Algae (
sing. alga) are a large and diverse group of simple, typically
autotrophic organisms, ranging from
unicellular to
multicellular forms. The largest and most complex marine forms are called
seaweeds. They are
photosynthetic, like plants, and "simple" because they lack the many distinct organs found in
land plants. Though the
prokaryotic cyanobacteria (commonly referred to as
blue-green algae) were traditionally included as "algae" in older textbooks, many modern sources regard this as outdated
[1][2] and restrict the term
algae to
eukaryotic organisms.
[3] All true algae therefore have a nucleus enclosed within a membrane and
chloroplasts bound in one or more membranes.
[1][4] Algae constitute a
paraphyletic and
polyphyletic group,
[1] as they do not all descend from a common algal ancestor, although their chloroplasts seem to have a single origin.
[5]Algae lack the various structures that characterize land plants, such as phyllids and rhizoids in nonvascular plants, or leaves, roots, and other organs that are found in tracheophytes. They are distinguished from protozoa in that they are photosynthetic. Many are photoautotrophic, although some groups contain members that are mixotrophic, deriving energy both from photosynthesis and uptake of organic carbon either by osmotrophy, myzotrophy, or phagotrophy. Some unicellular species rely entirely on external energy sources and have limited or no photosynthetic apparatus.
All algae have photosynthetic machinery ultimately derived from the cyanobacteria, and so produce oxygen as a by-product of photosynthesis, unlike other photosynthetic bacteria such as purple and green sulfur bacteria.
Algae are most prominent in bodies of water but are also common in terrestrial 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 are also found in other situations, such as on snow and on exposed rocks in symbiosis with a fungus as lichen.
