocean surface, helping to attenuate greenhouse gases.

Unveiled in a study in the journal Nature, the researchers admitted the project turned out to be something of a nightmare. The E. hux 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 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 and was eventually supported by 74 other researchers from a dozen countries.

“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 project in 2002.

What made E. hux 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 E. hux that the team was able to get a complete picture — the first algal pan-genome. Comparing and contrasting the 13 strains shows the alga has a core genome that accounts for about three-quarters of its DNA. 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 E. hux 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 also could lead to new composite materials for bone replacement.

Life beneath ocean floor


Micro-organisms are flourishing at great depths beneath the ocean floor, scientists report.

U.S. biologists looked for telltale scraps of genetic code in a core drilled deep into the sedimentary floor of the Pacific off Peru. They were hunting for traces of messenger RNA (mRNA), which hints at the presence and even the identity of living cells.

The results revealed a vast ecosystem living at all the subsea depths that were tested, from 5 to 160 meters, according to the study. The microscopic critters included bacteria, primitive single-cell organisms called archaea, and fungi.

The mRNA signatures point to cell proliferation and even movement. Some of the proteins they transcribe are for flagella — whiplike tails that help cells “swim” through fluid. “If there’s room to move, they move,” said William Orsi of Woods Hole Oceanographic Institution (WHOI), which shared the research with the University of Delaware. The paper is published in the journal Nature.

The population is so huge that climate scientists may have to take it into account in calculating Earth’s “carbon budget.”

“Cells are very abundant there, but they do not have high activity levels,” said WHOI microbial ecologist Virginia Edgcomb. “But it’s a huge biosphere, and when you do the math, you see we’re talking about a potentially significant contribution. Carbon is being turned over, and that has important implications for models of carbon and nitrogen cycling.”