Biogeochemistry

Little algae can have a big impact on the climate

Flynn, R.F., Haraguchi, L., McQuaid, J., Burger, J.M., Mutseka Lunga, P., Stirnimann, L., Samanta, S., Roychoudhury, A.N. and Fawcett, S.E., 2023. Nanoplankton: The dominant vector for carbon export across the Atlantic Southern Ocean in spring. Science Advances, 9(48), p.eadi3059. DOI: 10.1126/sciadv.adi3059 

Earth’s oceans are intimately connected with its climate. The oceans influence the amount of carbon in the atmosphere, affecting how quickly our planet is warming. The oceans are home to small algae—called phytoplankton—that perform photosynthesis, just like land plants. This process converts carbon dioxide into oxygen, removing carbon from our atmosphere and reducing the effects of global warming.

Some of the carbon used by phytoplankton sinks to the bottom of the ocean, in the form of organic material (such as the bodies of the algae themselves). This process of “carbon export” effectively cools the Earth by removing carbon from the atmosphere and storing it in the deep sea, where it can remain for a long time. Climate scientists want to understand exactly how much carbon sinks in this way, and which phytoplankton are most important for this process. This will help us make better predictions about future climate change.

Does size really matter?

An example of the small phytoplankton considered in this study: diatoms of the genus Chaetoceros. Image source; Richard Ingebrigtsen via Wikimedia Commons

Oceanographers think that large phytoplankton contribute more to carbon export than small phytoplankton. This is partly because large phytoplankton sink faster (but large is relative: “large” phytoplankton are around 1/3 the width of a human hair)! However, smaller phytoplankton may play a bigger role than expected. In a recent study, oceanographers asked how much different sizes of phytoplankton contribute to photosynthesis and carbon export in the Southern Ocean (near Antarctica).

The researchers collected seawater from various places in the spring. Using chemistry, they measured the amount of phytoplankton in the water as well as levels of nutrients that are important for phytoplankton. The scientists sorted the phytoplankton into big and small size classes and used a fancy machine called a “flow cytometer” to count and identify phytoplankton cells. Finally, they estimated photosynthesis rates by measuring oxygen (remember that photosynthesis converts carbon dioxide into oxygen). Together, these measurements let the researchers estimate how much carbon is exported at a given place and time.

Small but mighty

In the spring, the phytoplankton population explodes after a slow winter. During this spring bloom, there were far more small phytoplankton than the researchers expected. In fact, small phytoplankton made up most of the spring photosynthesis and carbon export; larger phytoplankton became more abundant later in the summer.

A lot of carbon in a short period of time 

Satellite image of a spring phytoplankton bloom near Antarctica. Image source: NASA.

Despite the relatively brief period of the spring bloom, it accounts for about 25% of the total carbon export for the year. This means that the small phytoplankton that dominate the spring bloom are big players in the Earth’s climate, much more so than previously thought.

The Southern Ocean is warming and changing, just like the rest of the planet. As it changes, it affects the phytoplankton that live there. Will these changes benefit or harm small phytoplankton? Scientists can build off of studies like this to understand whether future oceans will be friendly to different sizes of phytoplankton, and how that might affect rates of photosynthesis and carbon export in the future.

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