The hidden, essential biomass of the Arctic Mid-Ocean Ridge

Far below the reach of light, a community of deep-sea microbes is quietly shaping the planet. Unlike rainforests or coral reefs, this hidden life is tiny and out of reach—yet it manages Earth’s largest store of organic carbon, and it may harbour valuable new biochemistry with unknown potential. As STEFFEN LETH JØRGENSEN writes, we’ve only just begun to find out what’s there.

The scene unfolds more than 3,000 metres below the surface of the Arctic Ocean, halfway between Greenland and the Svalbard archipelago. In the rare moments when light reaches these depths along the Arctic Mid-Ocean Ridge—an underwater mountain chain formed where tectonic plates slowly pull apart—the seafloor appears in small, glowing fragments on the screens of a control room far above.

As the remotely operated submarine Ægir 6000 advances, whitish shrimp-like creatures called amphipods come into view against a rich, dark blue background shaded by sulphide-rich minerals. A few centimetres away, the ground turns bright white—not from snow or ice, but from dense mats of bacteria thriving on the chemicals that seep from beneath the seafloor.  

The camera drifts on, and a thicket of tubeworms appears, long and tangled like loose threads from an unravelled sweater. In a place without sunlight that is characterized by steep changes in temperature and chemistry, each shift in the landscape hosts its own specialized community. 

Similarly to how varying conditions around a hydrothermal vent result in a variety of specialized organisms, these varying conditions in the sediments of the seafloor trigger fascinating diversity. In the layers closest to the seafloor surface, for example, many microbes use oxygen. But as you go deeper and oxygen disappears, different microbes take over, and these use substances like manganese or iron instead to power their metabolisms. 

The habitat of these microbes stretches more than a kilometre down into the seabed, and their numbers are staggering. By weight, they account for roughly 1 per cent of all life on Earth—a far larger share than humans, who make up only 0.01 per cent. For geomicrobiologists, this deep biosphere is not just large: it is central to how Earth works. A crucial role of these microbes is to mediate the flow of elements between the Earth’s seafloor and the seawater above, essentially controlling which chemicals cross that interface and in what form. These chemicals include notorious greenhouse gases, such as carbon dioxide and methane.

A subtle change in the flow of methane or sulphur can alter which microbes thrive and which ones disappear, reshaping the entire food webs that were built upon them

—Steffen Leth Jørgensen, Associate Professor University of Bergen, Director of the Centre for Deep Sea Research

An urgent mystery

But what the microbes do for the planet is only part of the story. 

Because so many unique reactions take place in these extreme conditions, many of the microbes that live there have biochemical properties we have never seen before. Their enzymes can function at temperatures, pressures and chemical conditions that would destroy most familiar forms of life. These adaptations make them interesting not only for understanding ocean ecosystems, but also as potential sources of new molecules and tools for biotechnology, medicine or industrial processes. 

Yet we may not have long to observe them as they exist today. While the deep-sea processes these microbes take part in are slow, the changes happening in the Arctic are not. As sea ice retreats, currents, water temperatures and sedimentation patterns shift. These changes can influence the chemistry that sustains these microbes. For example, a subtle change in the flow of methane or sulphur can alter which microbes thrive and which ones disappear, reshaping the entire food webs that were built upon them.  

If we fail to understand this system now, we risk losing the chance to know how it functioned before the Arctic ice started disappearing—and to better understand how its smallest inhabitants help keep the planet in balance.