Brown Algae
(Chromista)
The third seaweed group is the one that builds forests. Brown algae include the kelps — organisms with no roots, no stems and no leaves that nonetheless grow 13 mm in a day, reach the surface from 15 m down, and shelter an entire ecosystem off our West Coast. They also sit in a different kingdom from the green and red seaweeds, for a genuinely strange reason: they appear to be the descendants of one cell that swallowed another and kept it.
There is a forest off Cape Town where the trees have no roots, no trunk and no leaves, grow a centimetre a day, and are held upright not by wood but by gas. Stranger still: the organism that builds it carries DNA from two completely different phyla — it is not one ancestor's descendant, but two. What is it, and how did that happen?
It is sea bamboo, Ecklonia maxima — the kelp that forms the West Coast Kelp Forest. It grips the reef with a holdfast (not a root — it only anchors), grows a hollow stipe up towards the light, and when it reaches the surface the top swells into a gas-filled pneumatocyst that floats the whole plant upright, holding its fronds in the brightest water. In good conditions it grows 13 mm per day.
The two-ancestor puzzle is the reason brown algae sit in Kingdom Chromista rather than with the greens and reds in Plantae. Chromista are thought to have arisen from an endosymbiotic relationship between two early eukaryotes — one of which was very probably related to modern red algae. A cell swallowed another cell, and instead of digesting it, kept it as a partner. The strongest physical evidence is inside every brown-algal cell: its chloroplasts are wrapped in four membranes, not two.
A Different Kingdom Entirely
Brown seaweeds are Kingdom Chromista → Division Ochrophyta → Class Phaeophyceae. Green and red algae are Plantae. The split is not about colour — it is about ancestry.
Red and green seaweeds evolved along more conventional lines. Chromista are thought to have evolved from an endosymbiotic relationship between two early eukaryotic organisms, one of which was very probably related to modern-day red algae. The result is an organism whose DNA clearly indicates ancestors from two different phyla — which is why they get a kingdom of their own.
Pigments and depth. All brown algae have chlorophyll-a and — uniquely among the three groups — chlorophyll-c. Most also carry the accessory pigment fucoxanthin, which gives them their brown colour and lets them use colours from the middle of the light spectrum. That is why they slot in between the greens above them and the reds below: they can grow at 40–250 m depending on water clarity, while still being extremely well represented in shallow water and into the lower intertidal, where wave action is significant.
Structure. Like all seaweeds they form multicellular thallus structures — but here is the difference from Topic 23: the larger varieties undergo some cell differentiation to form tissues. The kelps take that furthest, growing to enormous sizes (the Californian bladder kelp, Macrocystis, exceeds 50 m) with genuinely specialised body parts: holdfast, stipe, fronds and pneumatocysts. This is as close to "having organs" as an alga gets.
The Anatomy of a Kelp
A single sea bamboo, from the reef to the surface. Four differentiated body parts plus the structure that makes the next generation — tap each one. (CAPS asks you to label exactly these.)
Four Membranes — the Fingerprint of a Merger
This is the most interesting fact in the topic, and CAPS asks about it directly. Count the membranes around a brown alga's chloroplast and you can read its evolutionary history.
How to explain endosymbiosis in an exam
Endosymbiotic means a symbiotic relationship where one of the participants lives within the cells of the other. The argument runs like this: if a primitive heterotroph engulfed a primitive red alga and kept it alive inside itself as a partner, then that red alga's chloroplast — which already had its own double membrane — would end up wrapped in two extra membranes: the red alga's own cell membrane, and around that a membrane from the host that engulfed it. And that is precisely what we find: brown algal chloroplasts are bound by a quadruple membrane, the outer two associated with the endoplasmic reticulum. (Only the outermost of the four comes from the host; the other three came in with the red alga.) Add the DNA evidence pointing to ancestors from two different phyla, and the case is strong.
Endosymbiosis is not only ancient history
The booklet asks whether you can name a living marine endosymbiotic relationship. You already met one in Topic 22: the zooxanthellae — dinoflagellates living inside the ectodermal cells of coral polyps, photosynthesising on nutrients from the polyp's waste and feeding both partners. That is endosymbiosis happening right now, on every tropical reef. Roughly two billion years ago a similar arrangement became permanent, and the descendants of that merger are the kelp forests off our coast.
The Incredible Shrinking Gametophyte
In Topic 23 you learned the standard two-generation cycle. Brown algae show that same cycle being dismantled — step through four stages and watch one whole generation disappear.
Why would losing a generation be an advantage?
The booklet sets this as a discussion question, so have two reasons ready. (1) Vulnerability. A free-living gametophyte is a separate, delicate, microscopic organism that has to survive on its own in a place with wave action, grazers and desiccation. Every generation that does not have to survive independently is one fewer chance to be eaten or washed away. (2) Reliability of fertilisation. If gametes are made on the big, established sporophyte — or by a gametophyte tucked inside its tissue — the gametes are produced in a protected place, in numbers, at a predictable time, instead of depending on two microscopic strangers settling near each other first. A third, honest reason: speed — one fewer multicellular stage to build means a faster route back to a reproducing adult in a habitat that gets scoured by storms.
The Two South African Examples
CAPS names one for each side of the country. Both are worth knowing by structure, not just by name.
Sea bamboo
- The largest kelp on the South African coastline and the primary component of the West Coast Kelp Forest ecosystem.
- Distribution is limited to the western coastline, from Namibia to about Cape Agulhas — and recent research shows it is extending its range eastwards, having reached the De Hoop MPA some 50 km east of Agulhas.
- Grows in shallow water from the low-tide mark down to about 15 m, anchored firmly to reefs by holdfasts.
- It grows a hollow stipe until the surface is reached — after which the top expands into a gas-filled pneumatocyst.
- A short flat stalk on top of the pneumatocyst carries flat, strap-shaped fronds, which can grow up to 13 mm per day in ideal conditions. The pneumatocyst holds those fronds at the surface, in the best light.
Long-leafed sargassum
- A noticeably brown seaweed, abundant in deeper gullies and on rocks below the low-tide mark, all along the South and East Coast.
- It is the only seaweed in its genus and is endemic to the coast of South Africa — it grows nowhere else on Earth.
- A hard, twisted, stem-like base grows from a woody holdfast and flattens further along into blade-like fronds.
- The fronds have serrated margins with spur-like outgrowths. Alternating with the fronds are pneumatocysts for flotation, with leafy clusters at their ends.
- It has special reproductive branches resembling small clusters of berries — and, like others in its order, no alternation of generations at all: the berries on the sporophyte make gametes directly. The tiny eggs stay attached by strong mucus threads until fertilised, then disperse by wave action and grow into new sporophytes.
Green vs Red vs Brown — the Revision Table
The Two Oceans booklet ends this topic by asking you to build exactly this table across Topics 23 and 24. Here it is, complete — if you learn one thing off this page, learn this.
| Green algae | Red algae | Brown algae | |
|---|---|---|---|
| Division | Chlorophyta | Rhodophyta | Ochrophyta (Class Phaeophyceae) |
| Kingdom | Plantae | Plantae | Chromista |
| Membranes around the chloroplast | 2 (double) | 2 (double) | 4 (quadruple) — outer two linked to the ER |
| Chlorophyll types | a and b | a | a and c |
| Other photosynthetic pigments | Beta carotene, xanthophylls | Phycoerythrin (absorbs violet-blue) | Fucoxanthin (middle of the spectrum) |
| Food stored as | Starch, fats and oils | Floridean starch | Laminarin |
| Other characteristic chemicals | Cellulose cell walls | Cellulose + agars and carrageenan | Cellulose + alginic acid (alginate) |
| Typical depth | Surface and intertidal | Intertidal to 250 m | 40–250 m, plus shallow surf zones |
| Life-cycle notes | Alternation of generations; isomorphic generations, isogametes | Often a third generation — the carposporophyte | Ranges from full alternation → reduced/absent gametophyte |
~ scroll the table sideways on a phone →
Our Own Underwater Forest
Local connection — the Great African Seaforest
From the Namibian border down past Cape Point and east towards Agulhas lies one of the largest kelp forests on the planet, built mainly from sea bamboo (Ecklonia maxima) with Laminaria pallida in deeper water. Cold, nutrient-rich Benguela water (Topic 22) plus strong light makes it staggeringly productive — and it works exactly like a land forest: a canopy of fronds at the surface, a understorey of red algal epiphytes on the stipes and holdfasts (Topic 23), and a floor of urchins, abalone and rock lobster, with sevengill sharks and seals moving through it.
It is also an economy. Beach-cast and harvested kelp is processed for alginates — the same alginic acid in its cell walls — which thicken food, make dental moulds and coat pharmaceuticals; and fresh kelp is a major feed for South Africa's abalone farms. Meanwhile, unusually for kelp forests worldwide, ours are in places expanding: as parts of the West Coast have cooled and the species creeps eastward past Cape Agulhas, sea bamboo is claiming new reef. Most of the world's kelp forests are shrinking as seas warm. Watching this one is genuine, current South African science.
In the Lab — the CAPS Investigations
CAPS asks for "live samples of the prescribed examples" — fresh sea bamboo — to be examined, and a "visit to rocky shore or aquarium". A kelp washed up after a storm is a perfect specimen, and the practical sheet adds alginate, kelp growth and the four membranes.
Five investigations · a washed-up kelp, washing soda and some plastic bags
- 🌊 shore Examine a sea bamboo — measure, weigh and draw a beach-cast kelp, and work out how a half-litre float holds up 3.4 kg of fronds.
- 🌊 shore Brown seaweeds and look-alikes — nine specimens to sort, including a red seaweed that looks brown, a seagrass and a diatom film.
- 🟡 pharmacy Alginate in the kitchen — pull alginate out of kelp with washing soda, and turn it into jelly beads with calcium.
- 🟢 desk How fast does a kelp grow? — the divers' hole-punch method, with results supplied. Faster than bamboo? Not even close.
- 🟢 kitchen A cell inside a cell — a bag model of the endosymbiosis that gave brown algae four membranes, and the booklet's questions.
→ Open the Topic 24 practical sheet — printable, with supplied data for every task.
Key Vocabulary
Tap each card to flip it. The first six are the topic's examinable definitions; the last two are the chemicals CAPS names for this group.
Any large brown, cold-water seaweed of the family Laminariaceae, in the class Phaeophyceae. Kelps are the brown algae that grow big enough to show real differentiated body parts.
The stem-like structure on kelps. In sea bamboo it is hollow and grows upward until it reaches the surface — but it is not a true stem: there is no vascular tissue inside it.
Air bladders on seaweeds, which enable the fronds to be close to the surface or at the surface. In sea bamboo one large gas-filled pneumatocyst forms at the top of the stipe; in long-leafed sargassum many small ones alternate with the fronds.
An insoluble gelatinous polysaccharide found in the cell walls of brown seaweeds (also called alginic acid). It makes the thallus flexible and slippery enough to survive surf — and it is why kelp is harvested commercially.
A symbiotic relationship where one of the participants lives within the cells of the other. Chromista are thought to have arisen this way — an early eukaryote taking in another, probably related to modern red algae — leaving the quadruple-membrane evidence behind.
A multicellular plant structure which exhibits little or no cell differentiation. Brown algae form thallus structures too — but note the exception CAPS makes for this group: the larger varieties do undergo some cell differentiation to form tissues.
The accessory pigment that gives brown algae their brown colour. It lets them use colours from the middle of the light spectrum and so grow deeper than the greens — 40 to 250 m, depending on water clarity.
The complex polysaccharide in which brown algae store their food. Compare: green algae store starch, fats and oils; red algae store floridean starch; brown algae store laminarin.
Also worth knowing
Quadruple: having four parts or layers — as in the quadruple membrane around a brown algal chloroplast. · Serrated: having an edge that is notched like a saw — as on the fronds of long-leafed sargassum. · And the three names in order: Kingdom Chromista → Division Ochrophyta → Class Phaeophyceae.
Quick Check
Question 1
Well done!
Next: Vascular Plants — the last producers in the sequence, and the only ones that are actually plants: eel grasses and mangroves.