The Elephant in the Room: Rethinking Calcification in Ocean Carbon Removal

When discussing ocean-based carbon removal, one fundamental question has arisen and sparked significant debate amongst scientists: does marine calcification act as a source or a sink of CO₂? It’s a question that is especially important for us here at Blusink, as calcification is a key part of our solution, and it’s a topic that can be both controversial and complex, given the varied and dynamic interactions between carbonate chemistry and biological processes in marine ecosystems.

The Challenge: CO₂ Release During Calcification

Calcification—the process by which marine organisms like corals, shellfish, and coralline algae build their calcium carbonate structures— releases CO₂. This is because when calcium carbonate (CaCO₃) forms, the following reaction occurs:

At first glance, this seems counterproductive for carbon sequestration, as CO₂ is released back into the water and can potentially return to the atmosphere. However, this is only part of the story.

Beyond Simple Chemistry: The Ocean's Dynamic Carbon System

The ocean has different mechanisms for capturing carbon compared to terrestrial ecosystems. On land, we think of photosynthesis and its direct capture of CO₂ from the atmosphere. When considering technologies like Blusink that work on the ocean seafloor, we must account for something obvious but often forgotten: the water column.

Think of the water column as a complex system where multiple reactions occur simultaneously—photosynthesis, respiration, calcification, and dissolution—all driven by different organisms. Both photosynthesis and dissolution consume CO₂, while calcification and respiration release it.

When measuring CO₂ in the water column or at the surface, it's virtually impossible to distinguish which specific reaction is responsible for the produced or consumed the CO₂, as these processes are interconnected. Instead, scientists typically measure the sum of all these reactions combined. As an illustrative example:

1. If an area has some photosynthetic organisms but is dominated by organisms that respire (like fish), we will likely see increasing CO₂ in this area as more is being produced more than consumed. An example of this might be a coral reef.

2. Conversely, if there are more photosynthetic organisms than those that respire (for example an area dominated by seagrasses or kelp) we will see a decrease in CO₂, as it is being consumed more than produced.

3. In an area with many calcifying organisms but little dissolution and photosynthesis, we will see an increase in CO₂, as it is being produced more than being consumed.

4. Finally, in an area with abundant photosynthetic organisms, high dissolution rates, and low rates of calcification and respiration, there will be a decrease in CO₂, as it is being consumed in larger quantities than produced. Research indicates that Rhodolith beds often fall into this category, although these characteristics are always site dependent.

   
   

These examples illustrate just a few possible scenarios—marine ecosystems are complex and depend on many interacting factors, leading to countless potential combinations. Therefore, it is crucial to understand the system as a whole rather than individual reactions to grasp the overall effect on the carbon budget.

The movement of carbon between the ocean and atmosphere follows a basic principle: it flows based on CO₂ concentration differences between the two. When the ocean contains more CO₂ than the atmosphere, carbon flows upward into the atmosphere. Conversely, when atmospheric CO₂ levels are higher, carbon flows downward into the ocean, creating ocean carbon sinks. As stated above, the amount of CO₂ in the water column depends on the sum of all reactions: photosynthesis, respiration, calcification, and dissolution. This makes it far more complex than examining just one individual reaction!

Diving Deeper: How Blusink’s Technology Turns the Equation Around

At Blusink our technology has two main mechanisms of capturing carbon to lower the amount of CO2 in the water column, creating that downward flow from the atmosphere that is required for CO2 capture:

1. The first is through a purely chemical pathway: The dissolution of minerals in the Blusinkies increases ocean pH, capturing CO₂ in the process and turning into carbonate (CO3-)—similar to Ocean Alkalinity Enhancement (OAE) experiments. We'll explore this process in another blog post soon!

   
   

2. The second is through the colonisation of coralline algae that form rhodoliths. Rhodoliths are unique algae that, like other plants, photosynthesise—but unlike other plants, they also incorporate calcium carbonate into their structure through calcification. This process creates their distinctive hard, pink, stone-like structures. These organisms both consume and release CO₂, but here's the key: they consume more CO₂ than they produce, especially in an environment with high pH and abundant carbonates available to create their structure (exactly what we achieve in our first route for capturing carbon). This is because even when calcification rates increase, primary productivity (photosynthesis) also increases, which consumes CO₂ resulting in a net reduction of CO₂. You can read more about it in both if these articles: "Pink power”—the importance of coralline algal beds in the oceanic carbon cycle and The potential environmental response to increasing ocean alkalinity for negative emissions

   
   

Even better, the calcium carbonate structures created by these organisms represent one of the most secure forms of carbon storage on the ocean seafloor. They demonstrate exceptional stability and longevity, with virtually no risk of carbon release, as they form permanent, living rock structures. This storage method is particularly valuable because we can directly measure and verify the amount of sequestered carbon—providing concrete, quantifiable evidence of its effectiveness as a carbon removal solution.

Conclusion: Rethinking Marine Calcification

The idea that calcification is purely a CO₂-emitting process is an oversimplification that ignores the complexity of marine carbon cycling. While calcification releases CO₂ in isolation, the overall carbon balance in marine ecosystems depends on multiple interrelated processes—including photosynthesis, respiration, dissolution, and calcification.

Blusink's approach exploits this dynamic system to turn calcification into a net carbon sink. Through specific mineral dissolution from our Blusinkies, we not only enhance CO₂ capture but also create ideal conditions for coralline algae to thrive. These organisms uniquely combine photosynthesis and calcification, leading to a net reduction in CO₂. The calcium carbonate they produce locks carbon into highly stable structures on the seafloor, ensuring long-term sequestration and reliable carbon quantification.

Ultimately, ocean-based carbon removal isn’t about isolating single reactions—it’s about harnessing the full potential of marine ecosystems to create sustainable, measurable, and permanent carbon sinks.