Algae

# Overview
Algae have conventionally been regarded as simple plants within the study of botany. All are Eukaryota, though Chromophyta have Bacterial (see Blue-green algae) characteristics and some authorities consider them all to be Protists, however this view is now considered to be outdated. They may still be included in the algae as plants. Some authors often include the blue-green algae (Cyanophyta) but note that they are not eukaryote. Algae do not represent a single evolutionary direction or line but a level of organization that may have developed several times in the early history of life on Earth.
The protists are traditionally considered more animal-like (see Protozoa).
The prokaryotic forms, referred to as blue-green algae are only half-algae with a mixture of bacterial characteristics. However, they are quite distant from the bacteria and are referred to by some as Cyanochloronta. All other forms belong as true eukaryota algae within the study of Botany, they have a nucleus enclosed within a membrane. The protoctists are defined by some as eukaryotic microorganisms with the exception of animals and plants and including fungi and algae, slime moulds and other obscure eukaryotes. There is still some disagreement on some of these matters.
Algae range from single-cell organisms to multicellular organisms, some with fairly complex differentiated form and (if marine) called seaweeds. All lack leaves, roots, flowers, seeds and other organ structures that characterize higher plants (vascular plants). They are distinguished from other protozoa in that they are photoautotrophic although this is not a hard or vast distinction as 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 reduced or lost their photosynthetic apparatus.
All algae have photosynthetic machinery ultimately derived from the cyanobacteria, and so produce oxygen as a byproduct of photosynthesis, unlike non-cyanobacterial photosynthetic bacteria. It is estimated that algae produce about 73 to 87 percent of the net global production of oxygen - which is available to humans and other animals for respiration.

# Ecology
Algae are usually found in damp places or bodies of water and thus 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, however, 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  called phytoplankton  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, however some have been recorded to a depth of 300 m.Some are used as human food or harvested for useful substances such as agar or fertilizer.

# Study of algae
The study of marine and freshwater algae is called phycology or algology.
The US Algal Collection is represented by almost 300,000 accessioned and inventoried herbarium specimens.

# Classification

## Prokaryotic algae
Cyanobacteria have been included among the algae, referred to as the cyanophytes or Blue-green algae, (the term "algae" refers to any aquatic organisms capable of photosynthesis) though some recent treatises on algae specifically exclude them. Cyanobacteria are some of the oldest organisms to appear in the fossil record dating back to the Precambrian, possibly as far as about 3.5 billion years. Ancient cyanobacteria likely produced much of the oxygen in the Earth's atmosphere.
Cyanobacteria can be unicellular, colonial, or filamentous. They have a prokaryotic cell structure typical of bacteria and conduct photosynthesis on specialized cytoplasmic membranes called thylakoid membranes, rather than in organelles. Some filamentous blue-green algae have specialized cells, termed heterocysts, in which nitrogen fixation occurs. the perfect prokaryotic cell consist of miscalgnous sheath covering cellwall that consistof pectinic substance and sachride while the cellwall consist of 4 layer outer and inner and middle layer while the fourth layer is attached to plasma membrane and the protoplast consist of 2 part peripheral coloured partknown by chromatoplasm which contain the pigments in case of algae and contain photothynsis producte.g in cyanobacteria it contain chlorophylla, b-caroteinand c-phycocyanin and c-phycoerthyrin

## Eukaryotic algae
All other algae are eukaryotes and conduct photosynthesis within membrane-bound structures (organelles) called chloroplasts. Chloroplasts contain DNA and are similar in structure to cyanobacteria, presumably representing reduced cyanobacterial endosymbionts. The exact nature of the chloroplasts is different among the different lines of algae, reflecting different endosymbiotic events. The table below lists the three major groups of eukaryotic algae and their lineage relationship is shown in the figure on the left. Note many of these groups contain some members that are no longer photosynthetic. Some retain plastids, but not chloroplasts, while others have lost them entirely.
It was W.H.Harvey (1811  1866) who first divided the algae into four divisions based on their pigmentation. This is the first use of a biochemical criterion in plant systematics. Harvey's four divisions were: red algae (Rhodophyta), brown algae (Heteromontophyta), green algae (Chlorophyta) and Diatomaceae (Dixon, 1973 p.232).

# Forms of algae
Most of the simpler algae are unicellular flagellates or amoeboids, but colonial and non-motile forms have developed independently among several of the groups. Some of the more common organizational levels, more than one of which may occur in the life cycle of a species, are
- Colonial: small, regular groups of motile cells
- Capsoid: individual non-motile cells embedded in mucilage
- Coccoid: individual non-motile cells with cell walls
- Palmelloid: non-motile cells embedded in mucilage
- Filamentous: a string of non-motile cells connected together, sometimes branching
- Parenchymatous: cells forming a thallus with partial differentiation of tissues
In three lines even higher levels of organization have been reached, imma hit with full tissue differentiation. These are the brown algae some of which may reach 50 m in length (kelps)the red algae , and the green algae . The most complex forms are found among the green algae (see Charales and Charophyta), in a lineage that eventually led to the higher land plants. The point where these non-algal plants begin and algae stop is usually taken to be the presence of reproductive organs with protective cell layers, a characteristic not found in the other alga groups.
The first plants on earth were algae and these still thrive in a range of aquatic habitats today. The land plants evolved from the algae, more specifically green algae. Some 400 million years ago freshwater, green, filamentous algae invaded the land. These probably had an isomorphic alternation of generations and were probably heterotrichous. Fossils of isolated land plant spores suggest land plants may have been around as long as 475 million years ago.

# Fresh-water algae

# Algae and symbioses
Some species of algae form symbiotic relationships with other organisms. In these symbioses, the algae supply photosynthates (organic substances) to the host organism providing protection to the algal cells. The host organism derives some or all of its energy requirements from the algae. Examples include
- lichens: a fungus is the host, usually with a green alga or a cyanobacterium as its symbiont. Both fungal and algal species found in lichens are capable of living independently, although habitat requirements may be greatly different from those of the lichen pair.
- corals: algae known as zooxanthellae are symbionts with corals. Notable amongst these is the dinoflagellate Symbiodinium, found in many hard corals. The loss of Symbiodinium, or other zooxanthellae, from the host is known as coral bleaching.
- sponges: green algae live close to the surface of some sponges, for example, breadcrumb sponge (Halichondria panicea). The alga is thus protected from predators; the sponge is provided with oxygen and sugars which can account for 50 to 80% of sponge growth in some species.

# Life-cycle
Rhodophyta, Chlorophyta and Heterokontophyta, the three main algal Phyla, have life-cycles which show tremendous variation with considerable complexity. In general there is an asexual phase where the seaweed's cells are diploid, a sexual phase where the cells are haploid followed by fusion of the male and female gametes. Asexual reproduction is advantageous in that it permits efficient population increases, but less variation is possible. Sexual reproduction allows more variation but is more costly because of the waste of gametes that fail to mate, among other things. Often there is no strict alternation between the sporophyte and gametophyte phases and also because there is often an asexual phase, which could include the fragmentation of the thallus.