Phaeophyceae — Brown Algae
Imagine walking along a rocky seashore at low tide. The rocks are draped in slimy, olive-brown to dark greenish-brown strands that feel leathery, not soft like the green pond scum you might know. That is brown algae — Phaeophyceae. The colour is the first clue: it is not green, not red, but brown. Why? Because these algae live in colder, often turbulent coastal waters where green light penetrates deeper than red or blue. To capture that green light for photosynthesis, they have a special accessory pigment called fucoxanthin, which masks the green of chlorophyll and gives them that characteristic brownish hue.
The name Phaeophyceae comes from Greek phaios (dusky, brown) and phyton (plant). They are almost entirely marine, and most are large — some, like the giant kelp Macrocystis, can form underwater forests tens of metres long. They are not true plants (they lack true roots, stems, and leaves), but they have evolved a surprisingly plant-like body plan because they face the same physical challenges: staying anchored in waves, reaching toward sunlight, and transporting nutrients.
The Body Plan: Holdfast, Stipe, Frond
A typical brown alga is not a simple filament. It is differentiated into three distinct parts:
- Holdfast: a root-like structure that grips the rock or substrate. It does not absorb nutrients (unlike a true root); it is purely an anchor.
- Stipe: a stem-like stalk that is often flexible and tough, resisting the pull of waves. It may be hollow or solid, and in some species it can conduct photosynthates.
- Frond: the leaf-like blade that does the photosynthesis. It may be flat, branched, or ribbon-like, and often has a midrib or air bladders (pneumatocysts) to keep it floating toward the light.
This three-part structure is a classic exam point. Remember: holdfast anchors, stipe supports, frond photosynthesises.
Storage Products: Laminarin and Mannitol
Unlike green algae that store starch, or red algae that store floridean starch, brown algae store their food as laminarin (a β-1,3-glucan, a type of carbohydrate) and mannitol (a sugar alcohol). Mannitol is especially interesting — it is sweet-tasting and acts as a compatible solute, helping the alga cope with osmotic stress in seawater. You will often see these two named together as the characteristic storage products of Phaeophyceae.
Examples You Must Know
Two genera are textbook favourites:
- Laminaria — the kelp or oarweed. It has a long stipe and a large, flat, undivided frond. It is harvested commercially for alginate (a thickening agent used in ice cream and toothpaste).
- Fucus — the rockweed or bladder wrack. It has a branched, forked thallus with paired air bladders. It is common on rocky shores and is a classic example for studying alternation of generations (though that is a deeper topic).
Phaeophyceae = brown algae, marine, fucoxanthin pigment, food stored as laminarin + mannitol, body differentiated into holdfast, stipe, frond. Examples: Laminaria, Fucus.
A Quick Comparison (for clarity)
| Feature | Phaeophyceae (Brown) | Chlorophyceae (Green) | Rhodophyceae (Red) |
|---|
| Dominant pigment | Chlorophyll a, c, fucoxanthin | Chlorophyll a, b | Chlorophyll a, d, phycoerythrin |
| Colour | Olive-brown to dark brown | Grass green | Red to purplish |