Seaweeds (Macroalgae)
— Green and Red
Topic 22 was the invisible 95% of ocean productivity. This is the visible 5% — and it is the part you can walk up to and put your hand on. Seaweeds are not plants, they have no roots and no veins, and their colour tells you how deep they live. On a South African rocky shore they are also the entire foundation of the ecosystem: food, shelter, nursery and, in the Eastern Cape, a hand-picked export crop.
The jelly your Life Sciences class grows bacteria on. The smooth, non-icy texture of good ice cream. The soft capsule around some medicines. All three depend on the same substance — and in South Africa a large share of it is picked by hand off rocks near Gqeberha, at low tide, by people with buckets. What are they picking?
Red seaweed. The cell walls of red algae (Division Rhodophyta) contain cellulose plus two remarkable complex polysaccharides: agars and carrageenan. Agar is the gel in every petri dish in every microbiology lab on Earth; carrageenan is the thickener that stops ice cream going crystalline and keeps chocolate milk mixed. Those molecules exist because a seaweed needed a cell wall that is tough, slippery and able to hold water while being pounded by surf and dried by sun.
South Africa harvests Gelidium pristoides — a small red seaweed that grows on rocks and on limpet shells along the Eastern Cape coast — by hand at low tide, for agar production. Gracilaria is collected in Saldanha Bay for the same purpose. A seaweed's structural chemistry became an industry.
This topic is about that group and its green cousins: what a seaweed is, why colour predicts depth, how it survives a rocky shore, and the strange two-generation life cycle it uses to reproduce.
Not a Plant. Not a Weed. Not Even One Kingdom.
Macroalgae = all marine photosynthetic organisms easily visible to the naked eye. They are algae — simple photosynthetic organisms with no true roots, stems, leaves or vascular tissue.
Why not "sea plants"? Some texts use that term because they think "weed" implies something alien and problematic. But "plant" strictly means a member of the Kingdom Plantae — and not all macroalgae belong there. Current research classifies seaweeds under Chromista and Plantae; until recently they were all lumped into Protista. The classification is genuinely in flux, and that is worth noticing: it is a live example of biological knowledge changing as scientists get better tools.
Three groups, sorted by pigment. Seaweeds are classified as green, brown and red on the basis of their photosynthetic and accessory pigments. Those colours are usually visible — but they can be masked. A seaweed that looks brown may turn out, chemically, to be a red alga. Colour is a good first guess, not a diagnosis.
Small share, huge presence. Familiar as they are, macroalgae account for only about 5% of ocean productivity — the other 95% is the phytoplankton of Topic 22. But that 5% is concentrated exactly where we meet the sea: on the rocky shores and in the shallow reefs, where it builds the habitat everything else lives in.
Where Each One Lives
A South African rocky shore, from the spring high-tide mark down into deep water. Every seaweed here sits at a specific height for a specific reason. Tap each one.
Why Colour Predicts Depth
This is the single most examinable idea in the topic, and it is pure Topic 20 physics applied to biology.
Chlorophyll normally uses red and violet light. That works beautifully at the surface, where the full spectrum is available. But you already know what happens with depth: the long red wavelengths are filtered out first and only the short blue rays penetrate deep. So the chlorophyll of a deep-growing seaweed has no red light to work with — and would starve. The solution is accessory pigments: additional pigments that absorb the wavelengths that are available and pass that energy on to chlorophyll, so it can function normally.
The pigment trap that catches everyone
A pigment's colour is the light it reflects — the light it does not use. Green algae look green because they reject green light; experiments confirm they actually use the violet-to-blue and orange-to-red parts of the spectrum, exactly like land plants. Red algae look red because they reject red — which is easy, since at depth there is no red light to reject anyway — while their phycoerythrin absorbs the blue-green and green light that reaches them. So: red seaweed absorbs the light left at depth, it does not absorb red light. Get that the wrong way round and the whole answer collapses.
Which colour, exactly? CAPS says phycoerythrin "absorbs blue light", and your booklet says "violet-to-blue". In an exam, that wording is the expected answer. More precisely, phycoerythrin absorbs most strongly in the green and blue-green (about 495–570 nm) — the very colours chlorophyll on its own absorbs poorly, and the colours that reach deepest in green coastal water, where most red seaweeds grow. That is also why it looks red: it reflects the red end it cannot use. (Practical 23, Q10, makes you reason this out.)
...and the exception that proves you understood it
Generally reds grow deepest, browns shallower, greens at the surface and intertidally. But some red algae have evolved to live at or near the surface, and in doing so have lost their "true colours": red algae in the intertidal zone often appear brown, or even greenish. Hedgehog seaweed and tongue weed on our shores both look brown and are both red algae. That is why CAPS warns that colour is masked and cannot be the only identification tool — the group is defined by its pigments and chemistry, not by what your eye reports.
Built for a Rocky Shore
A seaweed has one structural job: hold on, and present a broad surface area to the light — while surviving everything the shore does to it.
Benthic, epiphytic, planktonic — three ways to live
Benthic = occurring at the bottom of a body of water; a seaweed attached to the seabed or to rock is benthic. Epiphytic = attached to or growing on another plant — many red algae grow on larger seaweeds, especially on kelp holdfasts and lower stipes. Crucially, epiphytes are not parasites: they use the host only for attachment and never penetrate its tissue. Planktonic (Topic 17) = drifting, not attached at all. A small group of seaweeds live this way — most importantly the floating sargassum beds of the tropical ocean, which form an entire ecosystem and breeding ground on the open sea.
Two Coasts, Two Kinds of Seaweed
Algae are very efficient producers, but they are limited in size and abundance where the sea is warm, or nutrients are low, or both. South Africa has one of each condition on either side of the country — which makes this a favourite comparison question.
| West Coast (cold, Benguela upwelling) | East Coast (warm, Agulhas Current) | |
|---|---|---|
| Size of seaweeds | Larger — kelps up to many metres | Smaller |
| Abundance | Larger (more biomass, dense beds) | Smaller |
| Species diversity | Smaller (fewer species, in huge numbers) | Larger (many more species) |
| Why | Cold water + upwelling = ample nutrients (Topic 22) | Warm water + low nutrient levels |
Big does not mean diverse
Read that table again: the cold coast wins on size and abundance, the warm coast wins on diversity. Nutrient-rich cold water lets a few well-adapted species grow enormous and dominate everything — the West Coast kelp beds. Warm, nutrient-poor tropical water cannot support that bulk, so instead you get many small species dividing the habitat between them. It is the same trade-off you met with plankton: productive water and diverse water are not the same water.
Alternation of Generations
To ensure diversity, seaweeds use a reproductive strategy with two multicellular generations — one diploid (2n), one haploid (n) — alternating with each other. CAPS asks you to be able to draw and label this cycle, so walk it one step at a time.
What the examiner wants you to say
In words: the (2n) sporophyte produces (n) spores through meiosis; the spores germinate into (n) male and female gametophytes; these produce (n) gametes, which fuse to form a (2n) zygote, which grows into the sporophyte. Normally the visible seaweed is the sporophyte and the gametophyte is reduced to a smaller structure — but not always: in purple laver it is the other way round. The advantage of this life cycle: it includes a sexual step (meiosis and fertilisation), which shuffles genetic material and ensures diversity — the raw material for adapting to a shore that changes with every tide.
Green Algae — Division Chlorophyta
Green and red algae are both regarded as Kingdom Plantae. Even when they form large multicellular structures, those are thallus structures — showing very little or no cell differentiation. No true tissues, no organs.
Sea lettuce
- Forms flat sheets, generally two cells thick, fastened to rock by a small disc-like holdfast.
- Found all along the South African coastline in calm gullies, on sheltered middle-shore rocks and in high-shore rock pools where wave action is minimal.
- Survives a wide range of temperatures, salinities and chemical concentrations — the adaptation that lets it live in high-shore pools that can be cut off from the sea for days during neap tides.
- All of its cells can photosynthesise and divide, so growth is rapid — making it an excellent pioneer in recently cleared areas after sand inundation, storm scouring or pollution.
- It grows thickly where there is organic pollution from fertiliser run-off or sewage — so a bright green carpet on a beach is often an indicator, not a decoration.
Red Algae — Division Rhodophyta
The big group: red algae have high species diversity, accounting for almost two thirds of all algae. They exhibit every typical seaweed form — many thick and rubbery, and some partly or highly calcified.
Purple laver
- Grows high up in the intertidal zone, close to the spring high-tide level, along most of the SA coastline except the far north-east. Seasonal — it favours the cooler months and is often absent in summer.
- The gametophytes are the prominent form (the opposite of the usual arrangement): clumps of overlapping thin, folded sheets or fronds up to 15 cm long. Male fronds have a yellowish fringe, female fronds a pink one.
- The red colour is masked — the fronds look purple-brown to yellow-green.
- Desiccation survival: exposed upper sheets dry out and resemble dark crinkled cellophane. It survives extreme drying by losing fluid from a gel-like material between the cells rather than from the cells themselves, and the dry top layer insulates the sheets below. Rewetted by the tide, it rehydrates rapidly.
- The sporophyte is microscopic and bores into mollusc shells, lying dormant through summer; when the water cools it releases spores and the leafy gametophytes recolonise the upper shore.
- It can grow lower down too — but there it is stripped by aggressive grazing.
Stalked roseweed
- Found mainly on lower shore rocks and into the infratidal zone — the region below the spring low-water mark, so always under water. Occurs along the entire SA coastline, but is particularly common on the KZN coast.
- A bright red seaweed — it lives low enough that its true colour is not masked.
- Spreads across the rock as a branched, rhizome-like stolon, a bit like spreading grass stems, from which "hairy" stalks grow.
- Those stalks flatten out into dichotomously forked fronds at the ends — dichotomous meaning the growing tip branches in two, repeatedly.
- Its life cycle includes the reduced carposporophyte generation; here the sporophyte is the visible generation and the gametophytes are small.
Why bother with a third generation? (beyond the exam, worth knowing)
In most seaweeds, gametes are released into open water and simply have to find each other — a lottery. In red algae with a carposporophyte, fertilised eggs stay inside small cysts within the female gametophyte's fronds, and the carposporophyte grows there, releasing numerous carpospores through continual mitosis. It is thought to reduce the risk of gametes being cast into the water column and never being fertilised — one fertilisation event is multiplied into many offspring. You will not be examined on the detail, but that logic is worth carrying.
Seaweed on Our Own Coast
Local connection — a crop you can walk to at low tide
South Africa runs a real seaweed industry, and it is built on exactly the biology in this topic. Gelidium pristoides — a small red alga that grows on rocks and on the shells of limpets along the Eastern Cape coast, including the shores around Algoa Bay — is hand-harvested at low tide and processed for agar. Gracilaria is collected from Saldanha Bay on the West Coast for the same purpose, and beach-cast kelp is harvested for alginates and for abalone feed (Topic 24).
Walk any rocky shore between Gqeberha and Cape Town at spring low tide and you can read this whole topic off the rock face: purple laver as a dry black crust right at the top, bright green sea lettuce in the sheltered pools, corallines like pink paint on the rock, and — right at the water's edge, only exposed on the biggest low tides — the deep-red fronds that spend their lives under water. The zonation is not random. It is desiccation, light and grazing writing the same three rules on every shore in the country.
In the Lab — the CAPS Investigations
CAPS sets a "visit to rocky shore or aquarium to see other seaweeds in situ" and "paper chromatography of crushed samples: a comparison to observe different pigments in the three groups", and asks for live sea lettuce and purple laver to be examined. The practical sheet does all three — and finds the one pigment chromatography cannot catch.
Four investigations · a mortar, a coffee filter, a quadrat and a packet of nori
- 🟡 pharmacy Paper chromatography of the three groups — separate the pigments of a green, a brown and a red seaweed with nail-polish remover and a coffee filter, then find where the red pigment went.
- 🌊 shore A rocky-shore seaweed transect — quadrats every 2 m from the top of the tide to the bottom. Why is the highest seaweed on the shore a red one?
- 🟢 kitchen Purple laver's drying trick — sushi nori is laver's close relative. Measure how fast it soaks up water, and whether a dry top layer protects the sheets below.
- 🟢 desk Life cycles and the booklet's questions — label the alternation of generations, and compare sea lettuce with purple laver.
→ Open the Topic 23 practical sheet — printable, with supplied data for every task.
Key Vocabulary
Tap each card to flip it. This topic has a long examinable list — these eight are the ones everything else hangs on.
All marine photosynthetic organisms easily visible to the naked eye. "Seaweed" is the common name for them. They are algae — not strictly "sea plants", because not all of them belong to the Kingdom Plantae.
A multicellular plant structure which exhibits little or no cell differentiation. Seaweeds can be large and multicellular, but they have no true tissues, organs, roots, stems or leaves — the whole body is a thallus.
The structures enabling seaweeds to anchor themselves firmly to a substrate — a pad of tissue or root-like structure. A holdfast only grips: it does not absorb water or nutrients the way a root does.
Occurring at the bottom of a body of water. A seaweed attached to rock or the seabed is benthic — as opposed to planktonic (drifting, Topic 17) or epiphytic (growing on another plant).
Plants attached to or growing on another plant, without being parasitic. Many red algae grow on kelp holdfasts and stipes. They use the host only for attachment and never penetrate its tissue.
A reproductive strategy in some organisms where there is a haploid multicellular phase (generation) in the life cycle, alternating with a diploid multicellular generation. Its advantage: the sexual step ensures genetic diversity.
A diploid (2n) multicellular structure which produces haploid spores through meiosis. In most seaweeds the sporophyte is the visible plant — but in purple laver it is microscopic and bores into mollusc shells.
A haploid (n) multicellular structure which grows from spores and produces gametes. Usually reduced to a small structure — but in purple laver the gametophytes are the prominent, leafy fronds you see on the high shore.
Also on the examinable list
Isomorphic: different structures which are visibly identical. · Isogametes: male and female gametes which look identical. · Coralline: seaweeds which incorporate calcium carbonate to make them coral-like and resistant to wave action and herbivores. · Carposporophyte: a third generation in the life cycle of red seaweeds. · Carpospore: a diploid spore produced by the carposporophyte, which germinates into new sporophytes. · Rhizome: a branching stem which grows horizontally, on or below the substrate. · Infratidal: the region below the spring low-water mark, so always under water.
And the general terms: dichotomous (a growing tip branches in two, repetitively) · encrustation (a hard growth covering a surface) · pioneer (the first organisms to establish in a newly available habitat) · flux (constantly changing).
Quick Check
Question 1
Well done!
Next: Brown Algae (Chromista) — the kelps. Same shore, same physics, but up to 50 m long and with actual differentiated body parts.