Marine protists

Marine protists are defined by their habitat as protists that live in marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. Life originated as single-celled prokaryotes (bacteria and archaea) and later evolved into more complex eukaryotes. Eukaryotes are the more developed life forms known as plants, animals, fungi and protists. Protists are the eukaryotes that cannot be classified as plants, fungi or animals. They are usually single-celled and microscopic. The term protist came into use historically as a term of convenience for eukaryotes that cannot be strictly classified as plants, animals or fungi. They are not a part of modern cladistics, because they are paraphyletic (lacking a common ancestor).

Protists are usually too small to be seen with the naked eye. They come in many shapes that range from the very simple to the intricacies of this marine radiolarian (from Ernst Haeckel's Kunstformen der Natur, 1904)

Most protists are too small to be seen with the naked eye. They are highly diverse organisms currently organised into 18 phyla, but not easy to classify.[1][2] Studies have shown high protist diversity exists in oceans, deep sea-vents and river sediments, suggesting large numbers of eukaryotic microbial communities have yet to be discovered.[3][4] There has been little research on mixotrophic protists, but recent studies in marine environments found mixotrophic protests contribute a significant part of the protist biomass.[5] Since protists are eukaryotes (and not prokaryotes) they possess within their cell at least one nucleus, as well as organelles such as mitochondria and Golgi bodies. Protists are asexual but can reproduce rapidly through mitosis or by fragmentation.

In contrast to the cells of prokaryotes, the cells of eukaryotes are highly organised. Plants, animals and fungi are usually multi-celled and are typically macroscopic. Most protists are single-celled and microscopic. But there are exceptions. Some single-celled marine protists are macroscopic. Some marine slime molds have unique life cycles that involve switching between unicellular, colonial, and multicellular forms.[6] Other marine protist are neither single-celled nor microscopic, such as seaweed.

Protists have been described as a taxonomic grab bag of misfits where anything that doesn't fit into one of the main biological kingdoms can be placed.[7] Some modern authors prefer to exclude multicellular organisms from the traditional definition of a protist, restricting protists to unicellular organisms.[8][9] This more constrained definition excludes many brown, multicellular red and green algae, and slime molds.[10]

Background

Phylogenetic and symbiogenetic tree of living organisms, showing a schematic view of the central position occupied by the protista (protists)

Trophic modes

Protists can be broadly divided into four groups depending on whether their nutrition is plant-like, animal-like, fungal-like,[11] or a mixture of these.[12]

Protists according to how they get food
Type of protist		Description	Example		Some other examples
Plant-like	Algae (see below)	Autotrophic protists that make their own food without needing to consume other organisms, usually by using photosynthesis		Green algae, Pyramimonas	Red and brown algae, diatoms and some dinoflagellates. Plant-like protists are important components of phytoplankton discussed below.
Animal-like	Protozoans (see below)	Heterotrophic protists that get their food consuming other organisms (bacteria, archaea and small algae)		Radiolarian protist as drawn by Haeckel	Foraminiferans, and some marine amoebae, ciliates and flagellates.
Fungal-like	Slime moulds and slime nets	Saprotrophic protists that get their food from the remains of organisms that have broken down and decayed		Marine slime nets form labyrinthine networks of tubes in which amoeba without pseudopods can travel	Marine lichen
Mixotrophs	Various (see below)	Mixotrophic and osmotrophic protists that get their food from a combination of the above		Euglena mutabilis, a photosynthetic flagellate	Many marine mixotrops are found among protists, particularly among ciliates and dinoflagellates[5]

micrograph

cell schematic

Choanoflagellates, unicellular "collared" flagellate protists, are thought to be the closest living relatives of the animals.[13]

• Single-celled and microscopic protists
• 
  Diatoms are a major algae group generating about 20% of world oxygen production.[14]
• 
  Diatoms have glass like cell walls made of silica and called frustules.[15]
• 
  Fossil diatom frustule from 32-40 mya
• 
  Radiolarian
• 
  Single-celled alga, Gephyrocapsa oceanica
• 
  Two dinoflagellates
• 
  Zooxanthellae is a photosynthetic algae that lives inside hosts like coral
• 
  A single-celled ciliate with green zoochlorellae living inside endosymbiotically
• 
  Euglenoid
• 
  This ciliate is digesting cyanobacteria. The cytostome or mouth is at the bottom right.

External video

How microscopic hunters get their lunch

Euglenoids: Single-celled shapeshifters

How do protozoans get around?

Mixotrophs

Mixotrophic radiolarians

Acantharian radiolarian hosts Phaeocystis symbionts

White Phaeocystis algal foam washing up on a beach

Mixotrophs have no single trophic mode. A mixotroph is an organism that can use a mix of different sources of energy and carbon, instead of having a single trophic mode on the continuum from complete autotrophy at one end to heterotrophy at the other. It is estimated that mixotrophs comprise more than half of all microscopic plankton.[16] There are two types of eukaryotic mixotrophs: those with their own chloroplasts, and those with endosymbionts—and others that acquire them through kleptoplasty or by enslaving the entire phototrophic cell.[17]

The distinction between plants and animals often breaks down in very small organisms. Possible combinations are photo- and chemotrophy, litho- and organotrophy, auto- and heterotrophy or other combinations of these. Mixotrophs can be either eukaryotic or prokaryotic.[18] They can take advantage of different environmental conditions.[19]

Recent studies of marine microzooplankton found 30–45% of the ciliate abundance was mixotrophic, and up to 65% of the amoeboid, foram and radiolarian biomass was mixotrophic.[5]

Phaeocystis is an important algal genus found as part of the marine phytoplankton around the world. It has a polymorphic life cycle, ranging from free-living cells to large colonies.[20] It has the ability to form floating colonies, where hundreds of cells are embedded in a gel matrix, which can increase massively in size during blooms.[21] As a result, Phaeocystis is an important contributor to the marine carbon[22] and sulfur cycles.[23] Phaeocystis species are endosymbionts to acantharian radiolarians.[24][25]

Mixotrophic plankton that combine phototrophy and heterotrophy – table based on Stoecker et. al., 2017[26]
General types	Description		Example		Further examples
Bacterioplankton	Photoheterotrophic bacterioplankton			Vibrio cholerae	Roseobacter spp. Erythrobacter spp. Gammaproteobacterial clade OM60 Widespread among bacteria and archaea
Phytoplankton	Called constitutive mixotrophs by Mitra et. al., 2016.[27] Phytoplankton that eat: photosynthetic protists with inherited plastids and the capacity to ingest prey.			Ochromonas species	Ochromonas spp. Prymnesium parvum Dinoflagellate examples: Fragilidium subglobosum,Heterocapsa triquetra,Karlodinium veneficum,Neoceratium furca,Prorocentrum minimum
Zooplankton	Called nonconstitutive mixotrophs by Mitra et. al., 2016.[27] Zooplankton that are photosynthetic: microzooplankton or metazoan zooplankton that acquire phototrophy through chloroplast retention^a or maintenance of algal endosymbionts.
	Generalists	Protists that retain chloroplasts and rarely other organelles from many algal taxa			Most oligotrich ciliates that retain plastids^a
	Specialists	1. Protists that retain chloroplasts and sometimes other organelles from one algal species or very closely related algal species		Dinophysis acuminata	Dinophysis spp. Myrionecta rubra
		2. Protists or zooplankton with algal endosymbionts of only one algal species or very closely related algal species		Noctiluca scintillans	Metazooplankton with algal endosymbionts Most mixotrophic Rhizaria (Acantharea, Polycystinea, and Foraminifera) Green Noctiluca scintillans
^aChloroplast (or plastid) retention = sequestration = enslavement. Some plastid-retaining species also retain other organelles and prey cytoplasm.

• Mixoplankton
• 
  Tintinnid ciliate Favella
• 
  Euglena mutabilis, a photosynthetic flagellate
• 
  Zoochlorellae (green) living inside the ciliate Stichotricha secunda

Protist locomotion

Another way of categorising protists is according to their mode of locomotion. Many unicellular protists, particularly protozoans, are motile and can generate movement using flagella, cilia or pseudopods. Cells which use flagella for movement are usually referred to as flagellates, cells which use cilia are usually referred to as ciliates, and cells which use pseudopods are usually referred to as amoeba or amoeboids. Other protists are not motile, and consequently have no movement mechanism.

Protists according to how they move
Type of protist		Movement mechanism	Description	Example		Other examples
Motile	Flagellates		A flagellum (Latin for whip) is a lash-like appendage that protrudes from the cell body of some protists (as well as some bacteria). Flagellates use from one to several flagella for locomotion and sometimes as feeding and sensory organelle.		Cryptophytes	All dinoflagellates and nanoflagellates (choanoflagellates, silicoflagellates, most green algae)[28][29] (Other protists go through a phase as gametes when they have temporary flagellum – some radiolarians, foraminiferans and Apicomplexa)
	Ciliates		A cilium (Latin for eyelash) is a tiny flagellum. Ciliates use multiple cilia, which can number in many hundreds, to power themselves through the water.		Paramecium bursaria click to see cillia	Foraminiferans, and some marine amoebae, ciliates and flagellates.
	Amoebas (amoeboids)		Pseudopods (Greek for false feet) are lobe-like appendages which amoebas use to anchor to a solid surface and pull themselves forward. They can change their shape by extending and retracting these pseudopods.[30]		Amoeba	Found in every major protist lineage. Amoeboid cells occur among the protozoans, but also in the algae and the fungi.[31][32]
Not motile		none			Diatom	Diatoms, coccolithophores, and non‐motile species of Phaeocystis[29] Among protozoans the parasitic Apicomplexa are non‐motile.

Difference of beating pattern of flagellum and cilium

Flagella are used in prokaryotes (archaea and bacteria) as well as protists. In addition, both flagella and cilia are widely used in eukaryotic cells (plant and animal) apart from protists.

The regular beat patterns of eukaryotic cilia and flagella generates motion on a cellular level. Examples range from the propulsion of single cells such as the swimming of spermatozoa to the transport of fluid along a stationary layer of cells such as in a respiratory tract. Though eukaryotic flagella and motile cilia are ultrastructurally identical, the beating pattern of the two organelles can be different. In the case of flagella, the motion is often planar and wave-like, whereas the motile cilia often perform a more complicated three-dimensional motion with a power and recovery stroke.

Eukaryotic flagella—those of animal, plant, and protist cells—are complex cellular projections that lash back and forth. Eukaryotic flagella are classed along with eukaryotic motile cilia as undulipodia[33] to emphasize their distinctive wavy appendage role in cellular function or motility. Primary cilia are immotile, and are not undulipodia.

Cilia performs powerful forward strokes with a stiffened flagellum followed by relatively slow recovery movement with a relaxed flagellum

Ciliates generally have hundreds to thousands of cilia that are densely packed together in arrays. Like the flagella, the cilia are powered by specialised molecular motors. An efficient forward stroke is made with a stiffened flagellum, followed by an inefficient backward stroke made with a relaxed flagellum. During movement, an individual cilium deforms as it uses the high-friction power strokes and the low-friction recovery strokes. Since there are multiple cilia packed together on an individual organism, they display collective behaviour in a metachronal rhythm. This means the deformation of one cilium is in phase with the deformation of its neighbor, causing deformation waves that propagate along the surface of the organism. These propagating waves of cilia are what allow the organism to use the cilia in a coordinated manner to move. A typical example of a ciliated microorganism is the Paramecium, a one-celled, ciliated protozoan covered by thousands of cilia. The cilia beating together allow the Paramecium to propel through the water at speeds of 500 micrometers per second.[34]

• Flagellate, ciliates and amoeba
• 
  Green algal flagellate (Chlamydomonas)
• 
  Paramecium feeding on bacteria
• 
  The ciliate Oxytricha trifallax with cilia clearly visible
• 
  Amoeba with ingested diatoms

External video

Paramecium: The White Rat of Ciliates

Protist shells

Diatoms

Diatoms, major components of marine plankton, have silica skeletons called frustules. "The microscopic structures of diatoms help them manipulate light, leading to hopes they could be used in new technologies for light detection, computing or robotics.[35]

SEM images of pores in diatom frustules[36]

Many protists have protective shells.

Diatom shells are called frustules, and are made from silica (glass). These glass structures have accumulated for the last 100 million of years, and left rich deposits of nano and microstructured silicon oxide in the form of diatomaceous earth around the globe. The evolutionary causes for the generation of nano and microstructured silica by photosynthetic algae have not yet been established. However, in 2018 it was shown that reflection of ultraviolet light by nanostructured silica protects the DNA in the algal cells, and this may be an evolutionary cause for the formation of the glass cages.[36][37]

Fossil radiolarian

X-ray microtomography of Triplococcus acanthicus

This is a microfossil from the Middle Ordovician with four nested spheres. The innermost sphere is highlighted red. Each segment is shown at the same scale.[38]

Benefits of coccolithophore calcification

(A) Accelerated photosynthesis includes CCM (1) and enhanced light uptake via scattering of scarce photons for deep-dwelling species (2). (B) Protection from photodamage includes sunshade protection from ultraviolet (UV) light and photosynthetic active radiation (PAR) (1) and energy dissipation under high-light conditions (2). (C) Armor protection includes protection against viral/bacterial infections (1) and grazing by selective (2) and nonselective (3) grazers.[39]

Energetic costs of coccolithophore calcification

Energetic costs are reported in percentage of total photosynthetic budget. (A) Transport processes include the transport into the cell from the surrounding seawater of primary calcification substrates Ca2+ and HCO3− (black arrows) and the removal of the end product H+ from the cell (gray arrow). The transport of Ca2+ through the cytoplasm to the CV is the dominant cost associated with calcification (Table 1). (B) Metabolic processes include the synthesis of CAPs (gray rectangles) by the Golgi complex (white rectangles) that regulate the nucleation and geometry of CaCO3 crystals. The completed coccolith (gray plate) is a complex structure of intricately arranged CAPs and CaCO3 crystals. (C) Mechanical and structural processes account for the secretion of the completed coccoliths that are transported from their original position adjacent to the nucleus to the cell periphery, where they are transferred to the surface of the cell. The costs associated with these processes are likely to be comparable to organic-scale exocytosis in noncalcifying haptophyte algae.[39]

Relative sizes of coccolithophores

Size comparison between the relatively large coccolithophore Scyphosphaera apsteinii and the relatively small but ubiquitous coccolithophore Emiliania huxleyi[40]

• Xu, K., Hutchins, D. and Gao, K. (2018) "Coccolith arrangement follows Eulerian mathematics in the coccolithophore Emiliania huxleyi". PeerJ, 6: e4608. doi:10.1126/science.aaa7378.

Algae

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.[41] 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,[42] mostly multicellular and including many notable seaweeds.[42][43] Brown algae form a class containing about 2,000 recognised species,[44] 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.[45] 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.

• 
  Chlamydomonas globosa, a unicellular green alga with two flagella just visible at bottom left
• 
  Chlorella vulgaris, a common green microalgae, in endosymbiosis with a ciliate[46]
• 
  Centric diatom
• 
  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.[47] 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.[48]

Diatoms

Diatomes come in many shapes

Diatoms are photosynthetic unicellular algae populating the oceans and waters around the globe. They form a (disputed) phylum containing about 100,000 recognised species. Diatoms generate about 20 percent of all oxygen produced on the planet each year,[14] take in over 6.7 billion metric tons of silicon each year from the waters in which they live,[49] and contribute 45% of the primary production of organic material found in the oceans.[36]

• 
  Diatoms are one of the most common types of phytoplankton
• 
  Their protective shells (frustles) are made of silicon
• 
• 

Diatoms

Centric

Pennate

Diatoms have a silica shell (frustule) with radial (centric) or bilateral (pennate) symmetry

Diatom shapes

          Drawings by Haeckel 1904 (click for details)

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.[50] The frustules of dead diatoms drift to the ocean floor where, over millions of years, they can build up as much as half a mile deep.[51]

• 
  Silicified frustule of a pennate diatom with two overlapping halves
• 
  Guinardia delicatula, a diatom responsible for algal blooms in the North Sea and the English Channel[52]
• 
  Fossil diatom
• 
  There are over 100,000 species of diatoms which account for 50% of the ocean's primary production

Views of the centric diatom Coscinodiscus wailesii

Light micrographs of a living cell in face view (a) and side view (b); many small discoid chloroplasts are visible. FESEM micrographs of a cleaned single valve, inner view (c) and outer view of a complete theca (d).[37]

Scale bars: 50 μm.

External video

Diatoms: Tiny factories you can see from space

Diatom 3D interference contrast

Structure of a centric diatom frustule[53]

Coccolithophores

Coccolithophores are minute unicellular photosynthetic protists with two flagella for locomotion. Most of them are protected by a shell covered with ornate circular plates or scales called coccoliths. The coccoliths are made from calcium carbonate. 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.[54]

Coccolithophores

...have plates called coccoliths

...extinct fossil

Coccolithophores build calcite skeletons important to the marine carbon cycle[55]

• 
• 
  Scyphosphaera apsteinii, scale bar 5 μm
• 
  Coccolithophores named after the BBC documentary series
  The Blue Planet
• 
  The coccolithophore Emiliania huxleyi
• 
  Algae bloom of Emiliania huxleyi off the southern coast of England

Dinoflagellates

See also: Mixotrophic dinoflagellate and Predatory dinoflagellate

Dinoflagellate shapes

Unarmored dinoflagellates Kofoid (1921)

Haeckel Peridinea (1904)

Dinoflagellates are usually positioned as part of the algae group, and form a phylum of unicellular flagellates with about 2,000 marine species.[56] 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.[28]

By trophic orientation dinoflagellates are all over the place. Some dinoflagellates are known to be photosynthetic, but a large fraction of these are in fact mixotrophic, combining photosynthesis with ingestion of prey (phagotrophy).[57] 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 Myrionecta rubra, which sequester their chloroplasts from a specific cryptophyte clade (Geminigera/Plagioselmis/Teleaulax)".[26]

• 
  Gyrodinium, one of the few naked dinoflagellates which lack armour
• 
  The dinoflagellate Protoperidinium extrudes a large feeding veil to capture prey
• 
  Nassellarian radiolarians can be in symbiosis with dinoflagellates
• 
  The dinoflagellate Dinophysis acuta

A surf wave at night sparkles with blue light due to the presence of a bioluminescent dinoflagellate, such as Lingulodinium polyedrum

Suggested explanation for glowing seas[58]

Dinoflagellates

        Armoured

        Unarmoured

Traditionally dinoflagellates have been presented as armoured or unarmoured

Dinoflagellates often live in symbiosis with other organisms. Many nassellarian radiolarians house dinoflagellate symbionts within their tests.[59] 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.[60] There is evidence from DNA analysis that dinoflagellate symbiosis with radiolarians evolved independently from other dinoflagellate symbioses, such as with foraminifera.[61]

Some dinoflagellates are bioluminescent. At night, ocean water can light up internally and sparkle with blue light because of these dinoflagellates.[62][63] 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.[64]

• 
  Tripos muelleri is recognisable by its U-shaped horns
• 
  Oodinium, a genus of parasitic dinoflagellates, causes velvet disease in fish[65]
• 
  Karenia brevis produces red tides highly toxic to humans[66]
• 
  Red tide
• 
  Noctiluca scintillans, a bioluminescent dinoflagellate[67]

Protozoans

Protozoans are protists which feed on organic matter such as other microorganisms or organic tissues and debris.[68][69] Historically, the protozoa were regarded as "one-celled animals", because they often possess animal-like behaviours, such as motility and predation, and lack a cell wall, as found in plants and many algae.[70][71] Although the traditional practice of grouping protozoa with animals is no longer considered valid, the term continues to be used in a loose way to identify single-celled organisms that can move independently and feed by heterotrophy.

Marine protozoans include zooflagellates, foraminiferans, radiolarians and some dinoflagellates.

Radiolarians

Radiolarian shapes

          Drawings by Haeckel 1904 (click for details)

Radiolarians are unicellular predatory protists encased in elaborate globular shells usually made of silica and pierced with holes. Their name comes from the Latin for "radius". They catch prey by extending parts of their body through the holes. As with the silica frustules of diatoms, radiolarian shells can sink to the ocean floor when radiolarians die and become preserved as part of the ocean sediment. These remains, as microfossils, provide valuable information about past oceanic conditions.[72]

• 
  Like diatoms, radiolarians come in many shapes
• 
  Also like diatoms, radiolarian shells are usually made of silicate
• 
  However acantharian radiolarians have shells made from strontium sulfate crystals
• 
  Cutaway schematic diagram of a spherical radiolarian shell

Turing and radiolarian morphology

Shell of a spherical radiolarian

Shell micrographs

Computer simulations of Turing patterns on a sphere
closely replicate some radiolarian shell patterns[73]

External video

Radiolarian geometry

Ernst Haeckel's radiolarian engravings

• 
  Cladococcus abietinus
• 
  Cleveiplegma boreale

Foraminiferans

Foraminiferan shapes

          Drawings by Haeckel 1904 (click for details)

Like radiolarians, foraminiferans (forams for short) are single-celled predatory protists, also protected with shells that have holes in them. Their name comes from the Latin for "hole bearers". Their shells, often called tests, are chambered (forams add more chambers as they grow). The shells are usually made of calcite, but are sometimes made of agglutinated sediment particles or chiton, and (rarely) of silica. Most forams are benthic, but about 40 species are planktic.[74] They are widely researched with well established fossil records which allow scientists to infer a lot about past environments and climates.[72]

Foraminiferans

...can have more than one nucleus

...and defensive spines

Foraminiferans are important unicellular zooplankton protists, with calcium tests

External video

foraminiferans

Foraminiferal networks and growth

• 
  section showing chambers of a spiral foram
• 
  Live Ammonia tepida streaming granular ectoplasm for catching food
• 
  Group of planktonic forams
• 
  Fossil nummulitid forams of various sizes from the Eocene
• 
  The Egyptian pyramids were constructed from limestone that contained nummulites.[75]

A number of forams are mixotrophic (see below). These have unicellular algae as endosymbionts, from diverse lineages such as the green algae, red algae, golden algae, diatoms, and dinoflagellates.[74] Mixotrophic foraminifers are particularly common in nutrient-poor oceanic waters.[76] Some forams are kleptoplastic, retaining chloroplasts from ingested algae to conduct photosynthesis.[77]

Amoeba

Shelled and naked amoeba

Testate amoeba, Cyphoderia sp.

Naked amoeba, Chaos sp.

                  Amoeba can be shelled (testate) or naked

• 
  Naked amoeba showing food vacuoles and ingested diatom
• 
  Shell or test of a testate amoeba, Arcella sp.
• 
  Xenogenic testate amoeba covered in diatoms (from Penard's Amoeba Collection)

External video

Amoebas

Testate amoebas

Feeding amoebas

Ciliates

• 
  Tintinnopsis campanula
• 
  Oxytricha chlorelligera
• 
  Stylonychia putrina
• 
  Holophyra ovum
• 
  Blepharisma japonicum

Ciliate shapes

          Drawings by Haeckel 1904 (click for details)

External video

Peritrich Ciliates

Macroscopic protists

• Macroscopic protists (see also unicellular macroalgae → )
• 
  The single-celled giant amoeba has up to 1000 nuclei and reaches lengths of 5 mm
• 
  Gromia sphaerica is a large spherical testate amoeba which makes mud trails. Its diameter is up to 3.8 cm.[78]
• 
  Spiculosiphon oceana, a unicellular foraminiferan with an appearance and lifestyle that mimics a sponge, grows to 5 cm long.
• 
  The xenophyophore, another single-celled foraminiferan, lives in abyssal zones. It has a giant shell up to 20 cm across.[79]
• 
  Giant kelp, a brown algae, is not a true plant, yet it is multicellular and can grow to 50m

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