The biological pump and the role of ocean plankton in mitigating global warming

The biological pump is a process by which the ocean, with the help of marine organisms, captures CO2 from the atmosphere and buries it in sediments, where it will remain for hundreds or thousands of years. Through this process, the ocean helps to mitigate the effects of global warming, as it captures and integrates into the living matter the same CO2 as all plants on the planet’s surface. And all this is done mostly by tiny unicellular beings called phytoplankton. Phytoplankton consists of microalgae of a few thousandths of a millimeter, but of great relevance because they are responsible for photosynthesis in marine planktonic trophic food webs.

Schematic representation of the biological pump. One of the many possible pathways has been exemplified. Drawing Albert Calbet

To understand how the biological pump works, imagine for a moment that we are a carbon atom that, together with two oxygen atoms, forms a molecule of CO2, the dreaded byproduct of burning fossil fuels. Maybe we came straight from a car’s exhaust pipe, we came out of the chimney of an industry, or just out of the lungs of our neighbor, no matter what. We, in the form of carbon, fly happily by the proximity of the sea enjoying the view, but one day, we enter into the water through a process called diffusion. In the water we are quickly trapped by a small algae that turns us into living matter with the help of the sun’s energy and some or other inorganic nutrient. Although we feel proud to be part of something bigger and more organized than a simple molecule (we are now part of a sugar chain), our joy does not last long, because a small mixotrophic dinoflagellate swallows us. Within the digestive vacuoles the complex thing we had become disintegrates again into small fractions and is used to create other complex structures. Well, not so bad, now we are part of something even bigger and that makes us happy. However, a ciliate that passed by makes us part of his diet and the digestion process starts over again. But the odds decided that, this time, we did not finish the digestion process because a copepod chews us and we end up in the beast’s stomach. With time and patience (i.e., by catabolic and anabolic processes), we move to a lipid chain that goes to the cephalothorax of the copepod in the form of a drop of fat. Our host migrates to s deeper zone during the day to avoid being consumed by fish, which as we all know are visual predators. At sunset, we ascend to shallower layers, where there are algae and other prey, but along the way, a euphausiacea (krill) attacks us and the copepod of which we were part ends up split in two. The part where we are is not ingested and we are slowly settling to the depths of the ocean — if what had attacked our guest had been a fish or a jellyfish we might still be wandering the food web and our history would be different. On the way to the abyss, bacteria and other microorganisms begin to decompose the remains of the copepod and each time we find ourselves into a smaller piece. Suddenly, there is a adrupt change of speed. Several decomposing particles have been added together and now we fall more quickly stuck to a piece of sea snow. Upon reaching the bottom, after what have seemed days, we still have some chance of being part of the benthic trophic network again by the action of crabs, worms, or other critters. However, whether by chance or because we were in a difficult-to-chew bite, our carbon atom is respected and little by little we go deeper into the sediment, where the lower pH will keep us for years, maybe decades, centuries or even millennia. By this way, that carbon atom that was part of a CO2 molecule has been trapped in the depths of the ocean.

Mixotrophic dinoflagellate. Ceratium sp. Photo by Albert Calbet.
Copepod. Calanus minor. Photo by Albert Calbet

The pathways by which a carbon atom passes from the atmosphere to the ocean floor are unlimited and of very different durations, from a few days, like the one I have represented here, to hundreds or thousands of years, if it never gets there. Of course, in the process, this carbon atom will surely contribute its grain of sand for life to continue.

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