Algae

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A seaweed (Laurencia) up close: the "branches" are multicellular and only about 1 mm thick. Much smaller algae are seen growing attached to the structure extending upwards in the lower right quarter

Algae (one Alga, several Algae) is a family of (usually) simple organisms. All of them photosynthesise. They capture light, and change it into energy. Normally, people think that algae are simple plants. Other kinds of algae however, seem to be more like simple animals. That is why, Algae form a group of their own.

Some algae live in the sea, they are usually called seaweeds.

Usually, Algae are photoautrophic, they produce energy from light, this is what tells them apart from other Protozoa. Protozoa are simple organisms (that usually live in the sea). The problem is that some algae are mixotrophic. Those algae also get energy from other sources than photosynthesis. This makes them almost like some bacteria.

Some algae have lost their capability of photosynthesis, and rely only on external energy sources.

All algae have photosynthetic machinery ultimately derived from the cyanobacteria, and so produce oxygen as a by-product of photosynthesis, unlike non-cyanobacterial photosynthetic bacteria.

Algae are usually found in damp places or water. For this reason they are common in terrestrial as well as aquatic environments. However, terrestrial algae are usually rather inconspicuous and far more common in moist, tropical regions than dry ones, because algae lack vascular tissues and other adaptations to live on land. Algae can endure dryness and other conditions in symbiosis with a fungus as lichen.

The various sorts of algae play significant roles in aquatic ecology. Microscopic forms that live suspended in the water column are called phytoplankton. They provide the food base for most marine food chains. In very high densities (so-called algal blooms) these algae may discolor the water and outcompete or poison other life forms. Seaweeds grow mostly in shallow marine waters. Some are used as human food or harvested for useful substances such as agar or fertilizer. The study of algae is called phycology or algology.

Some algae may harm other species.

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