What are plankton?

We often think of the sea as being dominated by fish and whales. But microscopic, single-celled plankton are, in fact, the main drivers of life in Earth’s oceans. The word plankton is derived from a Greek word – πλαγκτός (planktos). Plankton are organisms that inhabit all water bodies – from lakes and ponds to oceans. In size, plankton range from microscopic, bacteria-sized single-celled organisms to multi-celled animals such as krills, jellyfish, crab larvae and juvenile fish. Even some single-celled plankton can swim from the surface to deeper waters, perhaps 10 metres up and down each day – not bad for an organism so small that you cannot see it without a microscope!

For decades, the accepted scientific view has been that the single-celled microscopic plankton can be divided broadly into two types. Food producing “phytoplankton” (also known as microalgae) act rather like tiny marine plants. Animal-like single-celled plankton “protozooplankton”, on the other hand, eat the phytoplankton, and are in turn eaten by bigger organisms going up the food chain all the way to fish and whales. This division of these microscopic plankton is thus akin to the plant-animal split in terrestrial food webs. However, we now know that this division is wrong. It transpires that many of the “plant-like phytoplankton”, and half the “animal-like protozooplankton” are mixoplankton!

The term “mixoplankton” was coined in 2019 and is defined as plankton capable of obtaining nourishment via photo-auto-trophy and phago-osmo-heterotrophy.

For the last century, science has been incorrectly studying the ecology of the largest ecosystem on Earth – the Ocean. Indeed, most biology and ecology text books, and programmes like BBC’s Blue Planet, when discussing oceanic plankton still refer to the plant-like and animal-like descriptors. Now we know that this represents a flawed description, that marine science has been studying the wrong things, in the wrong organisms!