In what might yet prove to be the 21st century’s David vs Goliath, scientists have turned their attention towards plankton poo in the search for a potential – albeit unconventional – solution to combat atmospheric carbon dioxide and mitigate climate change. 

Yes, this study has seen researchers from Dartmouth College wading through plankton excrement to reveal that these microscopic marine microorganisms – aided by clay minerals – could just enhance carbon sequestration, simply by eating and pooping. Who knew that climate change heroics could come this easy?  

While it sounds an easy life for plankton, these results haven’t been easy to come by. And this novel strategy is one backed up by a lot of science. Recently published in Nature Scientific Reports, the study suggests that spraying clay dust on the surface of the ocean converts carbon into food that microscopic marine life, known as zooplankton, would eat, digest, and then send deep into the ocean as carbon-filled faeces.

The process, scientists suggest, would simply help speed up the ocean’s natural removal of carbon from the atmosphere, and lock it away in the biological matter as it journeys to the seabed. It’s been positioned as a natural enhancement of the ‘biological carbon pump’.

“Normally, only a small fraction of the carbon captured at the surface makes it into the deep ocean for long-term storage,” said Dr Mukul Sharma, the study’s corresponding author and a professor of Earth sciences. “The novelty of our method is using clay to make the biological pump more efficient – the zooplankton generate clay-laden poops that sink faster.”

Dr Sharma received a Guggenheim Award in 2020 to pursue the plankton poo project and presented the findings at the American Geophysical Union annual conference in Washington DC earlier this month.

So, what’s the ordure of things here? Well, the process begins with spraying clay dust at the end of an algae bloom. Blooms like these can grow to cover hundreds of square miles and remove about 150 billion tonnes of carbon dioxide from the atmosphere each year, converting it into carbon particulates. All good so far.

However, once the bloom dies, marine bacteria then devour the particulates, releasing most of the captured carbon back into the atmosphere in the process.

In a bid to redirect that carbon downwards, Dartmouth College researchers discovered that clay dust can attach to carbon particulates before they re-enter the atmosphere, placing them back into the marine food chain as tiny sticky pellets that (notoriously ravenous) plankton will then consume and later excrete at lower ocean levels.

“This particulate material is what these little guys are designed to eat,” said Dr Sharma. “Our experiments showed they cannot tell if it’s clay and phytoplankton or only phytoplankton – they just eat it. And when they poop it out, they are hundreds of metres below the surface and all that carbon is, too.”

Plankton plays a crucial role in the process of moving carbon from the atmosphere and into the depths of the ocean. Earlier this year, it was reported that climate change is currently impacting upon plankton’s ability to complete this process, suggesting that continued hinderance could lead to a devastating effect on the ocean driven through the climate crisis.

The study saw the team carry out laboratory experiments on water collected from the Gulf of Maine during a 2023 algae bloom. They discovered that when clay attaches to the organic carbon released when a bloom dies, it prompts marine bacteria to produce “a kind of glue” that causes the clay and organic carbon to form little balls, called flocs. Irresistible to zooplankton – a plankton which will routinely “gorge” on these flocs – this process present to them a veritable ‘smorgasbord’ of particulates to munch on. Once digested, the flocs embedded in the animals’ faeces sinks, “potentially burying the carbon at depths where it can be stored for millennia.”

Experiments conducted by the Dartmouth College team found that clay dust captured as much as 50% of the carbon released by dead phytoplankton before it could become airborne. They also found that adding clay increased the concentration of sticky organic particles – which would collect more carbon as they sink – by as much as ten times.

This is all a natural enhancement of a process known as the biological pump, in which descending flocs play an essential role as ‘marine snow’. Marine snow is the “constant shower of corpses, minerals, and other organic matter” that falls from the surface of the ocean, bringing food and nutrients to the deeper ocean.

Zooplankton accelerate this process by performing a daily practice known as the diel vertical migration. This is when – in darkness – the animals will rise hundreds, even thousands of feet from the deep ocean in one mass to feed on the nutrient-rich water near the surface. By morning, the animals will begin their descent back to the depths, where they will deposit flocs as faeces.

So, plankton poop… is this a dung deal? Well, not quite. The next step for Sharma is to take this hypothesis outside of the lab and into the field, something he plans to do by spraying clay on phytoplankton blooms off the coast of Southern California. By placing sensors at various depths offshore, Sharma hopes to capture how different species of zooplankton consume the clay-carbon flocs to begin to process just how much carbon is being locked away.

“It is very important to find the right oceanographic setting to do this work,” continued Dr Sharma in a statement. “You cannot go around willy-nilly dumping clay everywhere. We need to understand the efficiency first at different depths so we can understand the best places to initiate this process before we put it to work. We are not there yet, we are at the beginning.”

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