Beneath the sunlit surface of the open ocean, trillions of bacteria are quietly producing methane in waters long assumed to be too oxygen-rich for it. A study published in the Proceedings of the National Academy of Sciences in early 2026 identifies the trigger: a shortage of phosphate, one of the nutrients these microbes need to survive. When phosphate runs low, bacteria scavenge phosphorus from organic compounds and release methane as a byproduct. As climate change heats the ocean and cuts off the supply of deep nutrients to the surface, that shortage is expected to worsen, potentially turning a little-known biological process into a meaningful feedback loop for global warming.
“Phosphate scarcity is the control knob,” said Katja Fennel, the study’s senior author and an oceanographer at Dalhousie University who collaborated with a team led by researchers at the University of Rochester. “When you remove phosphate from the equation, these organisms switch to breaking down methylphosphonates, and methane is what comes out.”
The ocean methane paradox, solved at global scale
Scientists have puzzled over methane in oxygenated surface waters for decades. Methane production was traditionally associated with oxygen-free environments like wetlands, rice paddies, and deep ocean sediments. Finding it in well-mixed, sunlit water seemed contradictory, earning the phenomenon the nickname “the ocean methane paradox.”
The answer turned out to be microbial resourcefulness. The most abundant bacteria in the ocean, members of the SAR11 clade (also called Pelagibacterales), can break apart organic phosphorus compounds called methylphosphonates when inorganic phosphate is scarce. That chemical reaction frees the phosphorus the bacteria need but releases methane as a side product. A 2014 laboratory study published in Nature Communications confirmed the mechanism and showed it shuts off when phosphate is plentiful.
What the new PNAS paper adds is global scope. Using ocean observations and a methane cycle model, the research team demonstrated that phosphate scarcity is the dominant environmental control on aerobic methane production across the world’s open oceans, not just in isolated lab flasks or single ocean basins. Field data from the nutrient-poor North Atlantic, published separately in Nature Communications, corroborate the model by linking methylphosphonate cycling and methane formation to biological productivity in that region.
Why warming makes it worse
The feedback mechanism works through ocean stratification. As global temperatures climb, the upper ocean warms faster than the deep, creating a more stable layered structure that resists vertical mixing. That mixing is what normally delivers phosphate and other nutrients from the deep ocean to the surface. When it weakens, surface waters become more nutrient-starved, and the conditions that trigger bacterial methane production expand.
The vast nutrient-poor gyres of the subtropical oceans, which already cover roughly 40 percent of Earth’s surface, are the zones most susceptible. If stratification intensifies under continued warming, those gyres could grow, bringing a larger share of the ocean’s surface into the phosphate-depleted state that switches on methane production.
A separate complication comes from the atmosphere. A 2026 study led by researchers at Fudan University and published in Nature Communications found that nitrogen pollution carried by wind and rain into the ocean can stimulate microbial phosphonate decomposition under phosphorus-limited conditions, potentially amplifying methane emissions beyond what ocean physics alone would produce. “Atmospheric nitrogen deposition enhances the phosphorus stress that marine microbes already face, effectively widening the window for methane production,” said Yan Zhang, the study’s corresponding author. The interaction between atmospheric nitrogen deposition and ocean stratification has not yet been modeled together, but the implication is that human activity may be compounding the problem from two directions at once.
What scientists still do not know
The PNAS study establishes the mechanism and its global relevance but stops short of projecting how much additional methane the ocean will produce under specific warming scenarios. Without those numbers, it is difficult to compare this source against the dominant methane emitters. For context, the ocean’s total methane contribution is currently estimated at roughly 6 to 12 teragrams per year, a small fraction of the approximately 580 teragrams released annually from all sources, according to the most recent global methane budget assessments. Whether the aerobic production pathway could meaningfully shift that balance remains an open question.
Regional gaps also persist. The strongest observational data come from the North Atlantic. Whether the same dynamics play out at similar intensity in the South Pacific, Indian Ocean, or other oligotrophic regions has not been confirmed with equivalent field measurements.
The Arctic presents a related but distinct challenge. NASA has documented that Arctic waters can be a significant methane source due to seasonal ice dynamics and physical release from subsea deposits, but those emissions involve different pathways than the biotic, oxygen-rich production described in the PNAS study. Both contribute to the ocean’s total methane budget, and neither is fully captured in current Earth system models.
What this means for climate projections
Most climate models treat the ocean primarily as a carbon sink, absorbing CO₂ from the atmosphere and buffering warming. The possibility that warming simultaneously turns the ocean into a growing methane source complicates that picture. Methane is roughly 80 times more potent than CO₂ as a greenhouse gas over a 20-year period, so even modest increases in ocean emissions could matter for near-term warming trajectories.
For researchers building the next generation of Earth system models, the practical implication is clear: aerobic ocean methane production driven by phosphate scarcity may need to be included as an active feedback rather than treated as a negligible background process. Whether modeling groups working on upcoming climate assessments will incorporate this mechanism remains to be seen, but the scientific foundation, a confirmed biochemical pathway, a global observational constraint, and a physically plausible feedback to warming, is now in place.
The scale of the risk is still being measured. What is no longer in doubt is that the ocean’s sunlit surface, once considered an unlikely source of methane, is producing it right now, and the conditions that drive that production are heading in the wrong direction.
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*This article was researched with the help of AI, with human editors creating the final content.
