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In a study recently published in Nature Communications Sustainability, researchers Mojtaba Fakhraee and Noah J. Planavsky have identified a powerful, hidden mechanism by which seaweed farming can combat climate change.

While seaweed is already celebrated for its ability to absorb carbon dioxide (CO₂) through photosynthesis, this new research suggests that its most significant climate impact might actually be happening beneath the waves, in the seafloor sediments.

The “Hidden” Carbon Sink: Sedimentary Alkalinity

Traditionally, the climate value of seaweed farming has been measured by how much biomass is harvested or sunk to the deep ocean. However, this study, titled “Seaweed farms enhance alkalinity production and carbon capture,” uses advanced geochemical modeling to show that seaweed farms act as catalysts for a process called sedimentary alkalinity production.

When organic matter from seaweed – such as falling fronds or metabolic waste – settles on the seafloor beneath a farm, it stimulates microbial activity in the sediment. This activity triggers chemical reactions (like sulfate reduction and the dissolution of minerals) that produce alkalinity.

Why Alkalinity Matters

Alkalinity is essentially the ocean’s “antacid.” When the alkalinity of seawater increases, its capacity to absorb and chemically “lock away” CO₂ from the atmosphere grows. Unlike organic carbon, which can be easily released back into the atmosphere if the seaweed decays, carbon stored via alkalinity is extremely stable and can remain sequestered in the ocean for thousands of years.

Key Findings of the Study

• Significant CO₂ Drawdown: The researchers’ stochastic simulations suggest that this enhanced alkalinity production could remove an average of 0.85 tonnes of CO₂ per hectare per year. Depending on local conditions, this figure can range from 0.1 to over 2 tonnes.
• Beyond Photosynthesis: This process represents an additional carbon sequestration pathway that has been largely overlooked in previous blue carbon accounting.
• A Solution for Ocean Acidification: By increasing local alkalinity, seaweed farms don’t just capture carbon; they also help neutralize the rising acidity of coastal waters, potentially protecting shellfish and coral reefs in the surrounding ecosystem.
• Potential for Enhancement: The study suggests that if seaweed aquaculture is combined with “mineral addition” (such as adding limestone to the farm area), the rates of alkalinity production—and thus carbon capture—could be amplified even further.

Implications for the Future

The study highlights seaweed farming as a multi-functional tool for “Carbon Dioxide Removal” (CDR). Beyond providing food and sustainable feedstocks for industry, these underwater forests serve as “alkalinity factories.”

However, the researchers emphasize that this isn’t a “one-size-fits-all” solution. The efficiency of this carbon capture depends on factors like water temperature, salinity, and how well the sediment “breathes” (bio-irrigation).

As the world looks for scalable, nature-based solutions to meet net-zero goals, this research provides a compelling argument for the rapid expansion of seaweed aquaculture. By farming the seas, we aren’t just cleaning the water – we are fundamentally altering the chemistry of the ocean to better protect our planet’s atmosphere.