Green, brown or red: seaweed colour is chemistry plus ancestry. A guide to algal pigments, the three seaweed lineages, and what colour says about depth.
Walk a rocky shore at low tide and the seaweeds arrange themselves like a paint chart: bright grass-green sea lettuce near the top, leathery brown wracks and kelps in the middle, and — if you look under ledges or wade deeper — the dark reds and near-blacks of dulse and its relatives. That colour sequence is one of the oldest observations in marine biology, and one of the most frequently mangled. The colours are real, the depth pattern is real-ish, and the explanation involves some of the strangest photochemistry on Earth. Here is how it actually works.
A seaweed's colour is the sum of its pigments — molecules that absorb particular wavelengths of light and either put that energy to work in photosynthesis or throw it away as protection. Three pigment families matter most.
All seaweeds photosynthesise, and all carry chlorophyll a, the near-universal reaction-centre pigment that absorbs red and blue light and reflects green. Green algae (becoming land plants' ancestors) add chlorophyll b. Brown and red algae instead use chlorophyll c or chlorophyll d — variants tuned slightly differently, one of several signs that these groups took separate evolutionary roads.
Brown algae — the kelps, wracks and rockweeds — are loaded with fucoxanthin, a carotenoid that absorbs strongly in the blue-green part of the spectrum. Blue-green light happens to be the light that penetrates seawater best (see our explainer on light underwater), so fucoxanthin is superbly placed to harvest what actually reaches a kelp frond at ten metres' depth. It absorbs so much green light that the plant reflects mostly brown-gold — hence the colour. We cover how kelp puts fucoxanthin to work in how kelp photosynthesises.
Red algae carry phycoerythrin and its relatives — water-soluble proteins that absorb green and blue-green light with extraordinary efficiency. Packed into antenna complexes called phycobilisomes, they can capture a large fraction of the dim green light that filters to deep water. Too much phycoerythrin and the alga looks red; so much that almost nothing is reflected, and it looks nearly black — which is why deep-water red seaweeds hauled up in a net can look like wet leather.
The colour groups are not just a paint job over one body plan. Green, brown and red algae are separate evolutionary lineages that became photosynthetic in different ways, and "seaweed" is a guild, not a family.
This is why colour is genuinely useful to a field naturalist: it is a quick read of deep ancestry, not merely lighting or mood.
The tidy textbook story goes like this: green algae live shallow, where light is bright and full-spectrum; brown algae take the middle depths, harvesting penetrating blue-green light; red algae live deepest, their phycoerythrin soaking up the last thin residue of green. This is the chromatic adaptation hypothesis, and it contains real physics — light does filter predictably through the water column (see the euphotic zone for how far down the ocean runs on light).
But field biologists have spent decades adding footnotes:
The modern synthesis: pigments explain how efficiently a seaweed uses whatever light it gets, and zonation emerges from that efficiency interacting with competition, grazing, temperature and turbulence. Colour is part of the story, never the whole storey-cast.
For the beachcomber and the diver, colour remains a decent first sort:
But never identify on colour alone. Bleaching by summer sun can turn a brown kelp straw-yellow; deep-shade greens can look black; and many groups have members wearing the "wrong" pigment load. Colour reads ancestry and light strategy — identification still needs morphology.
Pigment groups frame real questions in ocean change. As waters warm and stratify, the light and nutrient regimes that favour each lineage shift. As forests thin, red-algal turf and green opportunists often claim the space first — a colour change on the reef floor is one of the earliest visual symptoms of a kelp forest in trouble. And for restoration, the pigment story is a reminder that "seaweed" is not interchangeable: a reef seeded with kelp rebuilds a brown canopy, but the red crusts, green films and all their tenants are what make the canopy a forest.
Sunlight, heat and oxidation break down the accessory pigments faster than hardy chlorophyll a. A dried brown kelp loses its fucoxanthin overcoat and the underlying green shows through; dried red algae can darken to near-black as their proteins denature.
No. Brown algae are stramenopiles, a lineage separate from both plants and red/green algae. They converged on plant-like forms — blades, stems, even a holdfast — entirely independently, which is why we say kelp reinvented the tree rather than descended from one.
They hold the depth records, yes — red crusts have been collected below 250 metres in clear tropical water, living on light intensities a fraction of a percent of surface brightness. But the deepest kelps are still brown algae; depth records and forest depth are different claims.
Fucoxanthin concentration varies with light exposure, season and species. South-facing fronds in bright water synthesise more protective pigment; shaded or fast-growing tissue stays greener and more golden. "Golden kelp" (Ecklonia radiata) earns its name this way.
Many. Fucoxanthin is studied as an antioxidant, phycoerythrin is a standard fluorescent label in biomedical research and flow cytometry, and carrageenan-producing red algae supply food science — one more reason pigment chemistry is more than a beach curiosity.
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