Scientists have uncovered new evidence that the deep ocean plays a more complex role in storing carbon than previously thought, shedding fresh light on how marine ecosystems help regulate the Earth’s climate. The world’s oceans are already recognised as one of humanity’s strongest allies in tackling global warming. United Nations figures show they absorb more than 30 per cent of carbon dioxide released into the atmosphere and around 90 per cent of the excess heat generated by those emissions. Yet while much of this carbon capture is known to occur near the surface, researchers have long suspected that important processes are also taking place far below.
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At the surface, microscopic phytoplankton fix inorganic carbon through photosynthesis, much like plants on land. In the deep ocean, however, where sunlight does not reach, carbon fixation relies on entirely different biological pathways. For years, scientists believed this task was carried out almost exclusively by microbes known as ammonia-oxidising archaea, which use chemical energy rather than light.
But a new study published in Nature Geoscience suggests this explanation was incomplete. Researchers found a mismatch between the amount of carbon fixation measured during ocean expeditions and the energy thought to be available to these ammonia-oxidising organisms.
“There was a clear gap between what we were measuring at sea and what existing models suggested was possible,” said Dr Alyson Santoro, a microbial oceanographer at the University of California, Santa Barbara, and senior author of the study. “The numbers simply didn’t add up.”
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Instead of assuming these microbes were more efficient than previously believed, the research team explored another possibility: that other organisms were also involved. By suppressing the activity of ammonia oxidisers using a chemical inhibitor, the scientists expected carbon fixation rates to fall sharply. When they did not, it became clear that additional microbes were contributing.
The study points to deep-sea heterotrophs — organisms that normally consume organic matter rather than producing it — as unexpected partners in fixing inorganic carbon. Working alongside autotrophs, these microbes appear to play a significant role at the very base of the deep-ocean food web.
“This gives us, for the first time, a clearer picture of how carbon is being fixed in the deep ocean and by whom,” Dr Santoro said. “It helps explain how this hidden ecosystem functions and how carbon moves through it.”
The findings arrive amid growing concern about the limits of the ocean’s ability to buffer climate change. Recent research suggests that even under optimistic scenarios, parts of the Southern Ocean could release stored heat back into the atmosphere for decades to come.
Even so, scientists stress that improving our understanding of ocean carbon storage remains vital. While the seas cannot solve the climate crisis alone, insights into how deep-ocean microbes operate may prove crucial in refining climate models — and in reinforcing the urgency of reducing emissions before the planet’s natural defences are pushed too far.
