Phytoplankton: The understated hero

I was in first grade the first time I remember hearing the word phytoplankton. It was in one of my science classes and I was mesmerized how something so little could be so important. Phytoplankton are microscopic marine algae that are a fundamental part of oceans, seas and freshwater ecosystems. They are the first link in the food chain; and besides their importance providing food to sea creatures, phytoplankton play a key role in regulating the climate of our planet. Of course, I did not learn this in first grade. They only told us the food chain part because no one talked about climate back then.

Much evidence exists to prove that climate-warming trends over the past century are linked to human activities. Anthropogenic carbon dioxide (CO₂) emissions have increased at a rapid rate since the mid-twentieth century. CO₂ and other greenhouse gases trap heat in our atmosphere causing global temperatures to rise making oceans’ temperatures warmer, ice sheets shrink, glacial retreat, and sea levels rise.

What do these microscopic creatures have to do with climate? Phytoplankton live in the oceans, which are a major sink for CO₂. Approximately 30% of CO₂ anthropogenic emissions are removed from the atmosphere and absorbed by oceans thanks to phytoplankton. They perform half of all photosynthesis on Earth making them vital mediators of the biological carbon pump. The biological pump is a natural process that regulates atmospheric CO₂ levels. Organic and inorganic carbon fixed by phytoplankton are transferred from the euphotic zone of the ocean (closest layer to the surface that receives enough sunlight for photosynthesis) to the bottom to underlying sediments. The carbon taken out from the atmosphere is held in dissolved form and in oceans organisms for thousands of years maintaining significantly lower CO₂ levels in the atmosphere than there would be otherwise.

The Earth naturally removes CO₂ from the atmosphere by many mechanisms including oceans (phytoplankton), soil, plants and even rocks; but the rate on which humans emit it is much faster. CO₂ concentration in the atmosphere is currently about 400 ppm and it is expected to increase. That is why scientist are working on ways to accelerate the CO₂ removing process. One way is carbon sequestration, which means capturing carbon emissions and diverting them into a secured storage. There is geologic carbon sequestration and oceanic carbon sequestration. For oceanic carbon sequestration, there are two strategies. One is direct injection of liquefied CO₂ and the other is to “prime” the biological pump by fertilizing the ocean.

Direct injection is a strategy that consists: first, in capturing the CO₂ from the air; second, liquefy it; and third, inject it. Some industrial plants capture their emissions, which are later processed to obtain high purity CO₂ which is easier to liquefy, transport or use. When CO₂ is in liquid form, it is denser than water and may be possible to store it at the bottom of the ocean as liquid or trapped in ocean sediments or ice-like solids called hydrates. As a liquid, it would basically form a CO₂ lake at the bottom of the ocean. It sounds dangerous as CO₂ is toxic. Additionally, leaks of CO₂ increase ocean acidity as it reacts with water forming carbonic acid. The increased acidity of oceans affects their balance and biodiversity. A major leak could have catastrophic consequences, reason why this strategy needs to be studied thoroughly and tested in a controlled environment.

The second strategy consists on fertilizing the oceans to allow phytoplankton to bloom. Some areas of the ocean are rich in nutrients like nitrogen and phosphorous but poor in phytoplankton. One limiting factor for phytoplankton growth is iron, and the major source of iron for the ocean is dust, which explains why ocean areas poor in phytoplankton are the ones furthest away from land. If the amount of phytoplankton increases, then there are more of them to perform photosynthesis, increasing the ocean’s productivity and the rate of the biological pump. But what happens if the phytoplankton grow out of control forming harmful algae blooms that produce toxic compounds that can harm fish and other marine creatures? Numerous iron-dumping trials have been conducted and have scientists worried because promoting phytoplankton growth can change the concentration of other gases in both water and air.

So far, the studies done on both of the above methods for oceanic carbon sequestration have not satisfactorily answered questions regarding the long-term effects; but scientists keep looking for solutions.

In any case, phytoplankton are doing the job by themselves helping reduce one third of the CO₂ in our atmosphere and are the key mediators of the biological carbon pump. For keeping CO₂ concentration much lower than it would be otherwise, we should thank them.

Sources:

M. Divya et al. A Study of Carbon Sequestration by Phytoplankton (2019), S. Basu and K.R.M. Mackey. Phytoplankton as Key Mediators of the Biological Carbon Pump (2018), NASA, Science Beat

Featured image: NOAA