It is common knowledge that photosynthesising plants produce the oxygen in the air we breathe. What most people don’t know, is that terrestrial plants only provide us with about half of the oxygen present in the air. Few of us actually know that marine phytoplankton (the plants that float in the surface waters of our oceans) provide at least half of the oxygen we breathe. And just like plants in the terrestrial environment phytoplankton form the basis of virtually all ocean food webs, supporting the productive and diverse marine ecosystems.
Phytoplankton, like terrestrial plants, need sunlight and inorganic nutrients (nitrates, phosphates and sulphur) to photosynthesis and grow. Most phytoplankton are naturally buoyant and keep themselves within the surface layer of the ocean, where sunlight can penetrate (the euphotic zone). The essential nutrients are gained from the surrounding water and just like soil, the nutrients available in the water is variable, which mean there are areas of high and low productivity in the oceans.
Phytoplankton take in carbon dioxide from the atmosphere and surrounding water. The carbon dioxide is turned into organic compounds, which when the phytoplankton die, are transported to the ocean depths. Some is recycled in the surface and mid-ocean waters by other animals and the rest falls to the ocean floor. These process form part of the Global Carbon Pump (below) and has in the past helped reduce the effect of burning fossil fuels on global temperature. However experts suspect we are reaching the point where the ocean can no-longer absorb the increasing level of carbon and is starting to cause ocean acidification.
In size, the larger phytoplankton are split into two main groups. Diatoms (left image of a coccolithophore) create elaborate skeletons from silica and rely on the ocean currents to transport them. Dinoflagellates (right image of a ceratium) on the other hand, have a tail (flagella) that help them move through the water column and can either possess or not possess a hard shell/skeleton. Dinoflagellates can also be heterotrophs (engulf other phytoplankton to gain energy) as well as autotrophs (photosynthesis). The other important but smaller phytoplankton, that were only discovered in the 1970’s when technology made them visible, are the photosynthetic bacteria. Although invisible to us, one tablespoon of seawater contains hundreds of thousands of cells.
The larger diatoms and dinoflagellates can have a negative effect on the environment. When a large amount of nutrients becomes available they bloom. They can cause the discolouration of large patches of the ocean (red tides) and some phytoplankton can produce harmful toxins and are monitored by marine scientists.
However, these few negative, short-term effects caused by phytoplankton are completely overshadowed by their important role in providing us with oxygen to breathe and creating productive oceans. Relatively little is known about phytoplankton, as the majority of species are incredibly hard to grow and study in the laboratory and it is difficult to assess influencing factors out at sea, as so many factors can affect phytoplankton. This is one of the many reasons why there should be global protection of the world’s oceans and continue investing in research.
Woods Hole Institute website
growingalgae.org (red tide)
Alison R Taylor (Coccolithophore)
teachoceanscience.net (phytoplankton and ceratium)
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