why algae may not be a magic bullet for carbon storage after all

In recent years, a lot of hope has been pinned on algae as a means of combating climate change.

The excitement came from studies suggesting that algae could be scaled up to capture and store massive amounts of carbon dioxide, taking advantage of rapid growth rates, large areas, and long-term storage in the deep ocean.

At present it is believed that the seaweed stores around 175 million tons annually of carbon, or 10% of the emissions of all the cars in the world. For many scientists, this has suggested the possibility that algae could join other blue carbon stores in mangroves and wetlands as a vital tool in the fight against climate change.

Although we are all ready for good climate news, there is almost always a “but” in science. Our new search has identified a major overlooked problem. Is it significant? Unfortunately yes. When we take this into account, our calculations suggest that on average algal ecosystems may not be a carbon sink after all, but a natural source of carbon.

How can this be?

There were good reasons to consider coastal algae as an important global carbon sink. Some species can grow up to 60 centimeters per day. Algae cover approximately 3.4 million square kilometers of our oceans. And when the wind and waves break up the fronds and bits of seaweed, some escape consumption and are instead swept to the depths of the ocean and deposited.

Read more: How farming giant algae can feed fish and fix the climate

Once the algae is in deep water or buried in sediment, the carbon it contains is safely locked away for several hundred years. That is to say the time it takes for the ocean circulation to carry the waters from the bottom to the surface.

So what’s the problem ?

As the surrounding coastal waters pass through the algal canopy, they bring large amounts of plankton and other organic matter from further out to sea. This provides additional food for filter feeders such as ascidians, crustaceans living among algae and bryozoan animals that eventually cover many algae fronds.

As these creatures consume this additional food supply, they exhale carbon dioxide in addition to that produced by eating algae. Individually, the quantity is negligible. But on an ecosystem scale, their numbers and their ability to filter large amounts of water are enough to skew what the researchers call the ecosystem’s net production – the balance between dioxide inputs and outputs. of carbon. And not just a little, but potentially a lot.

A: The previous model of carbon sequestration by algae, which did not include consumption of organic carbon by invertebrates. B: Our model, which includes additional carbon inputs (S¹ and S₂). Note: Es represents carbon locked up in long-term deep-sea storage. Schematic modified from our research paper.

How did we understand this? We collected global studies that directly measured or reported key elements of net ecosystem production, ranging from polar to tropical regions.

We found that algal ecosystems were natural sources of carbon, releasing an average of around 20 tonnes per square kilometer each year.

But it could be much higher still. When we included estimates of the amount of carbon returned to the atmosphere by algae carried to the deep sea only to decompose or be eaten first, we found that algae could be a much larger natural source.

We estimate that it could potentially be as high as 150 tonnes emitted into the atmosphere per km² each year, unlike previous estimates that algae absorb 50 tonnes per km². It should be stressed that this figure is surrounded by a certain uncertainty, given the difficulty of estimating the quantities concerned.

sea ​​spray over seaweed on a New South Wales beach.
Sea squirts and other filter feeders can alter the carbon balance.

Are we abandoning the carbon storage of algae?

In short, no. If we lose algae, what would replace it? These can be sea urchin barrens – large rocky outcrops dominated by sea urchins – or smaller seaweed species or mussel beds. Climate change is already showing us in some places, giant kelp dying off en masse due to sea heatwaves and background warming in Tasmania and being replaced by sea urchin barrens.

Read more: Move over, corn and soybeans: The next biofuel source could be giant kelp

To really take stock of what algae offer in terms of carbon storage, we need to consider what any replacement ecosystem would offer.

If a replacement ecosystem is an even greater carbon source or a smaller carbon sink than the original algal ecosystem, it follows that we must maintain or restore existing algal ecosystems to further reduce greenhouse gas emissions. However, to date, we have not found enough data to test whether all surrogate ecosystems are in fact more or less important carbon sources.

What does this mean for efforts to combat climate change? This means that we should not consider algae as a magic bullet.

Any effort to quantify algal carbon storage and mitigation for algal protection, restoration, or cultivation must make a full accounting of carbon inputs and outputs to ensure we are not unintentionally making the problem worse. rather than improving it.

Like some carbon trading systems seek to include seaweedthe ability of algae to store carbon should not be overestimated.

If we are wrong, we could see perverse results where industries offset their emissions by funding the preservation or restoration of algae – but in doing so are actually increasing their emissions rather than eliminating them.

Julio V. Miller