A novel catalyst for carbon-capturing algae

Rhodoliths, a group of bottom-dwelling ocean algae have a solid, rock-like appearance which is very far from the public’s conception of “seaweeds”. Unfamiliar and uncelebrated, rhodoliths quietly suffered large population losses in recent decades due to bottom trawling, but Blusink is changing the narrative on the species from trawler victim to climate hero – as the startup’s chief growth officer, Ellie Morris, explains. 

Before we get into the details of Blusink’s business model: what exactly are rhodoliths?

The classification is a bit vague, which causes confusion. We count 50 to 60 different species of coralline algae. Rhodoliths are typically the more rounded, rock-like forms that roll around on the seafloor. Then you have “maerl,” which are spikier and more plant-like, they actually look a bit like corals. There are also hybrids that fall somewhere in between. So, everything in this group is technically coralline algae, and whether something is called a rhodolith or maerl depends on its formation. In practice, though, the terms are often used interchangeably.

Regardless, many studies show that these habitats serve as important spawning grounds for various fish species and support a strong presence of other marine organisms. Rhodoliths really are a keystone species that support a wide range of marine life.

What led your team to focus on rhodoliths?

Initially, our work was in material science for coral restoration, developing substrates for corals to settle on. During our material testing, we noticed that these substrates were frequently colonised by rhodoliths.

This led us to explore rhodoliths as a blue carbon store, and we saw an overlooked opportunity. Among all blue carbon habitats, coralline algae are actually the most widely distributed on the planet, with significant advantages in the way they store carbon. There’s been increasing research over the past decade into their potential, but they’re still rarely considered in national blue carbon strategies. 

Ultimately, we chose rhodoliths because of their huge potential and the biodiversity benefits they provide. They’re essentially reef builders. Thanks to their structure, their freedom to move and the way they calcify, they create complex seafloor habitats that enhance biodiversity. We really liked that combination of carbon storage and marine biodiversity.

They’re quite slow-growing, though, is that right?

Yes, they are very slow-growing, although it depends on the region. For instance in the UK, we're looking at one to two millimetres a year. The upside here is that even with such slow growth, rhodoliths can live for hundreds or even thousands of years.

In tropical areas like Japan or Malaysia, growth rates are higher - up to five millimetres per year or more. So, water temperature and conditions make a big difference in how quickly they grow.

What business model are you considering?

We have three strands to our business model. 

The first is the carbon angle. What we’re trying to show is that rhodolith beds can be really significant carbon sinks if we operate at scale. However, we need an external certification body to verify our claims, and since no certification methodology currently exists, we have to co-develop that with a registry ourselves. 

This is why, while all our scientific work is aimed at eventually selling carbon credits, this is more of a long-term strategy for us. The good news is that, because the carbon is stored in calcium carbonate form, it’s extremely durable, lasting thousands of years, which means it should command a high price on the carbon market.

The second strand is biodiversity enhancement. Rhodoliths are ecosystem builders, and there are many applications for restoration. For example, having thriving rhodolith beds near coral reefs has been shown to improve the resilience of coral spawns. There’s also potential for reviving areas affected by bottom trawling. We pursue this through partnerships with governments and conservation charities; we are currently working with Conservation International.

After you put the rhodoliths in the water, you can see a positive effect within three months, so it’s easy to quantify and get started with. The challenge here is that there is no scientific consensus yet on how to monitor biodiversity as a whole, so we’re developing our own methodology for that. 

The third strand is aquaculture. Rhodoliths can slightly deacidify the water and raise the pH. Some aquaculture projects add large volumes of powdered aggregates or minerals to regulate pH. Since our product is in rock form, you could place it at the bottom of a tank and potentially regulate pH for much longer than powder. While we’re looking into this application, we don’t know the economics of this approach yet.

You mention that your substrate raises the pH of the water. That actually sounds a lot like ocean alkalinity enhancement, a more common marine carbon dioxide removal (CDR) technique.

You are right, that’s the final piece of this puzzle: it’s not just biology. To validate this, we’re currently running a mesocosm experiment with Plymouth Marine Laboratory to quantify the chemical impact of our substrate and combine it with the biological effects of the rhodoliths themselves. Essentially, the substrate’s material induces a chemical reaction, while the rhodoliths add their own de-acidifying effect as they grow. Together, we believe this dual approach amplifies the carbon sequestration potential.

How do you ensure successful growth on the ocean floor?

For the substrate to be successfully colonised, two factors are key. Firstly, the geometry and porosity of our substrate is optimised for rhodolith attachment, it also allows them to roll around and interlock, as they naturally do. 

Secondly, our pebbles are designed to slightly elevate the pH of the environment, and certain minerals added in the substrate make sure the algae have everything they need to get started. The algae are already there - they are just waiting for a substrate and the right conditions, which we provide.

Can you explain how your process works from an operational point of view?

We’re still in the pilot phase, but our general approach is to use apple-sized discs we call “Blusinkies” and deploy them in clusters on the seafloor. Before deployment, we conduct site-specific surveys to assess water conditions and existing ecology. This helps us avoid disrupting local ecosystems or smothering existing habitats. We typically place 40–100 discs per square metre, stacking them in quadrants of ~10 square metres for pilot projects.

To make this financially viable we need scale. For this we are currently looking at rock dumping vessels to cover areas between 100 square metres and a hectare. For now, though, deployment is manual: diving teams place and monitor the discs. 

To monitor we currently sample and analyse in the lab, and we use incubation chambers to measure water chemistry changes, but long-term we plan to shift to imaging technologies like photogrammetry to create 3D models of habitats and track biodiversity and carbon metrics.

We revisit sites annually to measure results against our estimates. For carbon, we measure the water’s chemistry. For biodiversity, we look at the rugosity (surface complexity) and species counts compared to baseline data. Typically, the projects will be between five years for tropical zones and ten years for temperate zones. 

You mentioned incubation chambers. Are you speeding up growth?

No, we don’t incubate to accelerate growth. We do it to isolate chemical interactions. Say we have our substrate. It's been colonised by a coralline algae: we are starting to see the rock go a bit pink and green. What we want to know is what the respiration and photosynthesis rates are, because that helps us to calculate the net carbon sequestration.

Calcifying organisms on their own are actually a net source of carbon, which our critics tend to pick up on first, but they miss that in balanced ecosystems with photosynthesis, they become a carbon sink. So we need to measure more to prove that – that’s what the chambers are for.

Previous ocean-based carbon removal startups, I am thinking of Running Tide in particular here, have faced extensive criticism from scientists and environmentalists. Obviously, your approach is very different, but do you anticipate similar pushback?

We definitely experience resistance on a daily basis, as I’m sure everyone in the blue carbon market does. 

Environmental groups have questions about the risks of shading, of changing the local species composition. The key difference with our approach is that we’re not introducing a new species. We’re simply placing a substrate in the water. If the target species isn’t already present in that environment, it won’t get colonised, and the project simply won’t work. We’re literally just putting in a piece of rock to see what happens.

But what if for example the substrate gets colonised by the wrong species, say, a fleshy algae instead of coralline algae? That could be a problem, and of course we have risk mitigation strategies in place for that.

We also get a lot of pushback on the mechanism. There is a general reticence from the carbon registries and certifiers around biological solutions. It’s really about education; people need to understand that not all biological mechanisms are created equal.

Some argue that because we use a calcifying organism, we’re actually contributing to carbon emissions through carbonate chemistry, which isn’t true if you look at the whole water column and all the processes our system is driving. The same with people who argue that carbon drawdown can only happen on the surface. Again, this is something we will just have to keep explaining.

Where are you now in terms of the development of the company? 

At the moment, we have two sites in Colombia, where the idea for Blusink started. Then we have one in the Maldives, and one off Madeira. Looking ahead, we’re working on deployments in the Red Sea and Indonesia. Additionally, we want to validate our approach in a temperate zone, so we’re talking to people for potential sites in Jersey, Wales and Scotland. 

We're hoping to announce some funding soon, which would give us a decent runway to gather much more data, especially to support the verification pathway for the carbon part of our model. The team will expand in the coming months to support this process. 

We want to have external validation to build credibility beyond our own data. For this we collaborate with a few universities. For example at KAUST University in Saudi Arabia, researchers are looking to further validate if and how our method can enhance coral growth. 

The focus now is on growing these partnerships and pilot sites to gather more data and build up our documentation for certification. We’re starting our first restoration projects with several NGOs, hopefully by the end of the year. 

Finally, what is the biggest challenge Blusink needs to overcome to make this a success? 

I think we have two big challenges. The first is something that applies to the entire marine carbon dioxide removal community: access to sea space, permitting and figuring out who owns the rights to sell credits from that space. There have been all sorts of creative ways that companies have tried to address this. For example, some ocean alkalinity enhancement companies look at river projects instead, because river permitting is easier. The issue of profit-sharing from sea space is really challenging. No one really owns the deep sea, so who benefits? It’s a nebulous space, and that affects all blue carbon projects. 

The second big challenge for us specifically is that we have a unique methodology. There’s no one else combining materials engineering, chemistry and biology in the way we do. It’s our key selling point, but it also makes things harder, because there’s no existing certification pathway. People don’t always know what we’re talking about, so our education gap – with customers, registries, policymakers, everyone – is much bigger. That’s probably our biggest problem: the unfamiliarity with our technology.