Algae

Algae are the simplest members of the plant kingdom. They carry chlorophyll, so they make their own food (they are autotrophic), yet their body stays simple and undifferentiated — it is a thallus, meaning it is not divided into true roots, stems and leaves. Most algae live in water, both fresh water and the sea, though they are far from being confined to it.

Where algae live

Water is the usual home, but algae turn up in many other damp places as well:

• On moist stones, in soils and on wood.
• In partnership with fungi, forming the composite organisms called lichens.
• In association with animals — for instance, growing on the fur of a sloth bear.

Range of form and size

The body of an alga varies enormously from one kind to another. Some are colonial, such as Volvox, where many cells live together as a coordinated ball. Others are filamentous, growing as long threads of cells, as in Ulothrix and Spirogyra. At the other extreme, some marine algae — the kelps — build very large, massive plant bodies.

How algae reproduce

Algae multiply in three broad ways: vegetative, asexual and sexual.

• Vegetative reproduction happens by fragmentation. The thallus simply breaks into pieces, and every fragment grows into a fresh, complete thallus.
• Asexual reproduction works through spores of various kinds. The commonest of these is the zoospore. Zoospores carry flagella, so they can swim (they are motile), and when one settles and germinates it gives rise to a new plant.
• Sexual reproduction takes place when two gametes fuse. The nature of the fusing gametes lets us recognise three patterns:
  • Isogamous — the two gametes look alike and are of the same size. They may both be flagellated and motile, as in Ulothrix, or both be non-flagellated and non-motile, as in Spirogyra.
  • Anisogamous — the two gametes are similar in kind but differ in size, as seen in species of Eudorina.
  • Oogamous — fusion occurs between one large, non-motile (static) female gamete and a much smaller, motile male gamete, as in Volvox and Fucus.

Why algae matter

Algae are useful to people, and to life on Earth, in several important ways.

• Through photosynthesis they carry out at least half of all the carbon dioxide fixation that happens on the planet.
• Because they photosynthesise, they raise the amount of dissolved oxygen in the water around them.
• They are prime primary producers, building the energy-rich compounds that support the food cycles of every aquatic animal.
• Many are eaten as food. Several species of Porphyra, Laminaria and Sargassum are among about seventy species of marine algae used as human food. …