Today, I will attempt to convince all of us that plankton is cool and worth our attention. We’ll talk about what plankton is and why it’s so important for our climate.
Plankton is cool. There, you can quote me on that. That established, I’m also pretty fed up with plankton. I’m more than half-way through my bachelor’s program and it feels like all we’ve discussed is plankton. But, and it’s a big but, while I would much rather talk about the bigger things, plankton is a kinda big deal when it comes to the climate. Duck, I just finished an entire class called Plankton & Climate which took a closer look at the connection—there are quite a few more essays on that waiting to be shared, of course.
But, to understand the connections between things, it’s essential to understand the components first. So, today, I’ll try to convince you of something I struggle with myself: Plankton is ducking cool!
If you are a little rusty on the basic divisions of life or need a refresher on words like eukaryote, prokaryote, bacteria or protists, check out the essay The Good, The Bad, The Ugly of Bacteria and Viruses.
What is plankton?
So, right, plankton. First, we should probably recap what plankton actually is. Let me Wikipedia that for you. No, I’m kidding, of course. I’ll tell you. Plankton is all that small living stuff that floats around the ocean currents with little say about where they go. The opposite is called nekton, so the organisms that can move of their own volition.
Some species stay plankton their entire life. They are called holoplankton. Holo- actually means “whole” or “entire.” If you paid more attention in chemistry than me, you might know that already. But for those of us who have brains like sieves, it might be easier to think of it as “whole” spelled by a very cool kindergarten kid. Whole-O!
Species that join the plankton crew for part of their live are called meroplankton. Mero- means “part” or “partial.” I guess they merely spend part of their life as plankton, so they are mere-o-plankton. I know, I know. If you have a better way to remember this, I’m all ears.
Another way to classify plankton is by how they get their energy. You might already know zooplankton with animals and protists. As I bet you didn’t actually look up what a protist is despite my introduction: Eukaryotes are divided into plants, animals, fungi (that’s mushrooms), and protists. Protists used to be defined as eukaryotes that aren’t plants, animals, or fungi. For now, that’ll work.
In addition to the zooplankton, there’s the phytoplankton. Very simply put, zooplankton is the animals of the plankton world and phytoplankton the plants. We’ll see why that’s way oversimplified even without talking about protists again later.
So, phytoplankton can photosynthesize, so take the sun and carbon dioxide and magically turn it into energy and oxygen. And the phytoplankton in the ocean plays an immense role in this. Forests and marshlands are nothing against the ocean’s phytoplankton gang.
When we talk about the primary producers, so the organisms that “fix” CO2 and produce oxygen, we are talking about way more than plants. We are talking about all photosynthetic organisms and even symbionts.
Quick vocabulary building discourse:
Autotrophs are organisms that don’t need to feed on anything to survive. Photosynthesis is one of the options they use to create energy instead of taking it in from food. Where there is no light (or not enough), chemosynthesis can be done instead. Remember how I told you life might have originated at the hydrothermal vents on the ocean floor? They usually get their energy from sulfuric compounds, so they are chemoautotrophs. The -troph ending comes from the Greek word for nourishment, but the only word I can think of is trophy, and you can’t really eat those. Ah, well.
The opposite of an autotroph is a heterotroph, so an organism that relies on feeding to gain energy. That’s us, again. Us and every animal on this planet. Guess our ancestors should’ve ingested some of those delicious cyanobacteria while they were still small enough to make them part of themselves.
And while I’m probably the only one thinking about flesh-eating plants right now, there’s also a combined form, of course. Some species can supplement their autotrophic energy supply with feeding. These organisms are called auto-heterotrophs or mixotrophs.
And then there’s the organisms that are heterotrophs (so feeding things) but live in symbiosis with autotrophs. Think of corals. Corals are animals, but thanks to photosynthetic algae that lives inside them, they get quite a bit of their energy supply through photosynthesis. Well, until they bleach and the photosynthetic symbionts panic so much that they become toxic and get kicked out by their coral host. But that’s another story for another day.
So, at least phytoplankton already has a leg up on us because they can photosynthesize. But how do they do that?
How does Photosynthesis Work?
Photosynthesis on a chemical level means taking carbon dioxide, water, and energy from the sun and turning that into glucose and oxygen. The opposite is respiration, which takes one of those sugar molecules and oxygen and turns it into carbon dioxide and water, as well as some heat (that’s energy, again). How exactly this works is too much chemistry for me, but if you care, look up photosynthesis and the Calvin cycle. I’m sure there are YouTube videos aplenty.
This coincidentally also answers a question most people would answer wrongly: How do you lose weight? The answer is pretty simple: breathe. That weight you lose gets lost through those carbon atoms you breathe out as part of your respiration. So, you don’t sweat out the weight, you breathe it out. Cool, right?
Anyway, this photosynthesis process happens in the chloroplast. If you recall from all that talk about endosymbionts, photosynthetic organisms got those organelles by ingesting cyanobacteria.
Inside the chloroplast, a green kidney-shaped organelle, are stacks of what looks a lot like pancakes—ironic that the photosynthetic equivalent of feeding happens in a place that looks like food. These stacks are called thylakoids and this is where the magic—er, photosynthesis—happens.
Did you know that even plants need oxygen? They can’t keep up with how much ATP their cells require, so they have to use their mitochondria (that other endosymbiotic example we talked about; the powerhouse of the cell) to make more. This process uses oxygen. It’s especially important when there is little or no light, like at night.
ATP is a compound with a boatload of energy stored in the bonds. You know, those three (tri) phosphates don’t seem to want to stick to that adenosine, so it takes a lot of energy to keep them together. When they break apart, that energy is released and can be used by the cell. The poor ATP loses a phosphate and becomes ADP, adenosine diphosphate.
But back to our phytoplankton and why it’s important for the climate. Primary production, so the fixing of carbon dioxide into oxygen, is divided roughly equally between the land and the ocean. In 2019, the ocean sank 9.64 Gt CO2, while the land sank 11.50 Gt CO2. But keep in mind, there’s a lot more than just trees on land. So, while trees get all the credit, they actually do much less than half the work. Underneath the ocean surface, there are also plants, mostly seagrasses, that work like land plants. Even when combined with kelp and other macro algae, they make up only 10 to 20 percent of global primary production. Phytoplankton does the rest. Let me say that again: that little green stuff in the ocean makes up 40 to 50 percent of global primary production. If I now tell you that phytoplankton accounts for only 1% of the total biomass on this planet, it’s even more impressive. So, they take up a lot less space while doing a lot more than the trees everyone is so focused on. Pretty impressive.
Groups of Plankton
The five major groups of plankton are diatoms, essentially little glass snuff boxes, coccolithophores, tiny hubcap-covered balls of cool, cyanobacteria, the bacteria that has led me to send multiple corrective letters to news outlets for calling them algae, green algae with a name so creative they describe themselves, and dinoflagellates, the organisms that cause red tides but also store their genetic material in seed banks for better times.
Diatoms
As I said, diatoms are algae in a glass house (Diatoms shouldn’t throw rocks, I guess) and have two halves of a shell that fit together much like one of those metal candy boxes lemon drops come in. Or snuff.
To make their glass houses (I mean shells), they need a lot of silicate in the water.
As diatoms make up about 40% of the ocean’s primary production, it’s pretty good to hear that they are robust to changes in pH, one of the main shifts in the ocean caused by climate change. They are also pretty diverse and exist in almost any aquatic ecosystem, saltwater or not.
Diatoms are pretty good at reproducing, thanks to both sexual and asexual reproduction. Their asexual reproduction actually makes me laugh, but if it works for them, great. When they split during asexual reproduction, each half keeps half of the shell. They then grow a smaller one to fit into the one they got from the parent cell. This means that with each split, they get a bit smaller. When these so-called frustules get frustrated by their small size, they flip over to actual sex, where a sperm cell fertilizes and egg to form an auxospore. But, unfortunately, diatoms are pretty vulnerable during this stage and ducking takes a lot longer than splitting in half, so it’s not the most efficient method for them.
Coccolithophores
I told you about the algae coccolithophores when we talked about the Ehux-86 virus that affects E. huxley, the most abundant coccolithophore. Coccos prefer a different style of house than diatoms and choose to fashion themselves in round plates that look a lot like hubcaps. As these plates are made from calcium carbonate, they are pretty quick to suffer during pH changes.
The role of coccolithophores in climate change is still a little up for debate. They aren’t as good at primary production as other phytoplankton groups, but the fact that they store carbon in their shells means that they are a carbon sink nonetheless. Also, they are white and when they die off after a bloom, they leave white clouds in the water that reflect some of the sunlight back to space.
Oh, and if you see white cliffs, they are likely long-dead coccolithophore shells. Cool, right? Especially if you consider how much carbon is stored in them. You didn’t think of pretty ocean-side cliffs when you heard about carbon storage, did you now?
Another reason scientists care about coccos so much is that they release DSM, an important gas involved in cloud formation. And clouds, like the dead-cocco ones in the water, have a higher albedo (reflectivity) than land or water. All in all, they play a pretty big role but they’ll be among the first to suffer with every bit the pH drops.
Cyanobacteria
Okay, of all the little stuff, cyanobacteria definitely comes closest to catching my interest. I mean, coccolithophores like Ehux are cool, but cyanobacteria are just so ducking omnipresent and just plain awesome.
First, cyanobacteria are nitrogen-fixing in addition to carbon-fixing due to specialized cells (Remember that cyanobacteria are bacteria and thus prokaryotes, so specialized cells are a pretty big deal.) This allows them to keep producing oxygen even when a lot of other organisms can’t.
Yeah, I know, cyanobacteria are also what causes toxic algae blooms like the one I talked about in one of my Youtube videos, but just because they can be bad in an overdose caused by humans, doesn’t mean they are bad.
Prochlorococcus is the pro among the chlorococcus, and while that sound a lot like ostreococcus (a green algae type), they are cyanobacteria. The reason their names are similar is because scientists aren’t always that good at things, especially very, very small things. In biology, coccus is used to describe round bacteria. Ostreo seems to mean oyster, so that’s fun. Whoever named the ostreococcus apparently thought they were round oyster bacterium things. Cool, but also wrong. But we’ll get to green algae soon.
First, let’s return to our chlorococcus, our green bacterium thing, as the name says. Prochlorococcus is thought to be the most abundant photosynthetic organism on the planet. Sylvia Earle, a pretty well known marine biologist estimates that this particular cyanobacterium species is likely responsible for 20% of earth’s oxygen, so every fifth breath you take. Pretty cool for a thing so small that millions fit into a drop.
Also, Prochlorococcus is a little different from other cyanobacteria and might have been involved in supplying oxygen to earth’s early ocean. And remember that cyanobacteria were likely the first to figure out how to photosynthesize. They are pretty damn cool.
But, as per usual, we are endangering them a lot. In this, case particularly with plastic leachates, so the stuff that leaches out of plastics into nature.
But, anyway, cyanobacteria really are everywhere. Lichen, the stuff that grows on trees in the forest in greens and blues and oranges, is a symbiosis. They are plants in symbiosis with fungi and bacteria. In the case of the green and green-blue ones, cyanobacteria are the answer, of course.
Simply put, cyanobacteria have managed to survive in pretty much any ecosystem on this planet. Pioneers, fighters, survivors. Even if you won’t believe that plankton in general is cool, I hope to have at least made my case for cyanobacteria. Though, that whole toxic bloom thing kinda puts a damper on them. But we have to keep in mind that those blooms are caused by humanity’s agricultural run-off and eutrophication. Without our interference, they wouldn’t be as bad, and they wouldn’t affect the larger wildlife in the areas as much.
But it’s time to let cyanobacteria be cyanobacteria, so we can get to the next plankton kind.
Green Algae
The first thing we learned about green algae in class was how they have sex and that we know very little about how they do it. For now, we can summarize it as complex life cycles in the larger ones and unknown but probably complex ones in the small kinds.
And then I was told that green algae primarily inhabit freshwater habitats, and lost interest. I’m a marine ecologist, for cod’s sake.
Dinoflagellates
Dinoflagellates are the mixotrophs responsible for red tides. Refresher: mixotroph means that they can do both chemo- or photosynthesis and supplement with feeding.
I talked about the dinoflagellate K. brevis in the video about blooms and tides above. They are a major issue. The reason dinoflagellates can photosynthesize is that there are cyanobacteria inside them. This also means that they are as toxic as the cyanobacteria they ingested.
They essentially have an active and a passive stage. They can go the easy route where one parent cell divides into two. Depending on the temperature, they might choose to reproduce sexually instead. In that case, two of them meet and fuse to form a zygote. Depending on how things are looking, they can then go back into the vegetative stage and merrily divide in two. But if things aren’t looking too good, they transform into resting cysts. They can stay dormant like this for a very long time. When things look better, they wake up—well, if you can call a vegetative stage waking up.
You know what else we learned in that class—other than a lot of plankton sex stuff, of course? There was an entire section about why size matters. Am I a 12-year old teenager? No. I still laughed.
But in the case of plankton, size does matter. Size matters for growth rates. Size matters for sinking velocity (how fast do you sink to the bottom if you die), nutrient uptake, the likelihood of becoming food yourself. So, yeah.
Apparently this is why functional phytoplankton groups don’t tell us everything about phytoplankton diversity. Apparently, it often makes more sense to characterize plankton by size rather than functional group.
We don’t know enough.
The problem (and also the solution, as I’ll explain) is that even with those five functional groups, there’s a boatload of diversity. We just don’t know enough. We don’t know which species will grow faster or slower when the temperature rises. We have a lot of individual experiments, often on individual species, but we know very little about how plankton globally will react to rising ocean temperatures and acidification.
On a more hopeful note—though I urge not to use this hope as an excuse to delay action!—the diversity is likely also what could save, if not us, life on this planet. There’s this thing called the insurance hypothesis that says that an ecosystem’s resilience is directly related to how diverse it is. And that makes sense, of course. If there are more different species, chances are some will cope with whatever gets thrown at them.
Hopefully, there will be enough diversity left to save the ocean. If plankton can’t make it, no one can.