What are mixotrophs?

All organisms feed. The term -trophy in biology refers to food or nutrition. Plants are phototrophic, using light to support photosynthesis to make sugars from which they make all else they need. Most other organisms are heterotrophic, using sources of organic matter (such as sugars, proteins, fats), often obtaining these by eating. However, some organisms combine, or mix, nutritional routes such as photosynthesising and eating; these organisms are mixotrophs.

A well-known example is the Venus fly trap plant. Far more important, however, are the mixotrophic single-celled organisms (plankton) that dominate life in our oceans.

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!

What are mixoplankton?

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 “microzooplankton”, 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 microzooplankton” are mixotrophs!

We define these organisms as “mixoplankton” – planktonic organisms that express, or have potential to express, phototrophy (using photosynthesis) and phagotrophy (eating by engulfing their prey) simultaneously.

For the last century, science has been incorrectly studying the ecology of the largest ecosystem on Earth – the oceans. Indeed, most biology and ecology text books, and programmes like BBC’s Blue Planet, when discussing ocean life 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, we need to do something about it.

MixITiN is a project that is training the next generation of marine scientists to work on the mixoplankton paradigm, where mixotrophic plankton rule the waves.