Beware the mixotrophs - they can destroy entire ecosystems 'in a matter of hours'

Bio luminescence. Illumination of plankton

AN INTERESTING article, which was first published in the online journal The Conversation, is reproduced in the current edition of Scientific American, writes Richard Collins

It concerns plankton, the multitude of mostly tiny organisms living in the oceans which are the basis of all marine life and from which, eons ago, all life on Earth evolved.

The plural noun “plankton”, comes from Greek meaning “to drift”. Most “plankters” are unable to swim against the current; they drift along with the flow. Tiny organisms, they live not only in the sea but also in rivers lakes and ponds. Even the great whales depend on them for sustenance.

Most are single-celled creatures, invisible to our eyes. Others are the larvae of crabs shrimps and fish. Jellyfish and krill are the top dogs of the plankton community. Out mindset has an in-built binary bias. We have two arms, legs, eyes, ears, lungs and kidneys, so it’s not surprising that we organise things into pairs; male and female, true or false, good and evil, Fianna Fáil and Fine Gael.

The living world, we used to think, consists of two great kingdoms, one of plants, the other of animals. Everything living, it seemed, could be fitted into one or the other. Mushrooms were treated as plants until we realised they belonged to a vast and vital kingdom of fungi. Nor can bacteria and protozoans, which make their disturbing presence felt when we fall ill, be accommodated in a dual classification scheme. Scientists argue as to how many distinct kingdoms should be recognised.

Plankton image under a microscope.
Plankton image under a microscope.

Plankton, we thought, could also be understood in binary terms; there were only plant items and animal ones, phytoplankton and zooplankton.

Aditee Mitra, of Swansea University, believes that this approach is mistaken. The traditional split, she argues, is no longer tenable”. Nor is this mere academic hair-splitting, angels on the head of a pin; there are important issues at stake here. As a result of the error, she thinks, even our “climate change models could be giving questionable results”.

Mitra wants to introduce “a new model for life in our oceans”. There is a third class of creatures, mixotrophs, which don’t fit neatly into either the plant or animal plankton categories. Mixotrophs can use the energy of the sun to photosynthesise, the way phytoplankton does but, like zooplankton, they also devour tiny animals. Like humans, mixothrophs are omnivorous. Arthur C Clarke considered John Wyndham’s 1951 novel, The Day of the Triffids, “one of the 13 best science-fiction horror novels”. Wyndham’s triffids were fictional flesh-eating plants, able to grow to a huge size and move about. Bioengineered in the old USSR, they had escaped into the wild and were spreading throughout the world, with dire consequences for humanity. They sprayed poison on every creature they encountered, rendering their victims blind. Soon, almost the entire human race was affected.

Mixotrophs under micrscope.
Mixotrophs under micrscope.

Like Wyndham’s fictional triffids, Mitra says mixothrops “engulf living prey, suck out their innards, poison them, harpoon them, make them explode, steal and reuse body parts”. They can destroy entire ecosystems “in a matter of hours”.

Devouring the sewage we dump into water-bodies, they produce toxins harmful to fish and molluscs. The brown foam of estuaries comes from mucus secreted by them. However, mixotrophs aren’t all bad. They provide food for young fish, especially during the summer months.

The mathematical models we use to predict environmental change, such as the effects of intensive fishing on ecosystems, may be flawed because we fail to take account of the presence of mixotrophs, “that we now realise comprise more than half of all microscopic plankton”.

Our climate-change models will be unsound until we do so. “We ignore these little triffids at our peril”.

  • Aditee Mitra. Strange Hybrid Killers, Growing like Plants and Eating like Animals, Rule the Oceans. Scientific American. November 2016.


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