Which marine carotenoid is abundant in brown seaweed, macroalgae and diatoms?

Correct — D. Fucoxanthin is the brown-olive xanthophyll that dominates brown seaweeds (Phaeophyceae), the macroalgae of that group and diatoms.

It masks the chlorophyll in those cells, which is why brown algae and diatom-rich water look brown rather than green, and it works as an accessory pigment, absorbing light chlorophyll misses and passing the energy on.

Because diatoms are so abundant in the sea, fucoxanthin is described exactly as the stem describes it — a marine carotenoid.

Why the others are wrong

• (a)
  β-cryptoxanthin — β-cryptoxanthin is the carotenoid of orange and yellow fruits and vegetables such as papaya, tangerine and red pepper, and it is a provitamin A compound. It is dietary and terrestrial, not a seaweed pigment.
• (b)
  Neoxanthin — Neoxanthin is a xanthophyll of green plant chloroplasts and a step towards the plant hormone abscisic acid. Its home is the green leaf, not brown seaweed.
• (c)
  Astaxanthin — Astaxanthin is marine, but it is the red-pink pigment of shrimp, salmon flesh and the microalga Haematococcus — not the brown pigment of seaweed and diatoms.

Carotenoids are the yellow, orange, red and brown accessory pigments of photosynthetic cells. They absorb light that chlorophyll cannot use, hand the energy on, and shield the cell from excess light.

They fall into two families: carotenes, pure hydrocarbons such as β-carotene and lycopene, and xanthophylls, which carry oxygen — lutein, zeaxanthin, astaxanthin and fucoxanthin among them.

The colour of an alga follows its dominant accessory pigment: fucoxanthin browns the Phaeophyceae, phycoerythrin reddens the Rhodophyceae.

Key facts

• Fucoxanthin is the dominant accessory pigment of brown algae and diatoms.
• Fucoxanthin is a xanthophyll, the oxygen-carrying family of carotenoids.
• Astaxanthin is the red-pink carotenoid of shrimp, salmon and Haematococcus algae.
• β-cryptoxanthin is a provitamin A carotenoid found in orange and yellow fruits.

Common traps

• Astaxanthin is marine too, so the word 'marine' alone does not settle the answer — the colour and the host organism do.
• All four names end in -xanthin, so the ending carries no information at all.

No directly related past PYQ was found.