Protists
Pick up a piece of chalk. It isn't a mineral that grew in the ground — it's a graveyard. Every gram is the crushed shells of trillions of single-celled organisms that drifted in an ancient sea and sank when they died. They're called protists — "the first of all" — and they have quietly been building the planet's rock and fuel for half a billion years.
The petrol in a Cape Town taxi. The cement in a Gqeberha apartment block. The chalk on a classroom blackboard. These three things seem completely unrelated. But they were all made by the same kind of organism — one so small you'd need a microscope to see it, and one that has been dead for millions of years. What is it?
Protists — specifically, the shelled marine protists called foraminifera and radiolarians. For hundreds of millions of years, these single-celled drifters have lived in the ocean, built tiny mineral shells around themselves, and then died and sunk to the seabed. Their shells pile up, and over geological time the deposits compress into rock. But the two groups build different rock, because they build different shells — and that distinction is the whole of Section three:
- Foraminifera build in calcium carbonate, so their graveyards become limestone (the cement in that apartment block) and chalk (the stick on the blackboard). The coccolithophores you meet in Topic 22 do the same.
- Radiolarians build in silica — glass — so their layers, up to 100 metres deep, become chert, a hard glassy rock. No radiolarian ever made a piece of chalk.
- And the petrol? That is the third answer: prehistoric protists, together with phytoplankton, are the organic source of the crude oil and natural gas South Africa drills for off Mossel Bay.
Protists are the "everything else" kingdom — single-celled (or simple multicellular) eukaryotes that don't fit neatly into plants, animals or fungi. In the ocean they are colossally important on two fronts: they form the base of the biggest food chains on Earth (most plankton are protists), and their mineral skeletons literally build the planet's sedimentary rock. They also happen to be one of the groups most at risk from ocean acidification — the topic that connects this single cell to the climate crisis.
The "Everything Else" Kingdom
Protists are single-celled (or simple multicellular) eukaryotes that don't fit plants, animals or fungi — and in the ocean they are the base of the food web and the builders of sedimentary rock.
The word protista was coined in the 1800s and means "the first of all" — because they were thought to be the most primitive organisms. They're all eukaryotic (proper nucleus, organelles — revise Topic 14), bound by a cell membrane, and most are microscopic. Some are autotrophic (they photosynthesise, like dinoflagellates); some are heterotrophic (they hunt and engulf food, like amoeba); some do both.
📌 That name turns out to be half right, and the half that is right is the interesting one. Protists are not "primitive" in any useful sense — many are extraordinarily sophisticated cells. But they are first in a real sense: they were the first eukaryotes, the point at which a cell acquired a nucleus and organelles through endosymbiosis (Topic 14). Everything with a nucleus that came afterwards — every plant, every animal, every fungus, including you — descends from protist stock. Plants, animals and fungi are branches; the protists are the trunk they all diverged from. That is why the kingdom looks like a leftovers bin: it is not one group of related organisms, it is everything that stayed behind when the three famous branches left.
Two roles make protists matter enormously for marine science. First, the food-chain role, and CAPS is careful about why: a huge number of marine animals spend a larval stage drifting in the plankton (the meroplankton of Topic 17). Those larvae — and many full-grown animals — feed on single-celled protist zooplankton and phytoplankton. That is the missing step in the usual claim. Protists are not at the base of the biggest food chain on Earth because there are a lot of them; they are at the base because almost everything else in the sea eats them, or eats something that did, at some point in its life. Second, the rock-building role: many protists build hard mineral shells — either of calcium carbonate (CaCO₃) or of silica (SiO₂, glass). When they die, those shells rain down and accumulate over millions of years into limestone, chalk and chert — and contribute to the formation of fossil fuels. The two most important shell-builders are foraminifera and radiolarians.
A Drop of Seawater, Magnified
Under a microscope, a single drop of water is a zoo of protists. Tap each one to see what it is, how it moves and feeds, where it lives, and why it matters to marine science. The two CAPS focus organisms — foraminifera and radiolarians — are the shelled stars of the show.
One honest caveat before you start. Amoeba, Paramecium and Euglena are here because they are the clearest possible illustrations of the three ways a protist can make a living — crawling, swimming, and photosynthesising. But all three of those genera are freshwater organisms; you will not scoop them out of Algoa Bay. What the sea has is their marine counterparts, built on exactly the same body plans — and each card below names its own. Learn the body plan from the famous organism, then attach the marine name to it.
The Two Shell-Builders
CAPS asks you to know these two specifically — their appearance, their distribution, and their significance. They look superficially similar but differ in almost every important way, starting with what their shells are made of.
Foraminifera
~ "the chalk-makers"
| Shell (test) | Calcium carbonate (CaCO₃), absorbed from seawater — or built from cemented sand/silt grains. Multi-chambered, often coiled like a tiny snail shell. |
|---|---|
| Appearance | Amoeboid — pushes out cytoplasmic threads/tubes to engulf food (phagocytosis). 100 µm to a few cm; the largest are multinucleate. |
| Distribution | Mostly in seafloor sediment (benthic) — but many are planktonic. |
| Diet | The planktonic forms feed on phytoplankton and zooplankton. A minority instead house symbiotic algae inside the cell membrane: the algae supply most of the foram's nutrition, and the foram returns the favour by producing or absorbing the nutrients the algae need to photosynthesise. Both sides gain — that is what makes it symbiosis rather than parasitism. |
| Significance | Their CaCO₃ tests build limestone & chalk over geological time. Their fossils help date rock layers and reconstruct past climates. |
Radiolarians
~ "the glass-blowers"
| Shell | Silica (SiO₂) — a complex, glass-like internal skeleton. Spherical, cone-shaped or multifaceted, with spine-like outgrowths. |
|---|---|
| Appearance | 30 µm to 2 mm. The spines increase surface area to slow sinking — the same buoyancy trick you met in Topic 17. |
| Distribution | Mostly planktonic — drifting in the open water column. Most are predators; some are filter feeders. |
| Significance | Dead shells form radiolarian ooze (up to 100 m thick) that hardens into glassy rock (chert). A key geological dating tool. |
One easy way to remember it
Foraminifera build with calcium carbonate (think: chalk, limestone — soft, white, sea-floor sediment). Radiolarians build with silica (think: glass, spiky, drifting in the plankton). "Forams sit in the sediment in calcium suits; radiolarians drift in glass armour with spines."
Radiolarian Ooze — Reading the Rock
When radiolarians die, their indestructible glass shells sink to the deep seafloor and pile up over millions of years into a thick layer called radiolarian ooze. This is a gift to geologists — it's a slow, layered tape-recorder of Earth's history.
~ from a living drifter to a continent's rock record
Why geologists love it
Radiolarian species changed slowly but constantly over millions of years. Because the silica shells survive intact in the rock, a geologist who finds a particular set of radiolarian shapes in a chert layer can say "this rock formed in such-and-such a period" — even on land, far from any ocean, in rock that's hundreds of millions of years old. The same logic that radiometric dating gave us in Topic 12, radiolarian ooze gives us biologically. Foraminifera fossils do the same job in limestone, and are also used to reconstruct past ocean temperatures and climates.
The Threat — Ocean Acidification
Here is where this microscopic topic collides with the biggest environmental issue of our time. The same protists that build the ocean's rock are now dissolving while they're still alive.
As humans burn fossil fuels, atmospheric carbon dioxide (CO₂) rises. The ocean absorbs a huge share of that CO₂. When CO₂ dissolves in seawater it forms carbonic acid, which makes the ocean steadily more acidic — a process called ocean acidification. The problem for protists is brutal in its simplicity: acid dissolves calcium carbonate. The CaCO₃ tests of foraminifera (and the shells of countless other marine organisms — corals, snails, plankton) literally start to dissolve in more acidic water. If the water gets acidic enough, these protists cannot build or maintain their shells, and they die. Lose the foraminifera and you damage the base of marine food webs and a key part of the planet's carbon cycle — all because of a chemical reaction between CO₂ and seawater.
Local connection — South African rock, fuel & red tides
- Fossil fuel: the natural gas that fed the PetroSA gas-to-liquids refinery at Mossel Bay — and the major Brulpadda and Luiperd gas discoveries in the deep water of the Outeniqua Basin off the South Coast — formed from the buried, compressed remains of ancient marine protists and plankton. Every litre of that fuel is fossilised single-celled life.
- Rock: the limestone of the West Coast (mined for cement near Saldanha and De Hoek) is a foraminifera-and-shell graveyard. So is much of the chalky soil of the Western Cape wheat belt.
- Red tides: when dinoflagellate protists bloom in huge numbers off the West Coast, they can colour the water red-brown and strip the oxygen out of it as they decay. These "red tides" have caused famous mass rock-lobster walkouts at Elands Bay and St Helena Bay, where thousands of lobsters crawl out of the oxygen-starved water onto the beach. A single-celled protist, in sufficient numbers, can empty a bay.
In the Lab — the CAPS Investigations
This block has the most equipment-heavy Resources column in the course — live specimens, a fresh plankton sample, microscopes and plankton ID sheets — and four investigations built on them. Three are genuinely doable from Gqeberha, and the fourth is the one that matters most, because it is kitchen-scale and quantitative.
What CAPS asks for, word for word
Investigations column, block 18: "Photographs, slide show or videos" · "Study live examples (aquarium or rock-pool study)" · "Example of a protist ID chart, used to classify protists in plankton samples" · "Collect and identify plankton samples, if possible". Resources: "Live specimens · Fresh plankton sample · Microscopes · Plankton ID sheets".
📌 And one more, hidden inside a key concept rather than the Investigations column. Concept 3 reads: "Many protists have CaCO₃ as part of their structure. (Investigate potential effects of ocean acidification.)" That parenthesis is an instruction, and it is easy to miss because of where it sits. It is Investigation 1 on the practical sheet — and it is the only experiment in this block you can run on a kitchen counter.
Four investigations · the acidification one needs nothing but vinegar
- 🟢 kitchen Dissolve a shell — weigh shell or chalk fragments, leave them in water and in three strengths of vinegar, and measure the mass lost each day. It puts a number on the mechanism that threatens every CaCO₃ protist in the sea.
- 🟡 net 🟠 microscope Collect and identify a plankton sample — tow the stocking net you built in Topic 17 through Algoa Bay, settle the sample, and classify what is in one drop. Described micrographs supplied if you have no microscope.
- 🟢 desk Build the protist ID chart — CAPS asks for a chart used to classify, so you write the branching key yourself and then run someone else's sample through it.
- 🌊 shore 🟢 desk Study live examples — a rock-pool study at Cape Recife, or the supplied described pool if you cannot get to the shore.
→ Open the Topic 18 practical sheet — printable, with supplied data for every task.
The honest limit of the vinegar experiment — and why it still works
Vinegar is vastly more acidic than any seawater will ever be. Household vinegar sits around pH 2.5; the ocean surface has moved from about pH 8.2 to 8.1 since the industrial revolution and is projected to reach roughly 7.8 by 2100. So the experiment is not a model of the rate, and any answer claiming shells will dissolve in the sea the way they do in a cup is wrong. What it does demonstrate, unambiguously and with a number, is the mechanism: acid dissolves calcium carbonate, and a shell exposed to more acid loses more mass. The sea is doing the same chemistry far more slowly to organisms that must build a shell faster than it is taken away. 📌 Knowing what your model does and does not represent is worth more than the result.
Key Vocabulary
Tap each card to flip it. These eight terms are examinable.
Generally single-celled (or simple multicellular) eukaryotic organisms that don't fit plants, animals or fungi. The word means "the first of all". In the ocean they form the base of food chains and build sedimentary rock.
Amoeboid protists enclosed by delicate calcium-carbonate shells, living mostly in ocean sediments (some planktonic). Their CaCO₃ tests build limestone and chalk over geological time.
Protists with a silica-rich, glass-like shell, living mostly in planktonic communities. Spiny shapes slow their sinking. Their shells form radiolarian ooze on the seabed.
The thin shells that enclose protist cells — made of calcium carbonate absorbed from seawater, or of cemented sand/silt grains. The foraminiferan test adds a new chamber as the cell grows.
Describes a cell capable of moving like an amoeba — flowing along by pushing out cytoplasmic extensions — and/or engulfing food by phagocytosis (wrapping around it). Foraminifera are amoeboid.
Massive accumulations of dead radiolarian shells on the sea floor — layers up to 100 m thick that eventually compress into glassy chert rock. A vital geological dating tool.
Single cells that contain more than one nucleus. The largest foraminifera (up to a few centimetres) become multinucleate as they grow — a single huge cell run by many nuclei.
The increasing acidity of seawater caused by the ocean absorbing higher levels of atmospheric CO₂ (forming carbonic acid). It dissolves the calcium-carbonate shells of forams, corals and shellfish.
Hydrocarbon fuels extracted from Earth's crust, all originating from the fossilised remains of prehistoric plants and organic ocean sediments. This is the second thing CAPS says prehistoric protists built — the first being limestone. On your booklet's general terminology list, and the reason the petrol in the hook is a protist question.
Engulfing food by flowing around it and enclosing it inside the cell, where it is digested in a food vacuole. It is how an amoeboid cell feeds, so it is the mechanism sitting inside the examinable definition of amoeboid. A foraminiferan does it by pushing out cytoplasmic threads and tubes.
The hard, fine-grained, glassy sedimentary rock that radiolarian ooze eventually solidifies into. ⚠️ Do not confuse it with limestone or chalk, which are calcium carbonate and come from forams and coccolithophores. Silica → chert. Calcium carbonate → limestone and chalk. One of the commonest slips in this topic.
Quick Check
Question 1
Well done!
Next: Sediments — how protist shells, river silt and rock fragments build the entire ocean floor.