Algae ( , ; : alga ) is an informal term for any
organisms of a large and diverse group of
photosynthetic organisms that are not
plant
Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...
s, and includes
species
A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
from multiple distinct
clade
In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
s. Such organisms range from
unicellular microalgae, such as
cyanobacteria, ''
Chlorella'', and
diatoms, to
multicellular macroalgae such as kelp or
brown algae which may grow up to in length. Most algae are aquatic organisms and lack many of the distinct cell and tissue types, such as
stomata,
xylem, and
phloem that are found in
land plants. The largest and most complex marine algae are called
seaweeds. In contrast, the most complex freshwater forms are the
Charophyta, a
division of
green algae which includes, for example, ''
Spirogyra'' and
stoneworts. Algae that are carried passively by water are
plankton
Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...
, specifically
phytoplankton.
Algae constitute a
polyphyletic group
because they do not include a
common ancestor, and although
eukaryotic algae with
chlorophyll-bearing
plastids seem to have a single origin (from
symbiogenesis with
cyanobacteria),
they were acquired in different ways. Green algae are a prominent example of algae that have primary
chloroplasts derived from
endosymbiont cyanobacteria.
Diatoms and brown algae are examples of algae with secondary chloroplasts derived from endosymbiotic
red algae, which they acquired via
phagocytosis. Algae exhibit a wide range of reproductive strategies, from simple
asexual cell division to complex forms of
sexual reproduction via
spores.
Algae lack the various structures that characterize
plant
Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...
s (which evolved from freshwater green algae), such as the phyllids (leaf-like structures) and
rhizoids of
bryophytes (
non-vascular plants), and the
roots,
leaves and other
xylemic/
phloemic
organs found in
tracheophytes (
vascular plants). Most algae are
autotrophic, although some are
mixotrophic, deriving energy both from photosynthesis and uptake of organic carbon either by
osmotrophy,
myzotrophy or
phagotrophy. Some unicellular species of green algae, many
golden algae,
euglenids,
dinoflagellates, and other algae have become
heterotrophs (also called colorless or apochlorotic algae), sometimes
parasitic, relying entirely on external energy sources and have limited or no photosynthetic apparatus. Some other heterotrophic organisms, such as the
apicomplexans, are also derived from cells whose ancestors possessed chlorophyllic plastids, but are not traditionally considered as algae. Algae have photosynthetic machinery ultimately derived from cyanobacteria that produce
oxygen as a
byproduct of splitting
water molecules, unlike other organisms that conduct
anoxygenic photosynthesis such as
purple and
green sulfur bacteria. Fossilized filamentous algae from the
Vindhya basin have been dated to 1.6 to 1.7 billion years ago.
Because of the wide range of types of algae, there is a correspondingly wide range of industrial and traditional applications in human society. Traditional
seaweed farming practices have existed for thousands of years and have strong traditions in
East Asian food cultures. More modern
algaculture applications extend the
food traditions for other applications, including cattle feed, using algae for
bioremediation or pollution control, transforming sunlight into
algae fuels or other chemicals used in industrial processes, and in medical and scientific applications. A 2020 review found that these applications of algae could play an important role in
carbon sequestration to
mitigate climate change while providing lucrative value-added products for global economies.
Etymology and study
The singular is the Latin word for 'seaweed' and retains that meaning in English. The
etymology is obscure. Although some speculate that it is related to Latin , 'be cold', no reason is known to associate seaweed with temperature. A more likely source is , 'binding, entwining'.
The
Ancient Greek
Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...
word for 'seaweed' was (), which could mean either the seaweed (probably red algae) or a red dye derived from it. The Latinization, , meant primarily the cosmetic rouge. The etymology is uncertain, but a strong candidate has long been some word related to the
Biblical (), 'paint' (if not that word itself), a
cosmetic eye-shadow used by the
ancient Egypt
Ancient Egypt () was a cradle of civilization concentrated along the lower reaches of the Nile River in Northeast Africa. It emerged from prehistoric Egypt around 3150BC (according to conventional Egyptian chronology), when Upper and Lower E ...
ians and other inhabitants of the eastern Mediterranean. It could be any color: black, red, green, or blue.
The study of algae is most commonly called
phycology (); the term
algology is falling out of use.
Description

The algae are a heterogeneous group of mostly photosynthetic organisms that produce oxygen and lack the reproductive features and structural complexity of land plants. This concept includes the cyanobacteria, which are prokaryotes, and all photosynthetic
protists, which are eukaryotes. They contain
chlorophyll ''a'' as their primary
photosynthetic pigment, and generally inhabit aquatic environments.
However, there are many exceptions to this definition. Many non-photosynthetic protists are included in the study of algae, such as the heterotrophic relatives of
euglenophytes
or the numerous species of colorless algae that have lost their chlorophyll during evolution (e.g., ''
Prototheca''). Some exceptional species of algae tolerate dry terrestrial habitats, such as soil, rocks, or caves hidden from light sources, although they still need enough moisture to become active.
Morphology

A range of algal
morphologies is exhibited, and
convergence of features in unrelated groups is common. The only groups to exhibit three-dimensional multicellular
thalli are the
reds and
browns, and some
chlorophytes.
Apical growth is constrained to subsets of these groups: the
florideophyte reds, various browns, and the charophytes.
The form of charophytes is quite different from those of reds and browns, because they have distinct nodes, separated by internode 'stems'; whorls of branches reminiscent of the
horsetails occur at the nodes.
Conceptacles are another
polyphyletic trait; they appear in the
coralline algae and the
Hildenbrandiales, as well as the browns.
Most of the simpler algae are unicellular
flagellates or
amoeboids, but colonial and nonmotile 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
lifecycle 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: nonmotile cells embedded in mucilage
* Filamentous: a string of connected nonmotile cells, sometimes branching
* Parenchymatous: cells forming a thallus with partial differentiation of tissues
In three lines, even higher levels of organization have been reached, 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 charophyte algae (see
Charales and
Charophyta), in a lineage that eventually led to the higher land plants. The innovation that defines these nonalgal plants is the presence of female reproductive organs with protective cell layers that protect the zygote and developing embryo. Hence, the land plants are referred to as the
Embryophytes.
Turfs
The term algal turf is commonly used but poorly defined. Algal turfs are thick, carpet-like beds of seaweed that retain
sediment and compete with foundation species like
corals and
kelps, and they are usually less than 15 cm tall. Such a turf may consist of one or more species, and will generally cover an area in the order of a square metre or more. Some common characteristics are listed:
* Algae that form aggregations that have been described as turfs include diatoms, cyanobacteria, chlorophytes, phaeophytes and rhodophytes. Turfs are often composed of numerous species at a wide range of spatial scales, but monospecific turfs are frequently reported.
* Turfs can be morphologically highly variable over geographic scales and even within species on local scales and can be difficult to identify in terms of the constituent species.
* Turfs have been defined as short algae, but this has been used to describe height ranges from less than 0.5 cm to more than 10 cm. In some regions, the descriptions approached heights which might be described as canopies (20 to 30 cm).
Physiology
Many algae, particularly species of the
Characeae, have served as model experimental organisms to understand the mechanisms of the water permeability of membranes,
osmoregulation,
salt tolerance,
cytoplasmic streaming, and the generation of
action potentials.
Plant hormones are found not only in higher plants, but in algae, too.
Life cycle
Rhodophyta,
Chlorophyta, and
Heterokontophyta, the three main algal divisions, have life cycles which show considerable variation and complexity. In general, an asexual phase exists where the seaweed's cells are
diploid, a sexual phase where the cells are
haploid, followed by fusion of the male and female
gamete
A gamete ( ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also referred to as s ...
s. Asexual reproduction permits efficient population increases, but less variation is possible. Commonly, in sexual reproduction of unicellular and colonial algae, two specialized, sexually compatible, haploid gametes make physical contact and fuse to form a
zygote
A zygote (; , ) is a eukaryote, eukaryotic cell (biology), cell formed by a fertilization event between two gametes.
The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individ ...
. To ensure a successful mating, the development and release of gametes is highly synchronized and regulated; pheromones may play a key role in these processes. Sexual reproduction allows for more variation and provides the benefit of efficient recombinational repair of DNA damages during
meiosis, a key stage of the sexual cycle. However, sexual reproduction is more costly than asexual reproduction. Meiosis has been shown to occur in many different species of algae.
Diversity
The most recent estimate (as of January 2024) documents 50,605 living and 10,556 fossil algal species, according to the online database
AlgaeBase. They are classified into 15
phyla or
divisions. Some phyla are not photosynthetic, namely
Picozoa and
Rhodelphidia, but they are included in the database due to their close relationship with
red algae.
The various algal phyla can be differentiated according to several biological traits. They have distinct morphologies, photosynthetic pigmentation, storage products, cell wall composition,
and mechanisms of carbon concentration.
Some phyla have unique cellular structures.
Prokaryotic algae
Among prokaryotes, five major groups of bacteria have evolved the ability to photosynthesize, including
heliobacteria,
green sulfur and
nonsulfur bacteria and
proteobacteria.
However, the only lineage where
oxygenic photosynthesis has evolved is in the
cyanobacteria,
named for their blue-green (cyan) coloration and often known as blue-green algae.
They are
classified as the
phylum Cyanobacteriota or Cyanophyta. However, this phylum also includes two
classes of non-photosynthetic bacteria:
Melainabacteria (also called Vampirovibrionia
or Vampirovibrionophyceae)
and Sericytochromatia
(also known as Blackallbacteria).
A third class contains the photosynthetic ones, known as
Cyanophyceae (also called Cyanobacteriia
or Oxyphotobacteria).
As bacteria, their cells lack membrane-bound organelles, with the exception of
thylakoids. Like other algae, cyanobacteria have chlorophyll ''a'' as their primary photosynthetic pigment. Their accessory pigments include
phycobilins (phycoerythrobilin and phycocyanobilin),
carotenoids and, in some cases, ''b'', ''d'', or ''f'' chlorophylls, generally distributed in
phycobilisomes found in the surface of thylakoids. They display a variety of body forms, such as single cells, colonies, and unbranched or branched filaments. Their cells are commonly covered in a sheath of
mucilage, and they also have a typical
gram-negative bacterial cell wall composed largely of
peptidoglycan. They have various storage particles, including
cyanophycin as aminoacid and nitrogen reserves, "cyanophycean starch" (similar to plant
amylose) for carbohydrates, and
lipid droplets. Their
Rubisco enzymes are concentrated in
carboxysomes. They occupy a diverse array of aquatic and terrestrial habitats, including extreme environments from hot springs to polar glaciers. Some are subterranean, living via hydrogen-based
lithoautotrophy instead of photosynthesis.
Three lineages of cyanobacteria, ''
Prochloraceae'', ''
Prochlorothrix'' and ''
Prochlorococcus'', independently evolved to have chlorophylls ''a'' and ''b'' instead of phycobilisomes. Due to their different pigmentation, they were historically grouped in a separate division,
Prochlorophyta, as this is the typical pigmentation seen in green algae (e.g., chlorophytes). Eventually, this classification became obsolete, as it is a
polyphyletic grouping.
Cyanobacteria are included as algae by most phycological sources
and by the
International Code of Nomenclature for algae, fungi, and plants,
although a few authors exclude them from the definition of algae and reserve the term for eukaryotes only.
Eukaryotic algae
Eukaryotic algae contain
chloroplasts that are similar in structure to cyanobacteria. Chloroplasts contain circular
DNA like that in cyanobacteria and are interpreted as representing reduced endosymbiotic
cyanobacteria. However, the exact origin of the chloroplasts is different among separate lineages of algae, reflecting their acquisition during different endosymbiotic events. Many groups contain some members that are no longer photosynthetic. Some retain plastids, but not chloroplasts, while others have lost plastids entirely.
Primary algae
These algae, grouped in the
clade
In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
Archaeplastida (meaning 'ancient plastid'), have "primary
chloroplasts", i.e. the chloroplasts are surrounded by two membranes and probably developed through a single endosymbiotic event with a cyanobacterium. The chloroplasts of red algae have
chlorophylls ''a'' and ''c'' (often), and
phycobilins, while those of green algae have chloroplasts with chlorophyll ''a'' and ''b'' without phycobilins. Land plants are pigmented similarly to green algae and probably developed from them, thus the
Chlorophyta is a sister taxon to the plants; sometimes the Chlorophyta, the
Charophyta, and land plants are grouped together as the
Viridiplantae.
There is also a minor group of algae with primary plastids of different origin than the chloroplasts of the archaeplastid algae. The photosynthetic plastid of three species of the genus ''
Paulinella'' (
Rhizaria –
Cercozoa –
Euglyphida), often referred to as a 'cyanelle', was originated in the endosymbiosis of a α-cyanobacterium (probably an ancestral member of
Chroococcales).
Secondary algae
These algae appeared independently in various distantly related lineages after acquiring a chloroplast derived from another eukaryotic alga. Two lineages of secondary algae,
chlorarachniophytes and
euglenophytes have "green" chloroplasts containing chlorophylls ''a'' and ''b''. Their chloroplasts are surrounded by four and three membranes, respectively, and were probably retained from ingested green algae.
* Chlorarachniophytes, which belong to the
phylum Cercozoa, contain a small
nucleomorph, which is a
relict of the algae's
nucleus.
* Euglenophytes, which belong to the phylum
Euglenozoa, live primarily in fresh water and have chloroplasts with only three membranes. The endosymbiotic green algae may have been acquired through
myzocytosis rather than
phagocytosis.
* Another group with green algae endosymbionts is the dinoflagellate genus ''
Lepidodinium'', which has replaced its original endosymbiont of red algal origin with one of green algal origin. A nucleomorph is present, and the host genome still have several red algal genes acquired through endosymbiotic gene transfer. Also, the euglenid and chlorarachniophyte genome contain genes of apparent red algal ancestry.
Other groups have "red" chloroplasts containing chlorophylls ''a'' and ''c'', and phycobilins. The shape can vary; they may be of discoid, plate-like, reticulate, cup-shaped, spiral, or ribbon shaped. They have one or more pyrenoids to preserve protein and starch. The latter chlorophyll type is not known from any prokaryotes or primary chloroplasts, but genetic similarities with red algae suggest a relationship there. In some of these groups, the chloroplast has four membranes, retaining a
nucleomorph in
cryptomonads, and they likely share a common pigmented ancestor, although other evidence casts doubt on whether the
heterokonts,
Haptophyta, and
cryptomonads are in fact more closely related to each other than to other groups.
The typical dinoflagellate chloroplast has three membranes, but considerable diversity exists in chloroplasts within the group, and a number of endosymbiotic events apparently occurred.
The
Apicomplexa, a group of closely related parasites, also have plastids called
apicoplasts, which are not photosynthetic.
The
Chromerida are the closest relatives of apicomplexans, and some have retained their chloroplasts.
The three
alveolate groups evolved from a common
myzozoan ancestor that obtained chloroplasts.
History of classification
Linnaeus
Carl Linnaeus (23 May 1707 – 10 January 1778), also known after ennoblement in 1761 as Carl von Linné,#Blunt, Blunt (2004), p. 171. was a Swedish biologist and physician who formalised binomial nomenclature, the modern system of naming o ...
, in ''
Species Plantarum'' (1753), the starting point for modern
botanical nomenclature, recognized 14 genera of algae, of which only four are currently considered among algae. In ''
Systema Naturae'', Linnaeus described the genera ''
Volvox'' and ''
Corallina'', and a species of ''
Acetabularia'' (as ''
Madrepora''), among the animals.
In 1768,
Samuel Gottlieb Gmelin (1744–1774) published the ''Historia Fucorum'', the first work dedicated to marine algae and the first book on
marine biology to use the then new binomial nomenclature of Linnaeus. It included elaborate illustrations of seaweed and marine algae on folded leaves.
W. H. Harvey (1811–1866) and
Lamouroux (1813)
were the first to divide macroscopic algae into four divisions based on their pigmentation. This is the first use of a biochemical criterion in plant systematics. Harvey's four divisions are: red algae (Rhodospermae), brown algae (Melanospermae), green algae (Chlorospermae), and Diatomaceae.
At this time, microscopic algae were discovered and reported by a different group of workers (e.g.,
O. F. Müller and
Ehrenberg) studying the
Infusoria (microscopic organisms). Unlike
macroalgae, which were clearly viewed as plants,
microalgae were frequently considered animals because they are often motile.
Even the nonmotile (coccoid) microalgae were sometimes merely seen as stages of the lifecycle of plants, macroalgae, or animals.
Although used as a taxonomic category in some pre-Darwinian classifications, e.g., Linnaeus (1753),
de Jussieu (1789),
Lamouroux (1813), Harvey (1836), Horaninow (1843), Agassiz (1859), Wilson & Cassin (1864),
in further classifications, the "algae" are seen as an artificial, polyphyletic group.
Throughout the 20th century, most classifications treated the following groups as divisions or classes of algae:
cyanophytes,
rhodophytes,
chrysophytes,
xanthophytes,
bacillariophytes,
phaeophytes,
pyrrhophytes (
cryptophytes and
dinophytes),
euglenophytes, and
chlorophytes. Later, many new groups were discovered (e.g.,
Bolidophyceae), and others were splintered from older groups:
charophytes and
glaucophytes (from chlorophytes), many
heterokontophytes (e.g.,
synurophytes from chrysophytes, or
eustigmatophytes from xanthophytes),
haptophytes (from chrysophytes), and
chlorarachniophytes (from xanthophytes).
With the abandonment of plant-animal dichotomous classification, most groups of algae (sometimes all) were included in
Protista, later also abandoned in favour of
Eukaryota. However, as a legacy of the older plant life scheme, some groups that were also treated as
protozoans in the past still have duplicated classifications (see
ambiregnal protists).
Some parasitic algae (e.g., the green algae ''
Prototheca'' and ''
Helicosporidium'', parasites of metazoans, or ''
Cephaleuros'', parasites of plants) were originally classified as
fungi
A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...
,
sporozoans, or
protistans of ''
incertae sedis'', while others (e.g., the green algae ''
Phyllosiphon'' and ''
Rhodochytrium'', parasites of plants, or the red algae ''
Pterocladiophila'' and ''Gelidiocolax mammillatus'', parasites of other red algae, or the dinoflagellates ''
Oodinium'', parasites of fish) had their relationship with algae conjectured early. In other cases, some groups were originally characterized as parasitic algae (e.g., ''
Chlorochytrium''), but later were seen as
endophytic algae. Some filamentous bacteria (e.g., ''
Beggiatoa'') were originally seen as algae. Furthermore, groups like the
apicomplexans are also parasites derived from ancestors that possessed plastids, but are not included in any group traditionally seen as algae.
Evolution
Origin of oxygenic photosynthesis
Prokaryotic algae, i.e.,
cyanobacteria, are the only group of organisms where
oxygenic photosynthesis has evolved. The oldest undisputed fossil evidence of cyanobacteria is dated at 2100 million years ago,
although
stromatolites, associated with cyanobacterial
biofilms, appear as early as 3500 million years ago in the fossil record.
First endosymbiosis
Eukaryotic algae are
polyphyletic thus their origin cannot be traced back to single hypothetical
common ancestor. It is thought that they came into existence when photosynthetic
coccoid cyanobacteria got
phagocytized by a
unicellular heterotrophic eukaryote (a
protist),
giving rise to double-membranous primary
plastids. Such
symbiogenic events (primary symbiogenesis) are believed to have occurred more than 1.5 billion years ago during the
Calymmian period, early in
Boring Billion, but it is difficult to track the key events because of so much time gap.
Primary symbiogenesis gave rise to three divisions of
archaeplastids, namely the
Viridiplantae (
green algae and later
plant
Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...
s),
Rhodophyta (
red algae) and
Glaucophyta ("grey algae"), whose plastids further spread into other protist lineages through eukaryote-eukaryote
predation, engulfments and subsequent endosymbioses (secondary and tertiary symbiogenesis).
This process of serial cell "capture" and "enslavement" explains the diversity of photosynthetic eukaryotes.
The oldest undisputed fossil evidence of eukaryotic algae is ''
Bangiomorpha pubescens'', a red alga found in rocks around 1047 million years old.
[
]
Consecutive endosymbioses
Recent
genomic and
phylogenomic approaches have significantly clarified plastid
genome evolution, the
horizontal movement of
endosymbiont genes to the "host"
nuclear genome, and plastid spread throughout the eukaryotic
tree of life.
It is accepted that both
euglenophytes and
chlorarachniophytes obtained their chloroplasts from
chlorophytes that became endosymbionts.
In particular, euglenophyte chloroplasts share the most resemblance with the genus ''
Pyramimonas''.
However, there is still no clear order in which the secondary and tertiary endosymbioses occurred for the "
chromist" lineages (
ochrophytes,
cryptophytes,
haptophytes and
myzozoans). Two main models have been proposed to explain the order, both of which agree that cryptophytes obtained their chloroplasts from
red algae. One model, hypothesized in 2014 by John W. Stiller and coauthors,
suggests that a cryptophyte became the plastid of ochrophytes, which in turn became the plastid of myzozoans and haptophytes. The other model, suggested by Andrzej Bodył and coauthors in 2009,
describes that a cryptophyte became the plastid of both haptophytes and ochrophytes, and it is a haptophyte that became the plastid of myzozoans instead.
In 2024, a third model by Filip Pietluch and coauthors proposed that there were two independent endosymbioses with red algae: one that originated the cryptophyte plastids (as in the previous models), and subsequently the haptophyte plastids; and another that originated the ochrophyte plastids, where the myzozoans obtained theirs.
Relationship to land plants
Fossils of isolated
spores suggest
land plants may have been around as long as 475
million years ago (mya) during the
Late Cambrian/
Early Ordovician period, from
sessile shallow
freshwater charophyte algae much like ''
Chara'',
which likely got stranded ashore when
riverine/
lacustrine water levels dropped during
dry seasons.
These charophyte algae probably already developed filamentous
thalli and
holdfasts that superficially resembled
plant stems and
roots, and probably had an isomorphic
alternation of generations. They perhaps evolved some 850 mya
and might even be as early as 1
Gya during the late phase of the
Boring Billion.
Distribution
The distribution of algal species has been fairly well studied since the founding of
phytogeography in the mid-19th century.
Algae spread mainly by the dispersal of
spores analogously to the dispersal of
cryptogamic
plant
Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...
s by
spores. Spores can be found in a variety of environments: fresh and marine waters, air, soil, and in or on other organisms.
Whether a spore is to grow into an adult organism depends on the species and the environmental conditions where the spore lands.
The spores of freshwater algae are dispersed mainly by running water and wind, as well as by living carriers.
However, not all bodies of water can carry all species of algae, as the chemical composition of certain water bodies limits the algae that can survive within them.
Marine spores are often spread by ocean currents. Ocean water presents many vastly different habitats based on temperature and nutrient availability, resulting in phytogeographic zones, regions, and provinces.
To some degree, the distribution of algae is subject to floristic discontinuities caused by geographical features, such as
Antarctica
Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean (also known as the Antarctic Ocean), it contains the geographic South Pole. ...
, long distances of ocean or general land masses. It is, therefore, possible to identify species occurring by locality, such as "
Pacific algae" or "
North Sea algae". When they occur out of their localities, hypothesizing a transport mechanism is usually possible, such as the hulls of ships. For example, ''
Ulva reticulata'' and ''
U. fasciata'' travelled from the mainland to
Hawaii
Hawaii ( ; ) is an island U.S. state, state of the United States, in the Pacific Ocean about southwest of the U.S. mainland. One of the two Non-contiguous United States, non-contiguous U.S. states (along with Alaska), it is the only sta ...
in this manner.
Mapping is possible for select species only: "there are many valid examples of confined distribution patterns." For example, ''
Clathromorphum'' is an arctic genus and is not mapped far south of there. However, scientists regard the overall data as insufficient due to the "difficulties of undertaking such studies."
Regional algae checklists

The ''Algal Collection of the US National Herbarium'' (located in the
National Museum of Natural History
The National Museum of Natural History (NMNH) is a natural history museum administered by the Smithsonian Institution, located on the National Mall in Washington, D.C., United States. It has free admission and is open 364 days a year. With 4.4 ...
) consists of approximately 320,500 dried specimens, which, although not exhaustive (no exhaustive collection exists), gives an idea of the order of magnitude of the number of algal species (that number remains unknown). Estimates vary widely. For example, according to one standard textbook, in the
British Isles, the ''UK Biodiversity Steering Group Report'' estimated there to be 20,000 algal species in the UK. Another checklist reports only about 5,000 species. Regarding the difference of about 15,000 species, the text concludes: "It will require many detailed field surveys before it is possible to provide a reliable estimate of the total number of species ..."
Regional and group estimates have been made, as well:
* 5,000–5,500 species of red algae worldwide
* "some 1,300 in Australian Seas"
* 400 seaweed species for the western coastline of South Africa, and 212 species from the coast of KwaZulu-Natal. Some of these are duplicates, as the range extends across both coasts, and the total recorded is probably about 500 species. Most of these are listed in
List of seaweeds of South Africa. These exclude
phytoplankton and crustose corallines.
* 669 marine species from California (US)
* 642 in the check-list of Britain and Ireland
and so on, but lacking any scientific basis or reliable sources, these numbers have no more credibility than the British ones mentioned above. Most estimates also omit microscopic algae, such as phytoplankton.
Ecology

Algae are prominent in bodies of water, common in terrestrial environments, and are found in unusual environments, such as on
snow
Snow consists of individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes.
It consists of frozen crystalline water througho ...
and
ice. Seaweeds grow mostly in shallow marine waters, under deep; however, some such as ''
Navicula pennata'' have been recorded to a depth of . A type of algae, ''Ancylonema nordenskioeldii'', was found in
Greenland in areas known as the 'Dark Zone', which caused an increase in the rate of melting ice sheet. The same algae was found in the
Italian Alps, after pink ice appeared on parts of the Presena glacier.
The various sorts of algae play significant roles in aquatic ecology. Microscopic forms that live suspended in the water column (
phytoplankton) provide the food base for most marine
food chains. In very high densities (
algal blooms), these algae may discolor the water and outcompete, poison, or
asphyxiate other life forms.
Algae can be used as
indicator organisms to monitor pollution in various aquatic systems.
In many cases, algal metabolism is sensitive to various pollutants. Due to this, the species composition of algal populations may shift in the presence of chemical pollutants.
To detect these changes, algae can be sampled from the environment and maintained in laboratories with relative ease.
On the basis of their habitat, algae can be categorized as:
aquatic (
planktonic,
benthic,
marine,
freshwater,
lentic,
lotic),
terrestrial,
aerial (subaerial),
lithophytic,
halophytic (or
euryhaline),
psammon,
thermophilic,
cryophilic,
epibiont (
epiphytic,
epizoic),
endosymbiont (
endophytic, endozoic),
parasitic,
calcifilic or
lichenic (phycobiont).
Symbiotic algae
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 are:
Lichens
Lichens are defined by the
International Association for Lichenology to be "an association of a fungus and a photosynthetic
symbiont resulting in a stable vegetative body having a specific structure". The fungi, or mycobionts, are mainly from the
Ascomycota
Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The def ...
with a few from the
Basidiomycota. In nature, they do not occur separate from lichens. It is unknown when they began to associate. One or more mycobiont associates with the same phycobiont species, from the green algae, except that alternatively, the mycobiont may associate with a species of cyanobacteria (hence "photobiont" is the more accurate term). A photobiont may be associated with many different mycobionts or may live independently; accordingly, lichens are named and classified as fungal species. The association is termed a morphogenesis because the lichen has a form and capabilities not possessed by the symbiont species alone (they can be experimentally isolated). The photobiont possibly triggers otherwise latent genes in the mycobiont.
Trentepohlia is an example of a common green alga genus worldwide that can grow on its own or be lichenised. Lichen thus share some of the habitat and often similar appearance with specialized species of algae (''
aerophytes'') growing on exposed surfaces such as tree trunks and rocks and sometimes discoloring them.
Animal symbioses
Coral reefs are accumulated from the
calcareous exoskeletons of
marine invertebrates of the order
Scleractinia (stony
corals). These animals
metabolize sugar and oxygen to obtain energy for their cell-building processes, including
secretion of the exoskeleton, with water and
carbon dioxide as byproducts. Dinoflagellates (algal protists) are often
endosymbionts in the cells of the coral-forming marine invertebrates, where they accelerate host-cell metabolism by generating sugar and oxygen immediately available through photosynthesis using incident light and the carbon dioxide produced by the host. Reef-building stony corals (
hermatypic corals) require endosymbiotic algae from the genus ''
Symbiodinium'' to be in a healthy condition. The loss of ''Symbiodinium'' from the host is known as
coral bleaching, a condition which leads to the deterioration of a reef.
Endosymbiontic green algae live close to the surface of some sponges, for example, breadcrumb sponges (''
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.
In human culture
In
classical Chinese, the word is used both for "algae" and (in the modest tradition of the
imperial scholars) for "literary talent". The third island in
Kunming Lake beside the
Summer Palace in Beijing is known as the Zaojian Tang Dao (藻鑒堂島), which thus simultaneously means "Island of the Algae-Viewing Hall" and "Island of the Hall for Reflecting on Literary Talent".
Cultivation
Seaweed farming
Bioreactors
Uses
Biofuel
To be competitive and independent from fluctuating support from (local) policy on the long run, biofuels should equal or beat the cost level of fossil fuels. Here, algae-based fuels hold great promise, directly related to the potential to produce more biomass per unit area in a year than any other form of biomass. The break-even point for algae-based biofuels is estimated to occur by 2025.
Fertilizer

For centuries, seaweed has been used as a fertilizer;
George Owen of Henllys writing in the 16th century referring to drift weed in
South Wales:
Today, algae are used by humans in many ways; for example, as
fertilizers,
soil conditioners, and livestock feed. Aquatic and microscopic species are cultured in clear tanks or ponds and are either harvested or used to treat effluents pumped through the ponds.
Algaculture on a large scale is an important type of
aquaculture in some places.
Maerl is commonly used as a soil conditioner.
Food industry

Algae are used as foods in many countries: China consumes more than 70 species, including ''
fat choy'', a cyanobacterium considered a vegetable; Japan, over 20 species such as ''
nori'' and ''
aonori''; Ireland,
dulse;
Chile,
cochayuyo.
Laver is used to make
laverbread in
Wales
Wales ( ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by the Irish Sea to the north and west, England to the England–Wales border, east, the Bristol Channel to the south, and the Celtic ...
, where it is known as . In
Korea,
green laver is used to make .
Three forms of algae used as food:
* ''
Chlorella'': This form of alga is found in freshwater and contains
photosynthetic pigments in its
chloroplast.
*
Klamath AFA: A subspecies of Aphanizomenon flos-aquae found wild in many bodies of water worldwide but harvested only from
Upper Klamath Lake, Oregon.
* ''
Spirulina'': Known otherwise as a cyanobacterium (a
prokaryote
A prokaryote (; less commonly spelled procaryote) is a unicellular organism, single-celled organism whose cell (biology), cell lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Gree ...
or a "blue-green alga")
The oils from some algae have high levels of
unsaturated fatty acids. Some varieties of algae favored by
vegetarianism and
veganism
Veganism is the practice of abstaining from the use of animal products and the consumption of animal source foods, and an associated philosophy that rejects the commodity status of animals. A person who practices veganism is known as a vega ...
contain the long-chain, essential
omega-3 fatty acids,
docosahexaenoic acid (DHA) and
eicosapentaenoic acid (EPA).
Fish oil contains the omega-3 fatty acids, but the original source is algae (microalgae in particular), which are eaten by marine life such as
copepods and are passed up the food chain.
The natural
pigments (
carotenoids and
chlorophylls) produced by algae can be used as alternatives to chemical
dyes and coloring agents.
The presence of some individual algal pigments, together with specific pigment concentration ratios, are taxon-specific: analysis of their concentrations with various analytical methods, particularly
high-performance liquid chromatography, can therefore offer deep insight into the taxonomic composition and relative abundance of natural algae populations in sea water samples.
Carrageenan, from the red alga ''Chondrus crispus'', is used as a stabilizer in milk products.
Gelling agents
Agar, a
gelatinous substance derived from red algae, has a number of commercial uses. It is a good medium on which to grow bacteria and fungi, as most microorganisms cannot digest agar.
Alginic acid, or alginate, is extracted from
brown algae. Its uses range from gelling agents in food, to medical dressings. Alginic acid also has been used in the field of
biotechnology
Biotechnology is a multidisciplinary field that involves the integration of natural sciences and Engineering Science, engineering sciences in order to achieve the application of organisms and parts thereof for products and services. Specialists ...
as a
biocompatible medium for cell encapsulation and cell immobilization.
Molecular cuisine is also a user of the substance for its gelling properties, by which it becomes a delivery vehicle for flavours.
Between 100,000 and 170,000 wet tons of ''
Macrocystis'' are harvested annually in
New Mexico for
alginate extraction and
abalone feed.
Pollution control and bioremediation
* Sewage can be treated with algae, reducing the use of large amounts of toxic chemicals that would otherwise be needed.
* Algae can be used to capture fertilizers in runoff from farms. When subsequently harvested, the enriched algae can be used as fertilizer.
* Aquaria and ponds can be filtered using algae, which absorb nutrients from the water in a device called an
algae scrubber, also known as an algae turf scrubber.
Agricultural Research Service scientists found that 60–90% of nitrogen runoff and 70–100% of phosphorus runoff can be captured from
manure effluents using a horizontal algae scrubber, also called an
algal turf scrubber (ATS). Scientists developed the ATS, which consists of shallow, 100-foot raceways of nylon netting where algae colonies can form, and studied its efficacy for three years. They found that algae can readily be used to reduce the nutrient runoff from agricultural fields and increase the quality of water flowing into rivers, streams, and oceans. Researchers collected and dried the nutrient-rich algae from the ATS and studied its potential as an organic fertilizer. They found that cucumber and corn seedlings grew just as well using ATS organic fertilizer as they did with commercial fertilizers. Algae scrubbers, using bubbling upflow or vertical waterfall versions, are now also being used to filter aquaria and ponds.
The alga ''
Stichococcus bacillaris'' has been seen to colonize silicone resins used at archaeological sites;
biodegrading the synthetic substance.
Bioplastics
Various polymers can be created from algae, which can be especially useful in the creation of bioplastics. These include hybrid plastics, cellulose-based plastics, poly-lactic acid, and bio-polyethylene. Several companies have begun to produce algae polymers commercially, including for use in flip-flops and in surf boards. Even algae is also used to prepare various polymeric resins suitable for
coating applications.
[Chandrashekhar K Patil, Harishchandra D Jirimali, Jayasinh S Paradeshi, Bhushan L Chaudhari, Prakash K Alagi, Pramod P Mahulikar, Sung Chul Hong, Vikas V Gite, Chemical transformation of renewable algae oil to polyetheramide polyols for polyurethane coatings, Progress in Organic Coatings 151, 106084, https://doi.org/10.1016/j.porgcoat.2020.106084]
Additional images
File:Algae bladder 4290.jpg, Algae bladder
See also
*
AlgaeBase
*
AlgaePARC
*
Eutrophication
*
Iron fertilization
* ''
Marimo'' algae
*
Microbiofuels
*
Microphyte
*
Photobioreactor
*
Phycotechnology
*
Plant
Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...
s
*
Toxoid – anatoxin
Notes
References
Bibliography
General
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Britain and Ireland
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Australia
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New Zealand
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Europe
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Arctic
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Greenland
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Faroe Islands
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Canary Islands
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Morocco
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South Africa
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North America
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External links
* – a database of all algal names including images, nomenclature, taxonomy, distribution, bibliography, uses, extracts
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{{Authority control
Endosymbiotic events
Polyphyletic groups
Common names of organisms