Deep Ocean Microbes Defy Expectations, Quietly Fixing Carbon in the Dark Depths

Researchers at the University of California - Santa Barbara and their collaborators have discovered that unexpected microbes are quietly fixing carbon in the dark depths of the ocean, challenging long-held ideas about how carbon dioxide is "fixed" in the deep sea. According to a new study published on December 10, 2025, experiments show that heterotrophs, a type of microbe, are doing far more of the work than previously expected in the ocean's carbon-fixing engine. The study, led by UCSB microbial oceanographer Alyson Santoro, reveals that heterotrophs are responsible for a significant portion of carbon fixation in the deep ocean, contrary to the long-held assumption that ammonia-oxidizing archaea dominated this process. This finding reshapes our understanding of how carbon moves through the deep ocean and stabilizes Earth's climate. "We were surprised to find that heterotrophs were playing such a significant role in carbon fixation," said Santoro. "This changes our understanding of the ocean's carbon cycle and has implications for our understanding of how the ocean stores carbon." The research team used a combination of laboratory experiments and field observations to study the microbial communities in the deep ocean. They found that heterotrophs, which obtain energy by consuming organic matter, were able to fix carbon at a rate that was previously underestimated. "This study highlights the importance of heterotrophs in the ocean's carbon cycle and suggests that they may be more important than previously thought," said Dr. Santoro. "It also underscores the need for further research into the role of heterotrophs in the ocean's carbon cycle." The discovery has significant implications for our understanding of how the ocean stores carbon and how it affects the climate. The ocean is a critical component of the Earth's carbon cycle, and understanding how it stores and releases carbon is essential for predicting climate change. The study's findings also have implications for the development of strategies to mitigate climate change. If heterotrophs are indeed playing a significant role in carbon fixation, then efforts to promote their growth and activity in the ocean may be an effective way to reduce carbon emissions. The research team plans to continue studying the role of heterotrophs in the ocean's carbon cycle and to explore the potential for using these microbes to mitigate climate change. In related news, the National Oceanic and Atmospheric Administration (NOAA) has announced plans to launch a new research initiative to study the ocean's carbon cycle and the role of heterotrophs in it. The initiative, which will be led by NOAA's Ocean Carbon and Biogeochemistry Program, will bring together researchers from around the world to study the ocean's carbon cycle and to develop new strategies for mitigating climate change. The study's findings were published in the journal Science and are available online.