We often consider trees and other terrestrial plants to be our lifeforce, the producers of the air we breathe. Yet this is only half true – phytoplankton produce roughly 50% of the oxygen we breathe. Phytoplankton are microorganisms that drift in the ocean and photosynthesize. They are often recognized for their role in harmful algae blooms (HABs), but they are so much more important than we give them credit for. In fact, they are quite cool and worth knowing more about. This article is meant to provide a bit more insight into what phytoplankton are, how they are impacted by climate change, and why you should care about them.

What are phytoplankton?

Phytoplankton are primary producers that form the foundation of the marine ecosystem. They literally translate to “plant drifter” in Latin since they photosynthesize and drift in the ocean. Phytoplankton are versatile creatures that are capable of many things, from oxygenating the planet, to being the foundation of the majority of marine food webs, and even influencing the weather. To understand what phytoplankton are, it is important to acknowledge where they came from. Phytoplankton can trace their lineage back to cyanobacteria, or blue-green algae. Cyanobacteria are prokaryotes that are credited with breathing life into our planet. They were the first and remain the only prokaryotes to perform oxygenic photosynthesis and they quite literally changed the world. Due to the biogeochemical changes they initiated on Earth by producing oxygen, cyanobacteria allowed for complex life to evolve in oceanic and terrestrial  environments. According to endosymbiotic theory, the phytoplankton that we’ve come to know and love today evolved from early cyanobacteria.

As microorganisms, phytoplankton are usually invisible to the naked eye and are primary producers. However, not all are made the same. Many groups of phytoplankton are recognized today, but two are prominent: dinoflagellates and diatoms (though coccolithophores might sound familiar!). Dinoflagellates are eukaryotes that are generally identified by their two flagella, one of which is similar to a little tail that helps the cell move. Diatoms are eukaryotes best known for their beautiful siliceous shells. Coccolithophores are single-celled organisms that have several calcite scales decorating themselves. All three of these are photosynthesizing, contributing to primary production in the ocean.

These tiny organisms that can barely be seen are capable of keeping our entire planet humming.

I want to take a moment here to re-emphasize how amazing this is – phytoplankton photosynthesize in the ocean and produce half of the entire planet’s oxygen. Through photosynthesis, the phytoplankton also assimilate carbon in the surface layer of the ocean. After dying, if they’re not eaten, phytoplankton then sink to the ocean floor. Some are then buried in the ocean’s sediment, storing carbon that would have otherwise been released back into the atmosphere during their decay^[1]. These tiny organisms that can barely be seen are capable of keeping our entire planet humming. That is amazing!

Where do phytoplankton live?

The highest concentrations of phytoplankton are found around the equator, at high latitudes, on continental shelves, and in coastal zones of the ocean, where sources of nutrients are highest. Like terrestrial plants, phytoplankton need sunlight and nutrients such as nitrogen, phosphorus, and iron to photosynthesize and grow. They tend to bloom seasonally when nutrient supplies are abundant. Upwelling, wind-blown dust, and river runoff all can increase nutrient supplies. Together with sunlight, these nutrient-rich areas promote phytoplankton growth and fuel primary production. This photosynthetic process produces half of the planet’s oxygen, making the ocean the Earth’s second lung. At the end of phytoplankton’s life cycle, they are often subject to being eaten by zooplankton or dying and sinking through the water column. When phytoplankton die at the surface level, they sink and cause marine snow. As they decompose, they release carbon and continue to provide food for other organisms. Additionally, as phytoplankton fall to the bottom of the ocean, they are buried in the sediment and sequester carbon. Phytoplankton are the powerhouse of the ocean!

How are phytoplankton being affected by climate change?

As we’ve learned, nutrients are one of the key components that allow phytoplankton growth. That being said, excess nutrients can result in excessive blooms, frequently referred to as harmful algal blooms. One example of a widely known HAB is the red tide that sweeps across Florida coastlines. These HABs have a strong, unpleasant odor that can irritate the eyes, nose, and throat and they are often deadly for sea life. However, not all algal blooms are inherently harmful, and instead are a natural process that often indicates a healthy ecosystem that is in sync with the ebbs and flows of seasonal changes.

So what does this have to do with climate change? Phytoplankton photosynthesize in the upper layer of the ocean where sunlight can penetrate. Warming waters and warming atmospheric temperatures can accelerate phytoplankton growth. Additionally, rates of precipitation are increasing with the warming climate, causing more rainfall that can flood rivers and cause increased river runoff. With the combination of warmer water, bright sunlight, and increased nutrients, some phytoplankton habitats are a little too suitable. With these changes occurring more frequently and with longer duration, algal blooms now sometimes last longer and become toxic. This can have devastating effects, for example by creating dead zones and fish kills. Instead of enabling primary production at a healthy rate, HABs can deplete the water column of oxygen, causing mortality among organisms such as fish that depend on it.

Additionally, increasing fossil fuel use results in higher carbon dioxide concentrations in the atmosphere which can influence phytoplankton. The ocean absorbs about thirty percent of the carbon dioxide in the atmosphere, and consequently the overall CO₂ concentration in the ocean has been steadily rising. This changes the pH of ocean water, which is normally about 8.1, or basic. The extra carbon dioxide acidifies water, lowering the pH. This can be harmful to many organisms that have evolved to live at higher pH. As a result, ocean acidification can slow the growth of some plankton. Some coccolithophores have been severely affected by this because it can be increasingly difficult to form their calcium carbonate coccoliths.

Other climate-change induced effects on phytoplankton include increased ice melt causing earlier or multiple blooms in the polar region, and stronger stratification that suppresses mixing in the water column. In polar regions, the formation of seasonal sea ice is a prominent phenomenon. Historically overlooked, phytoplankton blooms can occur under the ice where light is limiting. With the temperature of the polar regions warming, much of the seasonal ice is not as thick as in years past, potentially augmenting the frequency and/or strength of blooms in the region. At the same time, the upper layer of polar oceans is warming, which creates a larger temperature difference between the surface, middle, and bottom layers of the water column^[2]. This can impede upwelling in these regions, altering nutrient concentrations and influencing seasonal processes of importance to phytoplankton.

…so what?

At this point, I think it’s safe to say that you now think phytoplankton are extremely cool, though you might be wondering, “so what?” Since we know they are the foundation of the marine ecosystem, any change in phytoplankton’s natural processes has a ripple effect for other organisms, thus potentially changing the dynamic of the marine ecosystems where phytoplankton are present. Since phytoplankton are oceanic microorganisms, they often escape our immediate thought of climate change, but preserving phytoplankton is the key to preserving life on this planet and it’s essential that they are included in the conversation. Whether it’s talking about marine food webs or climate change impacts, make sure you’re not leaving out these little creatures! They provide half of our planet’s oxygen – so take a breath, now take another. You owe that to phytoplankton!

If you’re still interested in learning more about phytoplankton, I recommend checking out the resources on the NOAA, NASA, Washington Department of Ecology, or Plankton Chronicles pages!

^[1] “Phytoplankton as Key Mediators of the Biological Carbon Pump.” https://www.mdpi.com/2071-1050/10/3/869. Accessed 6 Feb. 2022.

^[2] “The effects of irradiance and nutrient supply on the productivity of ….” https://link.springer.com/chapter/10.1007/978-1-4020-9460-6_7. Accessed 6 Feb. 2022.

Contact Currents’ Editor-in-Chief for access to:

Basu, S., & Mackey, K. R. (2018). Phytoplankton as Key Mediators of the Biological Carbon Pump: Their Responses to a Changing Climate. Sustainability.

Tremblay, J.-E., & Gagnon, J. (2009). The effects of irradiance and nutrient supply on the productivity of Arctic waters: a perspective on climate change. Influence of Climate Change on the Changing Arctic and Sub-Arctic Conditions, 73-93.