Aditee Mitra, The Conversation
Have you ever wondered where the foam in the ocean comes from? Or why the sea can look clear on some days and green, brown, or even pink on others? And how fish get the ingredients to make those omega-fatty acids that we’re told are so good for us? Well, the single word answer to all of these questions is: “plankton”.
Plankton are organisms that inhabit all water bodies – from lakes and ponds to oceans. The word plankton is derived from a Greek word – πλαγκτός (planktos) – meaning “I drift”, and so while plankton can move between deeper waters and the surface and vice versa, they cannot swim against the current. So sometimes we have vast numbers of planktonic jellyfish, fantastic swimmers within the water columns but helpless against the tide, stranded on our beaches. In size, plankton range from microscopic, single-celled organisms to multi-celled animals such as krills, jellyfish, crab larvae and juvenile fish.
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. But how well do we really understand them?
For decades, the accepted view has been that these single-celled microscopic plankton can be divided broadly into two types. Food producing “phytoplankton” (also known as microalgae) are like tiny marine plants. “Microzooplankton”, on the other hand, eat the phytoplankton and are in turn eaten by bigger zooplankton, such as krills.
This division of microscopic plankton is akin to the plant-animal split in terrestrial ecosystems. However we now know that, beneath the waves, there is another microscopic plankton group – “mixotrophs” that combine features of “plant-like” phytoplankton and “animal-like” microzooplankton. And their mode of feeding is, but for their microscopic scale, the stuff of horror stories.
They are like miniature triffids, which can engulf living prey, suck out their innards, poison them, harpoon them, make them explode, and steal and reuse body parts. They can kill whole ecosystems in a matter of hours and alter the colour of the water – and yet they also shape the Earth’s atmosphere and support the growth of larval fish at critical stages of their life cycle.
For decades, these mixotrophs have been considered to be freaks of nature, prospering only when phytoplankton and microzooplankton are disadvantaged. Over the past five years, however, through a project funded by the Leverhulme Trust, we have established that the mixotrophs are far from freaks; indeed, mixotrophy is the norm rather than the exception.
This has major implications – it means that the base of the oceanic food web doesn’t follow the traditional “plant-animal” pattern. Instead, it is dominated by the activities of the single-celled mixotrophs, microscopic “triffids” which can photosynthesise like plants and eat like animals – all within the one cell.
A new type of life?
Based on our findings, we have proposed a new model for life in our oceans, arguing that the traditional split between the “plant-like” phytoplankton (microalgae) and the “animal-like” microzooplankton used to describe the oceanic food-web is no longer tenable. This model could overturn a century’s worth of our understanding of marine biology.
Indeed, mixotrophs have the potential to impact all of our lives, not least because they are major contributors to the food webs that support fisheries. This is especially true for the healthy growth of very young fish, which depend on them for food during the summer months.
Just like the triffids of John Wyndham’s classic sci-fi novel, however, mixotrophs can be dangerous, too – and to more than just other microplankton. The release of nitrates and organic nutrients, such as raw sewage or silage slurry, into coastal waters contributes to an imbalance of nutrient loads, which causes mixotrophs to produce toxins and mucus. The toxins can kill fish and close shell-fisheries. Muddy-coloured foam in estuaries during summer is the result of plankton secreting excess mucus – and this mucus can clog the gills of fish, effectively drowning them.
Mathematical models are used widely to aid environmental management, to study fisheries and to investigate the impacts of fishing and climate change on them. But such models do not take into account the presence and activities of the mixotrophs that we now realise comprise more than half of all microscopic plankton.
And this could result in serious flaws. We have shown that marine food web and climate change models that don’t include mixotrophs could be giving questionable results. Indeed, based on our modelling studies, we suggest that we start to take mixotrophs more seriously and include their remarkable impacts in mathematical models used to predict climate change and aid environmental management. They may be microscopic, but we ignore these little triffids at our peril.