Algae are a large and diverse group of simple, typically autotrophic organisms. Some have one cell and others have many cells. The largest and most complex marine algae are called seaweeds. They are like plants, and "simple" because they lack the many distinct organs found in land plants. For that reason they are "not" classified as plants.
Though the prokaryotic "cyanobacteria" (formerly referred to as "blue-green algae") were included as "algae" in older textbooks, it is not now. The term "algae" is now used for eukaryotic organisms. All true algae have a nucleus within a membrane and chloroplasts inside one or more membranes. However, algae are definitely "not" a monophyletic group, as they do not all descend from a common algal ancestor. Modern taxonomists propose splitting them up into monophyletic groups, but not everyone agrees how to do this.
Algae do not have the same structures that land plants do, such as leaves, roots, and other organs. Nearly all algae have parts that do photosynthesis the same way as cyanobacteria. They make oxygen, unlike other photosynthetic bacteria such as "purple" and "green" bacteria. Some unicellular species use only external energy sources and have limited or no photosynthetic parts.
Fossilized filamentous algae from the Vindhya basin have been dated back to 1.6 to 1.7 billion years ago.
Below are some important kinds of algae. The list is not complete.
Algae are usually found in damp places or water, and are common on land and water. However, algae on land are usually inconspicuous and are far more common in moist, tropical regions than dry ones. Algae do not have vascular tissues and other adaptations to live on land, but they 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. Kelp grows mostly in shallow marine waters. Some are used as human food or harvested for agar or fertilizer. Kelp can grow in large stands called kelp forests. These forests prevent some of the damage from waves. Many different species live in them, including sea urchins, sea otters, and abalone.
Some algae may harm other species. Some algae may reproduce a lot, and make an algal bloom. These algae may make protective toxins which kill fish in the water. Dinoflagellates make a compound that turns the flesh of fish into slime. Then the algae eats this nutritious liquid.
Algae have evolved a number of symbiotic partnerships with other organisms. The most famous is the plant-like lichen, which are each formed by a fungus with an alga. It is a highly successful life-form, and twenty thousand 'species' are known. In all cases the lichen are quite different in appearance and life-style from either constituent; it is possibly the most complete symbiosis known. Both constituents gain from their access to niches with low nutrient value, which is where lichen are found.
Less well known are the algal relationships with animals. Reef-building corals are basically social Cnidarian polyps. Corals are dependent on light, because the algae are important partners, and they require light. Corals have evolved structures, often tree-like, which offer the algae maximum access to light. The coral weakens the algal cell walls, and digests about 80% of the food synthesised by the algae. The corals' waste-products provide nutrients for the algae so, as with lichen, both partners gain from the association. The algae are golden-brown flagellate algae, often of the genus "Symbiodinium". A curious feature of the partnership is that the coral may eject the algae in hard times, and regain them later. The ejection of the algal partner is called "bleaching", because the coral loses its colour.
Other types of Cnideria, such as sea anemones and jellyfish, also contain algae. Jellyfish with algae behave so that their partners get the best light during the day, and descend to depths at night, where the water is rich in nitrates and brown with decay. Sea slugs and clams are also well known for harbouring algae. Both groups are molluscs. The sea slugs graze on coral, and are the same colour as the coral they graze. They are able to separate the algae from the polyp tissues they digest. The algal cells are moved to its tentacles, where they continue to live. The otherwise defenceless slug gains both camouflage and nutrition. The giant clam keeps algae in its mantle, which is revealed when the clam is open. The coloured mantle has places where the skin is transparent, and acts like a lens to concentrate light on the algae beneath. When the algae get too numerous, the clam digests them.
Various other marine invertebrate groups have members which symbiose with algae. Flatworms (Platyhelminths) and Polychaete worms (Annelids) are two such groups.
Alga (Korday district)
Alga (; ) is a hamlet in the Korday District of Kazakhstan. It is one of several communities in Kazakhstan with this name. The A-2 highway from Bishkek to Almaty runs through the town. A significant architectural feature is its mosque.
Kelp forest
Kelp forests are a type of marine ecosystem that is present around colonies of kelp; they contain rich biodiversity. Kelp can stretch 2-30 meters or more (up to 60 m in "Macrocystis pyrifera") from their anchors on the sea floor to the surface. It provides a vertical infrastructure that is home to many fish and invertebrate species. Kelp forests also often attract mammalian visitors, including whales, sea lions, sea otters, and SCUBA-diving humans.
Kelp forests draw their name from an analogy to forests on land.
In some places, sea urchins eat so many kelp plants that the kelp forest becomes an urchin barren.
Symbiosis
Symbiosis (pl. symbioses) means living together. It describes close and long-term relationships between different species. The term was
used by Anton de Bary in 1879, as "the living together of unlike organisms".
A symbiont is an organism living in a relationship with another species in which one or both get benefits. When one species lives inside another species, or a microscopic symbiont lives inside the cells of a host, it is called an endosymbiont.
The relevance of symbiosis is its frequency and its evolutionary significance. There appear to be no higher plants or animals without symbionts. Those symbionts are of great importance to the larger organisms, who in most cases would be unable to live as they do without their symbionts. Mycorrhiza in higher plants, and gut flora in insects and vertebrates are examples. Humans are no exception.
Furthermore, most of these associations are between organisms not just from different species, but from different kingdoms. And lastly, the cells of all eukaryotes contain organelles which are descendants of symbiotic relationships which began at least a billion years ago. Mitochondria and plastids are examples. The conclusion must be that symbiosis has been highly significant in the evolution of life.
The definition of symbiosis has been controversial. Some believe symbiosis should only refer to persistent mutualisms, while others believe it should apply to all kinds of lon-term biological interactions.
After 130 years of debate, current biology and ecology textbooks now use the latter "de Bary" definition or an even broader definition (where symbiosis means all species interactions). The restrictive definition (where symbiosis means mutualism only) is no longer used.
The widest definition includes parasitism (in which one organism is helped and the other organism is hurt), mutualism (in which both organisms are helped), commensalism (in which one organism is helped and the other is not affected), and competition (in which both organisms are hurt).
The various forms of symbiosis include:
Symbiosis does not always benefit both partners. This is what may happen: