Pyrosomes, siphonophores, bluebottles, dinoflagellates, diatoms and many other tiny inhabitants of the oceans are filigree titans, carrying the planet’s vegetation on their flagella, feelers, cilia and limbs.
Most of us only become interested in plankton when a mass bloom of cyanobacteria means that Baltic bathing beaches are shut, or when green soup instead of water in a lake makes us reconsider swimming there. Sailors also associate plankton with green flashes that delight the eye when the water is stirred. I, too, admired them once on the Atlantic, in the depths of night.
For others, it also carries a political association, like almost everything recently, from colourful atmospheric phenomena to apples and Budapest. At best, it arouses our indifference (as much as indifference can be ‘aroused’, but that’s a question for philosophers), and more frequently, our aversion. We are completely unaware that without plankton we would not be alive.
Knights’ helmets and Death Stars
Plankton is life in trillions of pieces. It is made up mostly of very small organisms (and even entities which don’t really deserve to be called organisms, such as viruses). But one doesn’t need to be microscopic to be part of plankton: it also contains the so-called megaplankton, or jellyfish with a diameter of many centimetres, thaliaceans (salps and pyrosomes), as well as cephalopods, amphipods and siphonophores, such as the Portuguese man o’war, also known as the bluebottle or the floating terror. All these fascinating creatures share one characteristic with their smaller siblings: they have very limited influence over the direction of their movement and are generally condemned to drift passively in the currents – it is that feature which determines whether an organism is part of plankton. This doesn’t mean, of course, that plankters are unable to move at all. Some regulate their displacement, others have cilia or flagella, and others still have bell-shaped bodies that can contract, which allows them to push themselves along. Plankton typically engages in diurnal vertical migration in the water column, depending on the light level. This allows photosynthetic organisms (which move more weakly) to make better use of daylight and the availability of nutrients close to the surface, while the organisms that feed on them and usually swim more efficiently can stuff themselves at night near the surface and hide in the depths during the day.
Here it might be worth explaining the more outlandish of the above-mentioned names. Thaliaceans are our distant relatives, although it’s not immediately obvious. (Of course everyone is our relative within nature, but we’re closer to them than, for example, to the famously intelligent cephalopods). They are tunicates and – like me or you, at least at some stage of our development – they have notochords. Their bodies look like transparent, gelatinous bells, into which delicious bits of food can float. Thaliaceans frequently form colonies. While salps float alone in their asexual generation, in the sexual one they join each other to form happy, orgiastic chains. Pyrosomes, in turn, always live together, frequently creating glowing, gelatinous ropes a few centimetres wide and sometimes more than a dozen metres long. In seawater they look like strange snakes, and American oceanographers give them the pet name ‘whale snot’. Siphonophores are cnidarians (and hence very distant cousins of the common jellyfish) with the ability of self-propulsion, and the bluebottle, or the Portuguese man o’war, is a unique siphonophore with a magnificent pneumatophore, or bladder, with a sail-shaped tip, and ropes of cnidocytes many metres long from the few dozen zooids forming the colony. Their cnidocytes are so powerful that they can be harmful to humans, although, of course, the zooids prefer to use them to sting fish and invertebrates, which the zooids then eat together.
So many taxonomic groups are part of plankton that it’s pointless to list them. In fact, apart from birds, mammals and turtles, everything that lives in the ocean spends either the entirety or at least the first stage of its life as plankton – including fish and sedentary invertebrates. So, for practical reasons, it’s easier to speak of two massive categories. The first is phytoplankton, consisting mostly of autotrophic organisms – those that photosythesize. This doesn’t mean only plants, but also cyanobacteria and plant-like protists. Protists are a taxonomic group that encompasses all organisms whose cells have a nucleus, but which couldn’t be included in the kingdoms of plants, animals or fungi (animal-like protists are the so-called protozoans). In ecological terms, representatives of this category are primary producers, because they produce organic matter out of mineral nutrients dissolved in water, and use solar energy.
The second group is the zooplankton, or animal plankton. These organisms get their nutrients from eating others. Of course – as usual – these distinctions mostly reflect a human way of thinking rather than what actually happens in nature. Our minds feel safe around clear definitions and categories, but nature doesn’t care. And so there are also, for example, mixotrophs, or organisms which might get in a little photosynthesis, but then also grab a bite, depending on the situation.
An example of such rebellion can be found in ‘predatory’ algae like dinoflagellates. Dinoflagellates are a huge group of plant-like protists, which includes, for example, the mysterious family Warnowiaceae. They’re difficult to study because they’re rare in samples, and when cultured they die and immediately decompose; their single-cell bodies have no shells. We do know, however, that they eat other dinoflagellates and eggs of plankton crustaceans, and they search them out with… an eye similar to ours! Although various protozoans have eyespots, they’re usually only able to differentiate whether light is present or not. Warnowiids, however, have structures analogous to the ones we have in our metazoan eyes, built from cell organelles – their ‘cornea’ and ‘lens’ are made of mitochondria, and the light-sensitive ‘retina’ is made up of plastids. This so-called ocelloid registers polarized light – the kind that has passed through the thecal plates of other dinoflagellates, or potential lunch. It is the most complex light-sensitive structure among all unicellular organisms. One genus, Erythropsidinium, also has a movable piston, used for palpating the environment and moving around at a speed of 1 mm/s, dizzying for an organism of this size.
While other dinoflagellates don’t have similarly sophisticated structures, the variety of their forms is astonishing. So far, almost 1500 species have been described, and they can be found in all aquatic environments, including snow and ice. Most of them move using flagella and have a two-part cellulose covering called the theca. The shapes of these coverings can be fantastic: they look like miniature armour, spacecraft straight from old science fiction films or the elaborate helmets of mythical warriors. Representatives of around 30 species can also produce hypnotizing, blue-green light. They sometimes make whole swathes of the ocean shine with an unearthly glow.
Diatoms, even more numerous than dinoflagellates, also have coverings, but their armour is made of silica and is more regularly shaped. These shapes are myriad: from shuttles and coffins to drums, samosas and elegant spheres. This is, incidentally, how they’re identified. There are so many diatoms that in certain places the layer of their remains (i.e., diatomaceous earth) can be even 800-metres deep, and is commercially extracted. It is used in thermal isolation, absorbents, filters and insecticides, but humanity’s technological history owes perhaps the most to their incredible absorbency. It allows them to stabilize nitroglycerine and create dynamite, which means it is thanks to them that we have the Nobel Prize – Alfred Nobel noticed it first. I’m not sure whether any Nobel Prize winner has honoured them appropriately, but someone should. However, their shapes have delighted many artists, including Ernst Haeckel. He was in awe of many miniature creatures, and his other prints also feature Foraminifera: animal-like protists, amoeboids with lace-like chitin shells, or tests. These tests have lots of miniature holes, through which, when searching for food, the Foraminifera extend the long threads of their pseudopoda, also called ectoplasm. Some are perfectly spherical, which makes them look like miniature Death Stars. Most of them live on the ocean floor, but they also include plankton species.
Monsters and angels
Phytoplankton provides food for zooplankton, i.e. animal-like protists and actual animals rooting around in the algae soup. Some of them content themselves mostly with plants, but others eat their animal plankton neighbours. They rarely exhibit the delightful harmony of filigree, arabesqued shells. Plankton predators might not be huge, but in reality many of them look like characters straight from a horror. Sometimes it even goes the other way and horrors are inspired by them. In almost all oceans, apart from the polar regions, live semi-transparent parasitoids (organisms that don’t attach themselves to the host permanently, but exploit it and throw it aside or kill it) from the genus Phronima. The appearance and lifestyle of these crustacean amphipods, a few centimetres long, inspired the creators of the film Alien. These tiny rascals search out the above-mentioned salps, enter their bell-shaped bodies and scrape out almost all of the host’s cells with sharp claws, leaving just enough not to kill it. Then they live in its perpetually dying bell, using it as a miniature submarine, a trap for food and a cradle for their eggs.
Other plankton predators also include the voracious chaetognaths, also called arrow worms, with elongated bodies and mouths armoured with many – frequently venomous – hooked spines. But the most bloodthirsty of all is Themisto, another genus equipped with long, spiny limbs. It’s no wonder it was named after the murderous queen Themisto from the Greek myth – if we consider all the species in this genus, the Themisto eats the largest amount of animal biomass on our planet. Although it’s not even two-centimetres long, it can be considered the biggest predator in the world. Second place belongs to us.
Zooplankton eats phytoplankton and other representatives of its own group, but it’s also eaten by larger sea inhabitants – from fish and birds to whales. It’s a plentiful food source; its biomass is much larger than the combined biomass of all vertebrates, including us. Of course, plankton doesn’t really enjoy being eaten, and it frequently adopts various strategies to prevent that.
One of my favourites was discovered in the Antarctic by James McClintock and starts with small plankton snails called pteropods, or the wingfooted. They are adorable, tiny spheres, energetically waving their transparent little wings in the watery depths. Some of them are customarily called ‘sea butterflies’, while the slightly larger ones are called ‘sea angels’. Interestingly, the angels hunt the butterflies, and, as it recently turned out, feed on them during copulation that lasts for many hours. I find the mere vision of angels making love and nibbling on butterflies delightful. But that’s not all. Angels from the genus Clione are not only beautiful, but also poisonous to fish. This is why they can afford to have both vividly red internal organs, visible through their transparent bodies, and a slow, majestic way of swimming. Researchers have noticed that if a fish swallows such an angel, it immediately spits it out.
Imagine, too, that amphipods from the genus Hyperia and their cousins Hyperiella snatch the Clione angels swimming past, and then tie them up with one of their many pairs of legs to carry them on their backs for weeks. All these animals are about two-centimetres long and an angel like this is a considerable weight for the amphipod, slowing down its movements by about 40%. But it’s a good investment – subsequent experiments have shown that even if a fish swallows an amphipod with a backpack, it spits it out, sensing the angel’s toxins. So there is such a thing as a guardian angel, but for it to be useful, one must catch and imprison it.
Small and important
The amount of plankton in the oceans is so large that when in 1914 sonar started to be used to measure depth, the operators of these new systems couldn’t understand why the ocean floor moves up and down every day. As it turned out later, the most minute organisms create a mass so dense that the sonar impulses don’t reach the true floor. Yearly plankton blooms are the basis of how marine ecosystems function – they are where the trophic pyramid begins. It then leads up through all the levels to the top, where whales float majestically.
In the world’s most productive waters (i.e. in the seas surrounding the southernmost continent), the keystone species is Antarctic krill, whose biomass is estimated to be around 400 million tonnes, and which can undoubtedly be considered the most important species of that ecosystem. Technically, krill is part of nekton, or organisms that actively swim, and not of plankton – but it feeds on plankton, and all the larger animals depend on it. If you live in the Antarctic, you either eat krill yourself or you eat those who eat krill.
For comparison, the biomass of all people in the world is estimated to be around 100 million tonnes, and farm animals – around 700 million tonnes. According to scientists, phytoplankton is responsible for the production of between 50-80% of oxygen on our planet, which means every other breath, or perhaps even two out of three breaths, come from the ocean. The share from diatoms alone is estimated to be between 20-50% of the world’s oxygen production. This also means that plankton binds huge amounts of the carbon dioxide we still thoughtlessly pump into the atmosphere. Some of this carbon circulates in nature, but about 30% is removed from the atmosphere for the long term, along with the dead bodies of plankters falling to the seabed. As the world’s greatest carbon containers, oceans delay the unfolding climate crisis, and healthy sea ecosystems assist them in that on all levels. One of the most interesting scientific discoveries of recent years are the so-called trophic cascades, in which events on the top level of the pyramid influence all the lower levels. In the case of oceans, these cascades start with whales – and specifically with their excrement, which literally flows over all the inhabitants of the oceans below.
After these massive mammals were almost completely killed off by whalers in the 20th century, one might have expected that the amount of plankton would grow, as whales eat it. But this didn’t happen. It wasn’t until the beginning of this century that scientists managed to explain this mystery, and since 2010 more and more articles have been written to describe the so-called whale pump. It turns out that whales feed not only near the surface, but also in the depths, where the high pressure doesn’t allow them to relieve themselves. This is why they do it at the surface, expelling plumes of pink or white faeces a few dozen metres long; the British science writer George Monbiot calls them ‘whale poonamis’. In this way, they provide the surface area with valuable nutrients, i.e. fertilize the phytoplankton, allowing it to develop. The phytoplankton is then eaten by zooplankton, which falls lower and there is eaten by whales, which transform it into poo and in this form take it back up. Whales, then, are the ocean’s gardeners, and their defecation – causing the increase in plankton mass – contributes to protecting the Earth from climate change, as plankton binds carbon, removing it from circulation for a time.
This carbon – unbound and dissolved in water – increases the acidity of seawater. More acidic waters flush calcium out of the shells of sea organisms, upset their reproductive processes and have other fatal results, the meaning of which we’re only starting to understand. Scientists call this increasingly worrying process ‘the other CO2 problem’. The first being, of course, global warming. We all know that as a result of the increase of CO2 content in the atmosphere, our world is warming up – the increasing temperature of the oceans kills coral reefs, harms the development of fish and numerous invertebrates, and, in general and completely literally, cooks up ever more serious problems.
Carbon dioxide is not plankton’s only problem. By overfishing, we’re turning ever-larger sea areas into lifeless deserts. We’ve all seen drastic photos of turtles tangled up in discarded packaging and whales or albatrosses caught in abandoned nets or dying with stomachs full of plastic bags. Things are similar with plankton: less spectacular, but omnipresent microplastics have been found in the bodies of all sea organisms, including the inhabitants of the Antarctic, the Arctic, and the bottom of the deepest oceanic trenches. Both tiny representatives of zooplankton and fish, reptiles, birds and mammals feed on plastic particles. As a result, so do we. We don’t know all the harmful results of this plastic diet yet, but it’s definitely not healthy for anyone.
Oceans, without which life on Earth couldn’t exist, are slowly beginning to die. By protecting them – from the littoral zone to open seas – we can also save ourselves. Only we have to work at it, and quickly.
Translated from the Polish by Marta Dziurosz
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