Climate change scientists have known for years that rising temperatures affect sea creatures, from the biggest fish to the microscopic plankton at the base of the ocean food chain. Now, a four-year expedition that sampled microbes from across the world's oceans is bringing the mechanisms of that change into focus. These tiny creatures (seen amazingly close up above and below), which may be among the oldest on Earth, together absorb carbon dioxide, make oxygen, break down waste and nourish other creatures. And they are profoundly affected by water temperature, according to a series of five studies published in Science about the voyage of the schooner Tara.
"Temperature is the most important environmental factor determining the composition of these communities," said Chris Bowler, an author on all five studies and a genomics expert with the Department of Biology of the École Normale Supérieure and the National Center for Scientific Research in France.
"This would imply that climate change, warming of the oceans, is going to have a strong impact on these organisms and the functions these organisms perform for the well-being of our planet," Bowler said.
The Tara expedition's findings, researchers said, have added an order of magnitude to what we know of the Tree of Life, vastly expanding its base. The microbes studied range from viruses and bacteria too small to see under a microscope to the single-celled amoebas or paramecia that children study in biology classes.
Research on the diversity of these creatures was virtually impossible until a few years ago, with the advent of relatively quick and affordable genetic analysis, said Stephen Palumbi, a Stanford University marine biologist who was not involved in the Tara studies but wrote a commentary that accompanied them.
Until about 50 years ago, scientists did not even realize that the specks they saw when they examined seawater under a microscope were alive, he said. The new research showed that these microbes are in a constant dance with one another, collaborating and battling just below the water's surface. Their interactions keep the ecosystem in balance, preventing any one species from dominating the seas.
"A lot of what we didn't really ever see before in the ocean are predators and parasites, zombies and vampires that are floating through this incredible set of diversity, battling it out," Palumbi said. "All these tiny little critters add up to something that is really a part of the way our planet operates."
The vast genetic diversity of the oceans impressed many of the scientists involved in the project, a consortium that involved 18 institutions. The researchers identified roughly 40 million genes in the upper layers of the world's oceans. The human gut microbiome, in comparison, is known to have only about 10 million genes, said Shinichi Sunagawa of the European Molecular Biology Lab in Heidelberg, Germany, who was a first author on one of the papers.
The raw data produced by the expedition should allow scientists eventually to predict how microbial life will change as a function of changes in water temperature, said Eric Karsenti, a cell biologist and scientific director of the consortium.
One of the papers tracks the effects warming waters have on bacterial diversity, suggesting that other microbes, like viruses and single-celled organisms, are probably affected as well. Future analysis should allow researchers to build predictive models for what will happen to microbial communities as water temperature changes, Karsenti said, and how much that will affect oxygen production and carbon dioxide absorption.
Life on Earth started in the oceans, so the Tara data also provides new insights into creatures directly descended from those of a billion years ago, Bowler said.
"By matching DNA-level information with what these organisms look like, we can learn more about them, more about how they work, and hopefully learn more about our own origins as well," he said.
The 110-foot research schooner Tara traveled as far north as the Arctic and as far south as Antarctica from 2009 to 2013. It had to evade pirates off the coast of Saudi Arabia, path-blocking ice in the Arctic and hurricane-force winds in the Magellan Straits linking the Atlantic and Pacific Oceans.
"It was an incredible time," said Romain Troublé, the Tara Foundation's executive director. As it journeyed across the oceans, Troublé and his team negotiated permission from more than 20 countries for the schooner to sample and dock.
Researchers had to follow precise protocols at each of 210 sites, where they remained for two days at a stretch, battling the wind and weather to scoop up samples from the fragile boundary between the sea and air.
As the Tara docked in ports along the way, expedition scientists turned into cheerleaders for the sea, educating children and adults about the creatures they live near but do not know.
The research team, which totaled more than 200 people, included experts from 35 countries and with more than 20 specialties. Members have begun analyzing their 35,000 samples, with just 579 explored in these five papers. The data will be made public and accessible to scientists across the globe.