The deep ocean has long been viewed as a quiet, shadowed world, but a new Nature Geoscience study shows it holds a surprising power. Researchers discovered that tiny microbes living hundreds of meters below the surface capture carbon in the dark, and their hidden activity may shape how the planet stores carbon today and in a warming future. This matters now because climate models need precise carbon budgets to forecast how quickly the ocean can lock away human produced carbon.
Fast Facts
- Project: Scientists measured how deep ocean microbes fix carbon in total darkness across the Eastern Tropical and Subtropical Pacific from 2021 to 2023.
- Goal: To determine how much ammonia oxidizers and sulfide oxidizers each contribute to dark carbon fixation.
- Key Insight: These two microbe groups drive most dark DIC fixation, providing new conversion factors for global carbon models.
- Why It Matters: The findings improve predictions of how much carbon the ocean can store as climate conditions shift.
- Broader Impact: Updated microbial data strengthens climate models used to forecast long term carbon sequestration in warming oceans.
The study finds that two groups of microbes, ammonia oxidizers and sulfide oxidizers, drive most carbon fixation in the dark ocean. Scientists already knew that some microbes could convert inorganic carbon into biomass without sunlight, but the scale and breakdown of who does what remained unclear. The new work provides the clearest picture yet by quantifying how much carbon each microbial metabolism contributes and offering updated conversion factors for global models.
To uncover this, the team performed ship based experiments across the Eastern Tropical and Subtropical Pacific Ocean from 2021 to 2023. They incubated deep water samples under controlled conditions, tracked dark dissolved inorganic carbon fixation rates, and paired these measurements with genetic and chemical analyses. The paper describes comparisons between DIC fixation yields in ammonia oxidizers and sulfide oxidizers, highlighting distinct metabolic contributions. The researchers then combined lab results with metagenomes and environmental data to estimate the relative influence of each microbe group across the water column.
These findings matter because dark carbon fixation supports food webs far from sunlight. The study gives scientists new numbers to plug into biogeochemical models, which improves predictions about how the mesopelagic zone stores carbon as climate conditions shift. Better models mean better estimates of how much carbon the ocean can absorb each year, a key factor in long term climate planning.
Experts note that the work fills a major gap. The authors explain that current models underestimate the role of dark carbon fixation and need updated microbial conversion factors to improve accuracy. Some researchers argue that the ocean twilight zone has been overlooked for too long, while others see these findings as a turning point for understanding carbon sequestration at depth.
The discovery also connects to broader issues. As climate change alters surface productivity and oxygen levels, deep ocean microbial communities may shift in ways that influence carbon storage. Changes in nutrient availability, ocean chemistry, and industrial pollution could also affect these bacteria, linking this research to marine ecology, fisheries, and global carbon policy.
The team plans to refine their estimates across more ocean regions and explore how microbial metabolism responds to changing oxygen concentrations. The paper notes that many open questions remain, including how these communities adapt under future warming and how much carbon they can fix at scale during large ocean shifts. Advanced sequencing and ship based incubations will help answer these questions in the years ahead.
Taken together, this study shows that unseen life in the deep ocean plays a powerful role in shaping Earth’s carbon balance. Understanding these microbes gives scientists a stronger foundation for predicting how the ocean will respond to climate change and how much carbon it can safely store.
Story Source: Materials provided by Nature Geoscience authors. Content may be edited for style and length.
Journal Reference: Bayer, B., Kitzinger, K., Poff, N.L., A.A., Santoro, A.E., et al. Quantifying metabolic pathways that fuel dark carbon fixation in the mesopelagic ocean. Nature Geoscience, 2025. Volume 18. DOI: 10.1038/s41561-025-01798-x.