Ocean-Based Greenhouse Gas Sequestration: The Role of Microbes in Mitigating Climate Change

The ocean is not just a vast body of water – it is one of Earth’s most powerful climate regulators. Every year, it absorbs enormous quantities of carbon dioxide from the atmosphere, slowing the rate of global warming. At the heart of this process are microorganisms, particularly photosynthetic bacteria like cyanobacteria, which drive carbon fixation on a scale that rivals all land-based plants combined. Understanding how these microscopic organisms sequester greenhouse gases – and what threatens their ability to do so – is central to understanding the future of our climate.

How oceans absorb carbon 🔗

According to NOAA, the ocean absorbs about 30% of the carbon dioxide released into the atmosphere each year. When CO₂ dissolves in surface seawater, some of it is used directly by photosynthetic organisms, while the rest reacts chemically with water to form carbonic acid. This dual process – biological uptake and chemical dissolution – makes the ocean a uniquely effective carbon sink.

The biological side of this process is largely driven by phytoplankton and cyanobacteria, microscopic photosynthetic organisms that fix dissolved CO₂ into organic matter using sunlight. Research published in Bioresource Technology Reports confirms that cyanobacteria are responsible for 20-40% of all carbon fixation in marine environments, making them indispensable to the ocean’s carbon cycle. Unlike land plants, these microbes evolved over 2.5 billion years in high-CO₂ environments, giving them exceptional CO₂-fixing capacity.

The biological carbon pump 🔗

Once cyanobacteria and other phytoplankton fix carbon into their cells, that carbon can be transported to the deep ocean through what scientists call the biological carbon pump (BCP). As Woods Hole Oceanographic Institution explains, a fraction of the organic matter produced in the sunlit surface ocean sinks to depth – either as dead cells, fecal pellets, or aggregates – where it is sequestered from the atmosphere for hundreds to thousands of years. Without this pump, the atmospheric CO₂ added by humans could be twice as high as it already is.

The BCP transports particulate organic carbon (POC) downward through the water column. A study published in Nature Communications found that sinking particles alone sequester approximately 3.9 petagrams of carbon in the California Current Ecosystem, illustrating just how significant this biological mechanism is at regional scales.

Impact of ocean acidification 🔗

The same CO₂ absorption that makes oceans valuable carbon sinks is also altering their chemistry in ways that threaten marine ecosystems. NOAA reports that since the beginning of the industrial revolution, surface ocean pH has dropped by 0.1 units – a seemingly small change that represents roughly a 30% increase in acidity, because the pH scale is logarithmic. The ocean’s average pH now sits around 8.1 and continues to decline.

When CO₂ dissolves in seawater, it forms carbonic acid (H₂CO₃), which then releases hydrogen ions (H⁺). These ions bond with carbonate ions (CO₃²⁻), reducing their availability. The U.S. Environmental Protection Agency notes that marine organisms face a two-fold challenge: decreased carbonate availability and increased acidity. Corals, oysters, mussels, and pteropods – all of which rely on calcium carbonate to build their shells and skeletons – are especially vulnerable. Shell dissolution, reduced calcification, lowered immune responses, and disrupted reproduction have already been documented across multiple species.

Threats to the carbon sequestration system 🔗

The damage from acidification goes beyond marine biodiversity – it directly undermines the ocean’s ability to sequester carbon. Studies show that reduced calcification from ocean acidification may weaken the biological pump by limiting the organisms that form carbonate-rich particles, which are key to transporting carbon to the seafloor. Calcifying organisms like coccolithophores and foraminifera contribute significantly to the sinking flux of carbon; when their shells thin or dissolve, that carbon transport pathway is disrupted.

The effects are already visible. NOAA’s ocean acidification reports describe shell dissolution in pteropods in the Southern Ocean and documented coral decline in the Great Barrier Reef, where living coral coverage has dropped significantly. In Antarctic waters, the conditions have become so corrosive that shelled organisms are dissolving outright, affecting the food web for fish, birds, and marine mammals.

Microbial contributions to oceanic carbon storage 🔗

Beyond the biological carbon pump, marine microbes contribute to carbon sequestration through a distinct and complementary mechanism: the microbial carbon pump (MCP). As described in Frontiers in Microbiology, the MCP works by transforming labile dissolved organic carbon (LDOC) – which would otherwise be quickly consumed and released back as CO₂ – into recalcitrant dissolved organic carbon (RDOC) that resists biological degradation and can persist in the ocean for decades to millennia.

What makes the MCP uniquely powerful is that it operates at every depth in the water column, independently of physical processes like sedimentation. It functions as a two-way regulator of the carbon cycle, making it fundamentally different from the BCP, which relies on particles physically sinking to depth. A 2024 review in Nature Reviews Microbiology highlights the MCP as a key component in integrated approaches for achieving ocean negative carbon emissions – a priority for climate change mitigation strategies.

Newly discovered microbes and carbon capture potential 🔗

Recent discoveries are expanding what scientists know about microbial carbon capture. Researchers from the Wyss Institute at Harvard and multiple partner institutions recently discovered a cyanobacterium nicknamed “Chonkus” (formally UTEX 3222), isolated from volcanic CO₂ seeps off the coast of Sicily. According to the published findings, this strain grows rapidly in high-CO₂ conditions, reaches 5 to 10 times the size of average microbial cells, and sinks quickly to the bottom after absorbing CO₂ – precisely the traits needed for effective biological carbon sequestration.

Stanford researchers have also found that certain cyanobacteria possess two forms of the carbon-fixing enzyme RuBisCO simultaneously – a rare genetic trait that may allow them to fix carbon more efficiently across both oxygen-rich and low-oxygen environments. This finding is particularly relevant as climate change expands oxygen minimum zones in the deep ocean, potentially allowing these dual-enzyme cyanobacteria to proliferate and store more carbon in regions previously considered less biologically productive.

At the ecosystem level, coastal blue carbon habitats also play a critical role. Research in Frontiers in Microbiology highlights that vegetated coastal ecosystems – including mangrove forests, salt marshes, and seagrass meadows – serve as intense blue carbon sinks at land-ocean transition zones, and that conserving and restoring them is an integral part of climate remediation strategies.

Future challenges and research needs 🔗

Despite the ocean’s immense carbon sequestration potential, significant uncertainties remain. Woods Hole Oceanographic Institution warns that oceans cannot continue absorbing CO₂ indefinitely, and there are early indications that the rate of absorption may already be slowing. As the ocean warms and stratifies, nutrient mixing between deep and surface waters decreases, limiting phytoplankton growth and therefore the efficiency of the biological carbon pump.

Ocean-based carbon dioxide removal (CDR) approaches – such as kelp farming, alkalinity enhancement, and iron fertilization – are being actively studied as interventions to boost marine carbon sequestration. However, NOAA’s CDR Research Strategy emphasizes that the scalability, effectiveness, cost, and ecological impacts of these approaches remain largely unknown. Some methods may inadvertently displace existing phytoplankton productivity or introduce unintended biogeochemical changes.

Acidification’s feedback on sequestration capacity 🔗

One of the most pressing concerns is the self-reinforcing feedback between acidification and reduced carbon sequestration. As acidification weakens calcifying organisms – the very organisms that help shuttle carbon to the seafloor – the efficiency of the biological carbon pump declines. Research published in Oceanography points out that even engineered marine CDR approaches that store carbon in the ocean may not fully alleviate acidification, since the location and stability of that stored carbon determines whether the problem is genuinely resolved or simply shifted elsewhere in the ocean system.

On a socioeconomic level, the stakes are enormous. The IAEA estimates that up to three billion people who depend on marine and coastal biodiversity for their livelihoods could be impacted by ocean acidification. A study in the United States found the country’s shellfish industry alone could face losses exceeding $400 million annually by 2100 if current trends continue. Understanding, protecting, and potentially augmenting the microbial processes that sustain oceanic carbon sequestration is therefore not just an ecological priority – it is an economic and humanitarian one.

The microbial world underpinning the ocean’s carbon cycle remains incompletely mapped. Key gaps persist in understanding how the MCP and BCP interact under changing climate conditions, how newly discovered microbes like Chonkus can be responsibly scaled for carbon capture, and how acidification’s cascading effects on marine food webs will ultimately alter the ocean’s sequestration capacity across different regions and depths. Bridging these gaps will require sustained investment in ocean monitoring, microbial ecology research, and international scientific collaboration.

What do you think? If marine microbes are already performing significant carbon sequestration naturally, should climate policy prioritize protecting existing ocean ecosystems over investing in engineered carbon removal technologies? And as ocean acidification increasingly disrupts the very organisms driving carbon storage, at what point does the ocean shift from being a climate solution to becoming a source of additional CO₂?

References

1. https://www.noaa.gov/education/resource-collections/ocean-coasts/ocean-acidification
2. https://www.sciencedirect.com/science/article/abs/pii/S1878818120300463
3. https://www.whoi.edu/ocean-learning-hub/ocean-topics/how-the-ocean-works/cycles/biological-carbon-pump-ocean-topic/
4. https://www.nature.com/articles/s41467-023-37771-8
5. https://www.epa.gov/ocean-acidification/effects-ocean-and-coastal-acidification-marine-life
6. https://en.wikipedia.org/wiki/Ocean_acidification
7. https://www.noaa.gov/ocean-acidification-high-co2-world-dangerous-waters-ahead
8. https://www.frontiersin.org/articles/10.3389/fmicb.2020.01039/full
9. https://www.us-ocb.org/mcp-new-insight-sequestration/
10. https://www.nature.com/articles/s41579-024-01018-0
11. https://phys.org/news/2024-10-newly-cyanobacteria-sequester-carbon-oceans.html
12. https://sustainability.stanford.edu/news/ocean-microbes-unusual-pair-enzymes-may-boost-carbon-storage-study-suggests
13. https://www.whoi.edu/ocean-learning-hub/ocean-topics/how-the-ocean-works/cycles/carbon-cycle/
14. https://oceanacidification.noaa.gov/carbon-dioxide-removal/
15. https://tos.org/oceanography/article/perspectives-on-marine-carbon-dioxide-removal-from-the-global-ocean-acidification-observing-network
16. https://www.iaea.org/bulletin/how-carbon-emissions-acidify-our-ocean