Algae is an informal term for a widespread and diverse group of photosynthetic
protists which are not necessarily closely related and are thus
polyphyletic. Marine algae can be divided into six groups:
green,
red and
brown algae,
euglenophytes,
dinoflagellates and
diatoms. Dinoflagellates and diatoms are important components of marine algae and have their own sections below.
Euglenophytes are a phylum of unicellular flagellates with only a few marine members. Not all algae are microscopic. Green, red and brown algae all have multicellular macroscopic forms that make up the familiar
seaweeds.
Green algae, an informal group, contains about 8,000 recognised species. Many species live most of their lives as single cells or are filamentous, while others form
colonies made up from long chains of cells, or are highly differentiated macroscopic seaweeds.
Red algae, a (disputed) phylum contains about 7,000 recognised species, mostly
multicellular and including many notable seaweeds.
Brown algae form a
class containing about 2,000 recognised species, mostly
multicellular and including many seaweeds such as
kelp. Unlike higher plants, algae lack roots, stems, or leaves. They can be classified by size as
microalgae or
macroalgae.
Microalgae are the microscopic types of algae, not visible to the naked eye. They are mostly
unicellular species which exist as individuals or in chains or groups, though some are
multicellular. Microalgae are important components of the marine protists
discussed above, as well as the phytoplankton
discussed below. They are very
diverse. It has been estimated there are 200,000–800,000 species of which about 50,000 species have been described. Depending on the species, their sizes range from a few micrometers (μm) to a few hundred micrometers. They are specially adapted to an environment dominated by viscous forces. File:Chlamydomonas globosa - 400x (13263097835).jpg|
Chlamydomonas globosa, a unicellular green alga with two
flagella just visible at bottom left File:Инфузории Ophridium versatile.jpg|
Chlorella vulgaris, a common green
microalgae, in
endosymbiosis with a
ciliate File:Centric diatom.jpg|Centric diatom File:Dinoflagellates.jpg|Dinoflagellates
Macroalgae are the larger,
multicellular and more visible types of algae, commonly called
seaweeds. Seaweeds usually grow in shallow coastal waters where they are anchored to the seafloor by a
holdfast. Like microalgae, macroalgae (seaweeds) can be regarded as
marine protists since they are not true plants. But they are not microorganisms, so they are not within the scope of this article. Unicellular organisms are usually microscopic, less than one tenth of a millimeter long. There are exceptions.
Mermaid's wineglass, a genus of subtropical
green algae, is single-celled but remarkably large and complex in form with a single large nucleus, making it a model organism for studying
cell biology. Another single-celled algae,
Caulerpa taxifolia, has the appearance of a vascular plant including "leaves" arranged neatly up stalks like a fern. Selective breeding in aquariums to produce hardier strains resulted in an accidental release into the Mediterranean where it has become an
invasive species known colloquially as
killer algae.
Diatoms Diatoms are photosynthetic unicellular algae populating the oceans and other waters around the globe. They form a (disputed) phylum containing about 100,000 recognised species. Diatoms generate about 20 per cent of all oxygen produced on the planet each year, They produce 25–45% of the total primary production of organic material in the oceans, owing to their prevalence in open-ocean regions when total phytoplankton biomass is maximal. Diatoms are enclosed in protective silica (glass) shells called
frustules. They are classified by the shape of these glass cages in which they live, and which they build as they grow. Each frustule is made from two interlocking parts covered with tiny holes through which the diatom exchanges nutrients and wastes. Dead diatoms drift to the ocean floor where, over millions of years, the remains of their frustules can build up as much as
half a mile deep. Diatoms have relatively high sinking speeds compared with other phytoplankton groups, and they account for about 40% of
particulate carbon exported to ocean depths.|alt=Guinardia delicatula, a diatom responsible for diatom blooms in the North Sea File:Pinnularia major.jpg| There are over 100,000 species of
diatoms accounting for 25–45% of the ocean's primary production File:CSIRO ScienceImage 7233 diatom.jpg| Linked diatoms File:Pennate diatom infected with two chytrid-like fungal pathogens.png| Pennate diatom from an Arctic
meltpond, infected with two
chytrid-like fungal pathogens. Scale bar = 10 μm.
Coccolithophores Coccolithophores are minute unicellular photosynthetic protists with two flagella for locomotion. Most of them are protected by calcium carbonate shells covered with ornate circular plates or scales called
coccoliths. The term coccolithophore derives from the Greek for a
seed carrying stone, referring to their small size and the coccolith stones they carry. Under the right conditions they bloom, like other phytoplankton, and can turn the ocean milky white. '' has an unusual shell with a regular dodecahedral structure about 10 micrometers across. File:Emiliania huxleyi.jpg|The
coccolithophore Emiliania huxleyi File:Cwall99 lg.jpg|
Algae bloom of
Emiliania huxleyi off the southern coast of England File:JRYSEM-247-05-azurapl.jpg|
Dinoflagellates Dinoflagellates are usually positioned as part of the
algae group, and form a phylum of unicellular flagellates with about 2,000 marine species. The name comes from the Greek "dinos" meaning
whirling and the Latin "flagellum" meaning a
whip or
lash. This refers to the two whip-like attachments (flagella) used for forward movement. Most dinoflagellates are protected with red-brown, cellulose armour. Like other phytoplankton, dinoflagellates are
r-strategists which under right conditions can
bloom and create
red tides.
Excavates may be the most basal flagellate lineage. Some species are
endosymbionts of marine animals and other protists, and play an important part in the biology of
coral reefs. Others predate other protozoa, and a few forms are parasitic. Many dinoflagellates are
mixotrophic and could also be classified as phytoplankton. The toxic dinoflagellate
Dinophysis acuta acquire chloroplasts from its prey. "It cannot catch the cryptophytes by itself, and instead relies on ingesting ciliates such as the red
Mesodinium rubrum, which sequester their chloroplasts from a specific cryptophyte clade (Geminigera/Plagioselmis/Teleaulax)". The nassellarian provides
ammonium and
carbon dioxide for the dinoflagellate, while the dinoflagellate provides the nassellarian with a mucous membrane useful for hunting and protection against harmful invaders. There is evidence from
DNA analysis that dinoflagellate symbiosis with radiolarians evolved independently from other dinoflagellate symbioses, such as with
foraminifera. Some dinoflagellates are
bioluminescent. At night, ocean water can light up internally and
sparkle with blue light because of these dinoflagellates. Bioluminescent dinoflagellates possess
scintillons, individual
cytoplasmic bodies which contain
dinoflagellate luciferase, the main enzyme involved in the luminescence. The luminescence, sometimes called
the phosphorescence of the sea, occurs as brief (0.1 sec) blue flashes or sparks when individual scintillons are stimulated, usually by mechanical disturbances from, for example, a boat or a swimmer or surf. File:Ceratium tripos.jpg|
Tripos muelleri is recognisable by its U-shaped horns. File:Archives de zoologie expérimentale et générale (1920) (20299351186).jpg|
Oodinium, a genus of
parasitic dinoflagellates, causes
velvet disease in fish. File:Karenia brevis.jpg|
Karenia brevis produces red tides highly toxic to humans. File:Algal bloom(akasio) by Noctiluca in Nagasaki.jpg|
Red tide File:Noctiluca scintillans unica.jpg|
Noctiluca scintillans, a bioluminescent dinoflagellate File:Ornithocercus heteroporus (probably).jpg|
Ornithocercus heteroporus - prominent lists on display ==Marine protozoans==