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Clockwise from top right: Amoeba proteus, Actinophrys sol, Acanthamoeba sp., Nuclearia thermophila., Euglypha acanthophora, neutrophil ingesting bacteria.

An amoeba or ameba /əˈmiːbə/ (pl.: amoebas or amebas (less commonly, amoebae or amebae /əˈmiːbi/)),[1] often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudopods.[2] Amoebae do not form a single taxonomic group; instead, they are found in every major lineage of eukaryotic organisms. Amoeboid cells occur not only among the protozoa, but also in fungi, algae, and animals.[3][4][5][6][7]

Microbiologists often use the terms "amoeboid" and "amoeba" interchangeably for any organism that exhibits amoeboid movement.[8][9]

In older classification systems, most amoebae were placed in the class or subphylum Sarcodina, a grouping of single-celled organisms that possess pseudopods or move by protoplasmic flow. However, molecular phylogenetic studies have shown that Sarcodina is not a monophyletic group whose members share common descent. Consequently, amoeboid organisms are no longer classified together in one group.[10]

The best known amoeboid protists are Chaos carolinense and Amoeba proteus, both of which have been widely cultivated and studied in classrooms and laboratories.[11][12] Other well known species include the so-called "brain-eating amoeba" Naegleria fowleri, the intestinal parasite Entamoeba histolytica, which causes amoebic dysentery, and the multicellular "social amoeba" or slime mould Dictyostelium discoideum.

Biology

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Pseudopods and movement

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The forms of pseudopodia, from left: polypodial and lobose; monopodial and lobose; filose; conical; reticulose; tapering actinopods; non-tapering actinopods

Amoebae do not have cell walls, which allows for free movement. Amoebae move and feed by using pseudopods, which are bulges of cytoplasm formed by the coordinated action of actin microfilaments pushing out the plasma membrane that surrounds the cell.[13] The appearance and internal structure of pseudopods are used to distinguish groups of amoebae from one another. Amoebozoan species, such as those in the genus Amoeba, typically have bulbous (lobose) pseudopods, rounded at the ends and roughly tubular in cross-section. Cercozoan amoeboids, such as Euglypha and Gromia, have slender, thread-like (filose) pseudopods. Foraminifera emit fine, branching pseudopods that merge with one another to form net-like (reticulose) structures. Some groups, such as the Radiolaria and Heliozoa, have stiff, needle-like, radiating axopodia (actinopoda) supported from within by bundles of microtubules.[3][14]

Naked amoeba in the genus Mayorella
Shell of the testate amoeba Cylindrifflugia acuminata
"Naked" amoeba of the genus Mayorella (left) and shell of the testate amoeba Cylindrifflugia acuminata (right)

Free-living amoebae may be "testate" (enclosed within a hard shell), or "naked" (also known as gymnamoebae, lacking any hard covering). The shells of testate amoebae may be composed of various substances, including calcium, silica, chitin, or agglutinations of found materials like small grains of sand and the frustules of diatoms.[15]

To regulate osmotic pressure, most freshwater amoebae have a contractile vacuole which expels excess water from the cell.[16] This organelle is necessary because freshwater has a lower concentration of solutes (such as salt) than the amoeba's own internal fluids (cytosol). Because the surrounding water is hypotonic with respect to the contents of the cell, water is transferred across the amoeba's cell membrane by osmosis. Without a contractile vacuole, the cell would fill with excess water and, eventually, burst. Marine amoebae do not usually possess a contractile vacuole because the concentration of solutes within the cell are in balance with the tonicity of the surrounding water.[17]

Nutrition

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Amoeba phagocytosis of a bacterium

The food sources of amoebae vary. Some amoebae are predatory and live by consuming bacteria and other protists. Some are detritivores and eat dead organic material.

Amoebae typically ingest their food by phagocytosis, extending pseudopods to encircle and engulf live prey or particles of scavenged material. Amoeboid cells do not have a mouth or cytostome, and there is no fixed place on the cell at which phagocytosis normally occurs.[18]

Some amoebae also feed by pinocytosis, imbibing dissolved nutrients through vesicles formed within the cell membrane.[19]

Size range

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Foraminifera have reticulose (net-like) pseudopods, and many species are visible with the naked eye.

The size of amoeboid cells and species is extremely variable. The marine amoeboid Massisteria voersi is just 2.3 to 3 micrometres in diameter,[20] within the size range of many bacteria.[21] At the other extreme, the shells of deep-sea xenophyophores can attain 20 cm in diameter.[22] Most of the free-living freshwater amoebae commonly found in pond water, ditches, and lakes are microscopic, but some species, such as the so-called "giant amoebae" Pelomyxa palustris and Chaos carolinense, can be large enough to see with the naked eye.

Species or cell type Size in micrometers
Massisteria voersi[20] 2.3–3
Naegleria fowleri[23] 8–15
Neutrophil (white blood cell)[24] 12–15
Acanthamoeba[25] 12–40
Entamoeba histolytica[26] 15–60
Arcella vulgaris[27] 30–152
Amoeba proteus[28] 220–760
Chaos carolinense[29] 700–2000
Pelomyxa palustris[30] up to 5000
Syringammina fragilissima[22] up to 200000

Sexual reproduction

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Recent evidence indicates that several Amoebozoa lineages undergo meiosis.

Orthologs of genes employed in meiosis of sexual eukaryotes have recently been identified in the Acanthamoeba genome. These genes included Spo11, Mre11, Rad50, Rad51, Rad52, Mnd1, Dmc1, Msh and Mlh.[31] This finding suggests that the ''Acanthamoeba'' are capable of some form of meiosis and may be able to undergo sexual reproduction.

The meiosis-specific recombinase, Dmc1, is required for efficient meiotic homologous recombination, and Dmc1 is expressed in Entamoeba histolytica.[32] The purified Dmc1 from E. histolytica forms presynaptic filaments and catalyses ATP-dependent homologous DNA pairing and DNA strand exchange over at least several thousand base pairs.[32] The DNA pairing and strand exchange reactions are enhanced by the eukaryotic meiosis-specific recombination accessory factor (heterodimer) Hop2-Mnd1.[32] These processes are central to meiotic recombination, suggesting that E. histolytica undergoes meiosis.[32]

Studies of Entamoeba invadens found that, during the conversion from the tetraploid uninucleate trophozoite to the tetranucleate cyst, homologous recombination is enhanced.[33] Expression of genes with functions related to the major steps of meiotic recombination also increase during encystations.[33] These findings in E. invadens, combined with evidence from studies of E. histolytica indicate the presence of meiosis in the Entamoeba.

Dictyostelium discoideum in the supergroup Amoebozoa can undergo mating and sexual reproduction including meiosis when food is scarce.[34][35]

Since the Amoebozoa diverged early from the eukaryotic family tree, these results suggest that meiosis was present early in eukaryotic evolution. Furthermore, these findings are consistent with the proposal of Lahr et al.[36] that the majority of amoeboid lineages are anciently sexual.

Ecology

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Pathogenic amoebae

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Trophozoites of the pathogenic Entamoeba histolytica with ingested red blood cells

Some amoebae can infect other organisms pathogenically, causing disease:[37][38][39][40]

Amoebae have been found to harvest and grow the bacteria implicated in plague.[41] Amoebae can likewise play host to microscopic organisms that are pathogenic to people and help in spreading such microbes. Bacterial pathogens (for example, Legionella) can oppose absorption of food when devoured by amoebae.[42] The currently generally utilized and best-explored amoebae that host other organisms are Acanthamoeba castellanii and Dictyostelium discoideum.[43] Microorganisms that can overcome the defenses of one-celled organisms can shelter and multiply inside them, where they are shielded from unfriendly outside conditions by their hosts.

History of knowledge and classification

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Conceptual origins

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The first illustration of an amoeboid, from Rösel von Rosenhof's Insecten-Belustigung (1755)

The earliest record of an amoeboid organism was produced in 1755 by August Johann Rösel von Rosenhof, who named his discovery "Der Kleine Proteus" ("the Little Proteus").[44] Rösel's illustrations show an unidentifiable freshwater amoeba, similar in appearance to the common species now known as Amoeba proteus.[45] The term "Proteus animalcule" remained in use throughout the 18th and 19th centuries, as an informal name for any large, free-living amoeboid.[46]

In 1822, the genus Amiba (from the Greek ἀμοιβή amoibe, meaning "change") was erected by the French naturalist Bory de Saint-Vincent.[47][48] Bory's contemporary, C. G. Ehrenberg, adopted the genus in his own classification of microscopic creatures, but changed the spelling to Amoeba.[49]

In 1841, Félix Dujardin coined the term "sarcode" (from Greek σάρξ sarx, "flesh", and εἶδος eidos, "form") for the "thick, glutinous, homogeneous substance" which fills protozoan cell bodies.[50]: 26  Although the term originally referred to the protoplasm of any protozoan, it soon came to be used in a restricted sense to designate the gelatinous contents of amoeboid cells.[10] Thirty years later, the Austrian zoologist Ludwig Karl Schmarda used "sarcode" as the conceptual basis for his division Sarcodea, a phylum-level group made up of "unstable, changeable" organisms with bodies largely composed of "sarcode".[51]: 156  Later workers, including the influential taxonomist Otto Bütschli, amended this group to create the class Sarcodina,[52]: 1  a taxon that remained in wide use throughout most of the 20th century.[53]

Traditional classification

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Examples of different kinds of amoebae
Amoeba proteus, a gymnamoeba
Actinophrys sol, a heliozoan
Naegleria lustrarea, a heterolobosean
Vampyrella lateritia, a proteomyxid
Euglypha ciliata, a filose testate amoeba
Hyalosphenia papilio, a lobose testate amoeba
Reticulomyxa filosa, a reticulose amoeba
Dictyostelium discoideum, an eumycetozoan

For convenience, all amoebae were grouped as Sarcodina and generally divided into morphological categories, on the basis of the form and structure of their pseudopods. Amoebae with pseudopods supported by regular arrays of microtubules (such as the freshwater Heliozoa and marine Radiolaria) were classified as Actinopoda, whereas those with unsupported pseudopods were classified as Rhizopoda.[54] The Rhizopods were further subdivided into lobose, filose, plasmodial and reticulose, according to the morphology of their pseudopods. During the 1980s, taxonomists reached the following classification, based exclusively on morphological comparisons:[55][53]

  • Sarcodina Schmarda 1871: all amoebae.[55]: 40 
  • Rhizopoda von Siebold 1845: amorphous amoebae that lack axopodia and move through pseudopodia.[55]: 41 [53]: 202 
  • Heterolobosea Page & Blanton 1985: amoebae with eruptive pseudopodia, similar to the lobose ones but with a distinct movement, and usually with flagellate life stages. It was traditionally divided into those which aggregate to form fruiting bodies (Acrasida) and those that do not (Schizopyrenida).[53]: 203–204 
  • Caryoblastea Margulis 1974: amoebae with sparse, non-motile flagella on the surface. This group only includes the order Pelobiontida,[53]: 207  which now belongs to the amoebozoan group Archamoebae together with some naked amoebae.[56]
  • Eumycetozoea Zopf 1885: plasmodial amoebae with filiform subpseudopodia that produce fruiting bodies.
  • Plasmodiophorea Cook 1928: endoparasitic plasmodial amoebae with minute pseudopodia. This group is now an order within Rhizaria, closely related to the endoparasites Phagomyxida.
  • Filosea Leidy 1879: amoebae with filose pseudopodia.
  • Aconchulinia de Saedeleer 1934: filose naked amoebae, sometimes covered in scales. This group included two unrelated taxa: the nucleariid amoebae, closely related to fungi; and most of the Vampyrellida, found in Rhizaria.
  • Testaceafilosia de Saedeleer 1934: filose testate amoebae. This group included taxa now found throughout Rhizaria, such as Gromiida and Euglyphida.
  • Granuloreticulosea de Saedeleer 1934: amoebae with delicate granular pseudopodia. This group included both the Foraminifera (now in Rhizaria) and some members of Vampyrellida.
  • Xenophyophorea Schulze 1904: plasmodial amoebae enclosed in a branched-tube system composed of a transparent organic substance. This group is now fully integrated into the Foraminifera.

Transitional period

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Eukaryotes

Archezoa

Percolozoa (Heterolobosea)  

other excavates

Eosarcodina

Neosarcodina

Sarcodina
The 'amoeboflagellate' hypothesis by Thomas Cavalier-Smith, where higher eukaryotes evolved from amoeboid phyla.[57]: 244 

In the final decades of the 20th century, a series of molecular phylogenetic analyses confirmed that Sarcodina was not a monophyletic group, and that amoebae evolved from flagellate ancestors.[10] The protozoologist Thomas Cavalier-Smith proposed that the ancestor of most eukaryotes was an amoeboflagellate much like modern heteroloboseans, which in turn gave rise to a paraphyletic Sarcodina from which other groups (e.g., alveolates, animals, plants) evolved by a secondary loss of the amoeboid phase. In his scheme, the Sarcodina were divided into the more primitive Eosarcodina (with the phyla Reticulosa and Mycetozoa) and the more derived Neosarcodina (with the phyla Amoebozoa for lobose amoebae and Rhizopoda for filose amoebae).[57]

Shortly after, phylogenetic analyses disproved this hypothesis, as non-amoeboid zooflagellates and amoeboflagellates were found to be completely intermingled with amoebae. With the addition of many flagellates to Rhizopoda and the removal of some amoebae, the name was rejected in favour of a new name Cercozoa. As such, both names Rhizopoda and Sarcodina were finally abandoned as formal taxa, but they remained useful as descriptive terms for amoebae.[58]: 238  The phylum Amoebozoa was conserved, as it still primarily included amoeboid organisms, and now included the Mycetozoa.[58]: 232 

Current classification

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Today, amoebae are dispersed among many high-level taxonomic groups. The majority of traditional sarcodines are placed in two eukaryote supergroups: Amoebozoa and Rhizaria. The rest have been distributed among the excavates, opisthokonts, stramenopiles and minor clades.[10][59]

The following cladogram shows the sparse positions of amoeboid groups (in bold), based on molecular phylogenetic analyses:[66]

Eukaryotes

Amoeboid cells in other organisms

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Neutrophil (white blood cell) engulfing anthrax bacteria

Amoeboid cell types in multicellular organisms

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Some multicellular organisms have amoeboid cells only in certain phases of life, or use amoeboid movements for specialized functions. In the immune system of humans and other animals, amoeboid white blood cells pursue invading organisms, such as bacteria and pathogenic protists, and engulf them by phagocytosis.[68] Sponges exhibit a totipotent cell type known as archaeocytes, capable of transforming into the feeding cells or choanocytes.[69]

Amoeboid dispersal stages

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Amoeboid stages also occur in the multicellular fungus-like protists, the so-called slime moulds. Both the plasmodial slime moulds, currently classified in the class Myxogastria, and the cellular slime moulds of the groups Acrasida and Dictyosteliida, live as amoebae during their feeding stage. The amoeboid cells of the former combine to form a giant multinucleate organism,[70] while the cells of the latter live separately until food runs out, at which time the amoebae aggregate to form a multicellular migrating "slug" which functions as a single organism.[8]

Other organisms may also present amoeboid cells during certain life-cycle stages, e.g., the gametes of some green algae (Zygnematophyceae)[71] and pennate diatoms,[72] the spores (or dispersal phases) of some Mesomycetozoea,[73][74] and the sporoplasm stage of Myxozoa and of Ascetosporea.[75]

References

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Amoeba

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An amoeba is a type of single-celled eukaryotic belonging to the supergroup , distinguished by its amorphous, changeable body shape and locomotion through temporary cytoplasmic extensions called . The term "amoeba" is often used informally for any amoeboid organism, though the genus Amoeba is specific within . These organisms are heterotrophic, obtaining nutrients by engulfing food particles via , in which the surrounds prey to form a food vacuole for digestion. Primarily asexual reproducers, amoebas divide by binary fission, splitting into two genetically identical daughter cells, though some species in the broader group exhibit sexual phases or complex multicellular structures. The genus Amoeba, exemplified by , typically inhabits freshwater environments such as clean, oxygenated ponds and streams, where they prey on , , and other microorganisms. Amoebas lack a fixed body form, with their plasma membrane enclosing a granular that includes a central nucleus, contractile vacuoles for , and mitochondria for energy production—though some related forms rely on bacterial endosymbionts instead. , which can be lobe-shaped or tubular, not only facilitate by anchoring to substrates and streaming forward but also aid in capturing food. , a well-studied species reaching up to 500 micrometers in diameter, exemplifies these traits and serves as a in research due to its large size and visibility under microscopes. While most amoebas are free-living and benign, certain pathogenic relatives in , such as , can cause diseases in humans, highlighting the group's medical significance. The diversity within extends beyond the naked amoebas of the Amoeba to include testate forms with protective shells, multinucleate giants like Pelomyxa (up to 5 mm long), and social slime molds that aggregate into multicellular slugs for reproduction. These organisms play key ecological roles as decomposers and predators in aquatic and soil ecosystems, contributing to nutrient cycling. Evolutionary studies place as an ancient eukaryotic lineage, with estimates suggesting origins over a billion years ago and fossils dating back approximately 800 million years, underscoring their fundamental position in understanding evolution and eukaryotic cell function.

General Characteristics

Definition and Overview

Amoebae are single-celled eukaryotic protists defined by their amoeboid locomotion, which involves the extension of temporary cytoplasmic projections called pseudopods for movement and feeding, and they belong to the diverse supergroup within the supergroup. This supergroup encompasses a broad array of organisms, including both amoeboid and forms, that lack a fixed shape and inhabit nearly all non-extreme environments, from aquatic to terrestrial habitats. Amoebae exist as free-living species that thrive in freshwater, , and other moist ecosystems, as well as parasitic forms that infect hosts including humans and animals. They play crucial ecological roles, such as acting as primary predators of in and freshwater systems, thereby regulating microbial populations and facilitating nutrient cycling through predation-induced mineralization of organic matter. Medically, certain parasitic amoebae, like , are significant pathogens causing amebiasis, an intestinal infection that can lead to severe and extraintestinal complications such as liver abscesses, affecting millions globally, particularly in developing regions. Due to their simplicity and accessibility, amoebae have served as key model organisms in since the , with enabling foundational studies on cellular , , and nucleus-cytoplasm interactions over more than a century of research. It is important to distinguish the Amoeba, which refers to specific naked lobose amoebae within the clade of (such as ), from the broader term "amoeboid," which describes any eukaryotic cell exhibiting pseudopod-based crawling movement, a trait shared across diverse groups and not limited to a single taxonomic category. This functional descriptor highlights the of locomotion rather than phylogenetic relatedness, as modern classifications rely on molecular data like 18S rRNA sequences to delineate true amoebozoans from superficially similar forms in other supergroups.

Morphology and Size

Amoebae are characterized by their lack of a fixed body shape, instead exhibiting a constantly changing, amorphous form enclosed by a thin plasma membrane that allows for flexibility and environmental interaction. The cell interior consists of granular divided into two distinct regions: the outer ectoplasm, a clear, gel-like layer that forms the advancing edge during shape changes, and the inner , a more fluid, sol-like region containing various organelles and inclusions. Key cellular components include typically a single, centrally located vesicular nucleus that controls genetic functions, though some species are multinucleate, one or more contractile vacuoles responsible for by expelling excess water in freshwater environments, and food vacuoles that enclose ingested particles for . These structures enable the amoeba's adaptability but are static features independent of active processes. Size in amoebae varies widely across species, reflecting their ecological roles and environmental pressures. Free-living species like typically measure 0.2–0.5 mm in length, with some individuals reaching up to 0.74 mm, making them visible to the under optimal conditions. Larger forms, such as those in the related genus Chaos, can extend to 2–5 mm, while most amoebae fall within a 0.1–0.5 mm range. In contrast, pathogenic species like Entamoeba histolytica are much smaller, with trophozoites ranging from 10–60 μm, adapted to parasitic lifestyles within hosts. Amoebae display significant morphological variability, shifting from irregular, elongated feeding stages to compact spherical cysts depending on conditions such as and , which influence cell volume and stability. This plasticity, driven by environmental factors, allows survival in diverse settings without altering core structural elements like the layers or vacuoles.

Biological Processes

Locomotion and Pseudopods

Amoebae achieve locomotion through the dynamic extension and retraction of pseudopods, which are actin-based protrusions that allow the cell to crawl across surfaces or through viscous media. This form of movement, known as amoeboid locomotion, relies on the coordinated reorganization of the and , enabling navigation in diverse environments without rigid structures like flagella or cilia. Pseudopods vary in form among amoeboid protists, with the type in the Amoeba, such as , being the —a broad, bulbous extension that advances the cell body forward. consist of a thick, rounded tip filled with flowing , providing stability and propulsion during movement. In contrast, other amoeboids employ , which are thin, thread-like projections used for probing and sensing, or reticulopodia, intricate net-like anastomosing strands that facilitate exploration over larger areas. The core mechanism of pseudopod-driven locomotion involves cytoplasmic streaming, a process where the inner, fluid-like endoplasm (sol state) surges forward into the pseudopod tip, expanding it, while the outer, gel-like ectoplasm (gel state) at the cell's rear contracts to pull the body along. This sol-gel transformation is reversible and regulated by actin polymerization at the leading edge, where globular actin (G-actin) assembles into filamentous actin (F-actin) networks, combined with myosin motor proteins that generate contractile forces. Calcium ions play a key role in initiating these changes by activating actin-binding proteins that promote gelation or solation as needed. Locomotion efficiency is reflected in typical speeds of 1–5 μm/s for under standard conditions, though rates can reach up to 10 μm/s on favorable substrates; these velocities depend on factors like surface and cytoplasmic . Environmental influences, such as chemotactic gradients from or nutrients, guide pseudopod formation toward positive stimuli, enhancing directed movement, while ions like calcium modulate extension rates by altering cytoskeletal dynamics.

Nutrition and Feeding

Amoebae are primarily heterotrophic organisms that obtain nutrients through , engulfing , , other protists, or organic as food sources. This process allows them to capture and internalize solid particles larger than 0.5 μm, serving as their main mode of in diverse environments. In nutrient-rich settings, some amoebae supplement with osmotrophy, directly absorbing dissolved organic molecules across their plasma membrane to meet energy needs. The feeding mechanism begins with the extension of pseudopods around prey, forming a food cup that encloses the particle and pinches off to create a food vacuole within the . Once formed, the food vacuole fuses with lysosomes, releasing hydrolytic enzymes such as proteases for protein breakdown and for , along with lysozymes that degrade bacterial cell walls. These enzymes facilitate , converting complex organics into soluble nutrients like , sugars, and fatty acids that diffuse into the for absorption. Digestion efficiency is enhanced by dynamic pH changes in the vacuole, shifting from near-neutral at formation to acidic (around 5.5) during peak enzymatic activity, which optimizes and killing before gradually neutralizing as digestion completes in 12–24 hours. Undigested residues, such as indigestible cell walls or waste, are expelled through when the vacuole approaches the cell surface and fuses with the plasma membrane. The absorbed nutrients fuel energy production via in the and in mitochondria, generating ATP for cellular functions. Amoebae typically ingest a volume of equivalent to 10–100% of their body size daily, supporting growth and maintenance in active individuals.

Reproduction and Life Cycle

Amoebae of the genus Amoeba, such as A. proteus, primarily reproduce asexually through binary fission, a process in which the nucleus first undergoes to produce two identical copies, followed by that divides the and into two daughter cells. This method ensures rapid clonal propagation under favorable conditions, with the entire fission process typically lasting 30 minutes to one hour, though the full generation time between divisions ranges from 24 to 72 hours. The life cycle consists of the stage, the motile and actively feeding form responsible for nutrient uptake and locomotion, and the stage, a resistant dormant form that facilitates survival and dispersal during environmental stress. Encystment into the stage is induced by factors like nutrient scarcity or , during which the amoeba retracts its pseudopods, secretes a protective wall, and reduces metabolic activity to withstand adverse conditions. In A. proteus, cysts form notably under prolonged after initial feeding, allowing the to endure until conditions improve for excystation and resumption of the phase. Sexual reproduction has not been observed in the genus Amoeba, including well-studied species like A. proteus. Reproduction and encystment are influenced by environmental factors, including , with optimal rates at 20–25°C where generation times are minimized and fission efficiency peaks. also plays a role, as higher densities increase competition for resources like , potentially slowing fission rates and promoting encystment to reduce metabolic demands.

Classification and Evolutionary History

Early Discovery and Conceptual Development

The earliest documented observations of amoeba-like organisms occurred in 1674, when Dutch microscopist Antony van Leeuwenhoek examined samples of pond water through his handmade single-lens microscope and reported seeing small, wriggling "animalcules" that moved in a fluid, changeable manner. These descriptions, detailed in letters to the Royal Society published starting in 1677, captured the dynamic motion of what are now recognized as free-living , including forms resembling amoebae, marking the initial glimpse into the microscopic world of single-celled life. By the 18th century, further microscopic studies built on these findings, with French microscopist Louis Joblot describing similar organisms in 1718 as part of the ""—a broad category for tiny creatures found in infusions like decaying plant matter or water, often viewed as the most primitive animals due to their amorphous, shape-shifting bodies. Joblot's work in Descriptions et usages de plusieurs nouveaux microscopes emphasized the infusoria's lack of fixed structure, portraying them as basal forms of animal life that challenged traditional notions of organization in living beings. This perspective highlighted the conceptual shift toward recognizing variability and simplicity as key traits of these entities. Advancements accelerated in the , beginning with French Félix Dujardin's 1835 description and naming of the Amoeba in his studies of rhizopods, coining the term from the Greek amoibē (change) to reflect the organism's protean form and pseudopodial movement. In 1838, German naturalist Christian Gottfried Ehrenberg advanced this by classifying amoebae within his comprehensive work Die Infusionsthierchen als vollkommene Organismen, treating them as fully formed animalcules () with complex internal structures, thereby affirming their status as independent animals rather than mere cellular fragments. The same year, Matthias Jakob Schleiden's observations on plant cells, followed by Theodor Schwann's 1839 extension to animal tissues, introduced cell theory, which reframed amoebae as quintessential single-celled organisms embodying the fundamental unit of life./02%3A_The_Cell/2.02%3A_Cell_Theory) These developments fueled ongoing debates about amoebae’s affinities, with some scholars aligning them with plants due to their engulfing nutrition and others with animals based on motility, creating uncertainty in early biological classifications. This ambiguity was partially resolved in 1866 when German biologist Ernst Haeckel proposed the kingdom Protista in Generelle Morphologie der Organismen, designating amoebae and similar forms as primitive, unicellular intermediates between plants and animals, thus establishing a dedicated conceptual space for such shape-shifting microbes./08%3A_Protists_and_Fungi/8.01%3A_Protist_Kingdom)

Traditional Taxonomic Frameworks

Traditional taxonomic frameworks for amoebae relied heavily on morphological characteristics, particularly the type and structure of pseudopods, as observed through light microscopy, from the 19th to the mid-20th century. The term Rhizopoda was first proposed by Félix Dujardin in 1835, encompassing amoebae and other sarcodine protists unified by their locomotion and feeding via cytoplasmic extensions known as pseudopods. This grouping emphasized the as a defining trait, placing Rhizopoda within the broader phylum and distinguishing it from other protozoan classes like flagellates and . By the early , amoebae were further organized under the class Sarcodina within the phylum , a category that included all pseudopod-bearing protists and highlighted their shared granular, streaming . In the , Thomas Cavalier-Smith revised this structure in his higher-level of , retaining Sarcodina as a class but subdividing it into major groups such as loboseans (exemplified by Amoeba-like forms with broad, lobe-shaped pseudopods) and filoseans (with slender, needle-like pseudopods). These revisions aimed to refine morphological distinctions while maintaining the traditional kingdom, incorporating ultrastructural details from electron microscopy to better delineate subgroups. A key contribution to this framework came from Eugene C. Bovee in 1985, who proposed a detailed system for naked lobose amoebae within the class Lobosea (under Rhizopoda). Bovee's classification emphasized locomotive forms and cytoplasmic flow patterns, organizing them into orders such as Amoebida, which included families like Amoebidae. Within Amoebida, genera were delineated by pseudopod morphology and body size; for instance, Amoeba species feature compact, monopodial pseudopods and smaller cell sizes (typically 100–600 μm), while Chaos species exhibit larger bodies (up to several millimeters) with more eruptive, branching pseudopods. This approach prioritized observable traits like the arrangement of ectoplasm and during locomotion to resolve taxonomic ambiguities. Despite these advances, traditional frameworks had significant limitations due to their dependence on light microscopy, which often failed to capture ultrastructural or subcellular details. This reliance led to polyphyletic groupings, such as the artificial lumping of unrelated naked (e.g., gymnamoebae) and testate (shelled) forms based solely on superficial pseudopod similarities, overlooking deeper evolutionary relationships that later molecular studies revealed. Consequently, categories like Sarcodina encompassed distantly related lineages, including those now placed in separate supergroups, highlighting the challenges of morphology-alone in resolving amoeboid diversity.

Modern Classification and Phylogeny

The supergroup was established in 1998 as a major clade of eukaryotic protists, encompassing diverse amoeboid organisms including lobose amoebae (such as the genus ), slime molds (myxogastrids and dictyostelids), and the anaerobic Archamoebae. This arose from molecular phylogenetic analyses that unified these groups based on shared ultrastructural and genetic features, distinguishing them from other amoeboid lineages like and . The genus is placed within the class (also known as Lobosea), a subclass of characterized by cylindrical or tubular pseudopods, with the belonging to the order Euamoebida and family Amoebidae. Phylogenetic reconstructions using small-subunit ribosomal RNA () and genes position within a of free-living lobose amoebae, branching closely with genera like Chaos and Pelomyxa in the . These analyses, supported by multigene datasets, confirm the monophyly of and reveal its divergence from the supergroup Opisthokonta (including animals and fungi) approximately 1.2 to 1.6 billion years ago during the era. The order Euamoebida (formerly grouped under Amoebida) includes around 100 described species of free-living amoebae, primarily freshwater forms with lobose pseudopods and no tests or scales. Genomic studies highlight distinctive features in Amoebozoa, including evidence of extensive horizontal gene transfer (HGT) from bacteria, which has contributed to metabolic adaptations in amoebae like Amoeba. Historical estimates suggested A. proteus possessed one of the largest known eukaryotic genomes at approximately 290 pg of DNA, but recent reassessments using flow cytometry and sequencing data from the 2020s indicate this value was overestimated by orders of magnitude, with the actual size likely in the range of 25-30 Mb—still large but comparable to other protists. Pathogenic genera such as Entamoeba (e.g., E. histolytica) are excluded from Tubulinea and placed in the Archamoebae clade due to their anaerobic lifestyle and secondary loss of conventional mitochondria, retaining only reduced mitosomes for iron-sulfur cluster assembly.

Ecology and Interactions

Habitats and Distribution

Free-living amoebae inhabit a wide array of environments, predominantly freshwater systems such as ponds and rivers, as well as soils and sediments where they navigate organic-rich substrates. Some species occupy marine habitats, including coastal seas and brackish waters, demonstrating their versatility across aquatic and terrestrial niches. These protists flourish under neutral conditions ranging from 6.5 to 7.5 and temperatures between 15°C and 30°C, which support active growth and metabolic processes. Their global distribution is cosmopolitan, spanning diverse climates from temperate to tropical regions, with the greatest concentrated in tropical soils due to favorable moisture and organic content. For example, , a well-studied , is prevalent in temperate freshwater bodies worldwide, often associated with benthic surfaces and . This widespread occurrence underscores their adaptability to varying biogeographic zones without reliance on specific host reservoirs. In ecological contexts, free-living amoebae function as apex predators within microbial webs, primarily consuming through bacterivory, which promotes nutrient recycling by mineralizing and releasing essential elements like back into the and . Population densities in can attain up to 10⁴ individuals per gram, particularly in vegetated arid zones during wet seasons, influencing bacterial community dynamics and overall . Amoebae exhibit notable adaptations, including cyst formation that confers tolerance to hypoxia and other stresses, allowing survival in low-oxygen sediments or fluctuating environments. Recent microbiome studies highlight their interactions with and fungi in biofilms, where they graze on these organisms while harboring bacterial symbionts, thereby shaping complex microbial consortia in natural habitats.

Pathogenic Species and Human Impact

Among the pathogenic amoebae, Entamoeba histolytica is a primary cause of amebiasis, a protozoan infection transmitted via the fecal-oral route through ingestion of cysts in contaminated food or water, leading to approximately 50 million symptomatic cases annually worldwide, with around 100,000 deaths, predominantly in tropical and subtropical regions. The parasite invades intestinal tissues by secreting cysteine proteases that degrade the mucus layer and extracellular matrix, enabling trophozoites to adhere to epithelial cells, lyse host cells through contact-dependent mechanisms, and form flask-shaped ulcers or abscesses in the liver and other organs. In immunocompromised individuals, Acanthamoeba species cause granulomatous amebic encephalitis, a rare but often fatal central nervous system infection affecting 3–12 cases per year in the United States, with an 82% mortality rate, while Acanthamoeba keratitis primarily impacts contact lens wearers, leading to severe corneal ulcers and potential vision loss. Similarly, Naegleria fowleri, a free-living amoeba, enters the body through the nasal mucosa during freshwater exposure and causes primary amebic meningoencephalitis (PAM), a rapidly progressing brain infection with a fatality rate exceeding 97%, as evidenced by only four survivors out of 167 known U.S. cases from 1962 to 2024. The global human burden of these infections is significant, particularly in low-resource settings where poor exacerbates E. histolytica transmission, contributing to disability-adjusted life years and economic strain in endemic areas. Treatment for invasive amebiasis typically involves , which effectively targets trophozoites by disrupting , often followed by a luminal agent like to eliminate cysts, achieving cure rates over 90% in uncomplicated cases. However, studies from the indicate emerging concerns over metronidazole resistance in some E. histolytica strains, linked to genetic adaptations like increased expression, though overall resistance remains low and clinical failures are rare. For Acanthamoeba and Naegleria infections, management is more challenging, relying on surgical intervention, , and combinations, with limited success due to delayed . Ecologically, pathogenic amoebae like and act as opportunistic predators in biofilms, where they graze on and disrupt microbial community structures in aquatic and environments, potentially altering dynamics by favoring resistant bacterial strains. exacerbates their impact by warming waters and expanding habitable ranges, as seen with N. fowleri proliferation in temperate regions previously too cool for survival, increasing exposure risks and threatening in evolving environmental contexts.

Amoeboid Features in Other Organisms

Amoeboid Cells in Animals

In multicellular , amoeboid cells represent specialized populations that exhibit shape-changing motility integrated within tissues, enabling coordinated responses to physiological needs such as immunity and development, in contrast to the independent locomotion of free-living amoebae. These cells primarily utilize actin-driven pseudopods for rapid navigation through extracellular matrices, often under the influence of tissue-specific cues like , to support organismal . Prominent examples include of the , such as macrophages and neutrophils, which employ pseudopod extension for migration and to engulf pathogens and debris. Macrophages, in particular, adopt an amoeboid mode characterized by actin-rich pseudopods at the leading edge and a contractile uropod, facilitating rapid infiltration into infected or damaged sites. Neutrophils exemplify classic amoeboid behavior, squeezing through endothelial barriers and tissues via low-adhesion, high-contractility mechanisms to initiate inflammatory responses. Beyond immunity, contributes to through the movement of fibroblasts, which can transition to an amoeboid state under confined conditions to remodel and close injury sites. In embryonic development, amoeboid migration allows neural progenitor cells, such as those forming the horizontal cell layer in the vertebrate , to navigate crowded tissues and establish precise laminar structures essential for visual circuitry. These processes highlight how amoeboid cells integrate with surrounding multicellular environments, responding to developmental gradients rather than solitary environmental foraging. The underlying mechanisms involve polymerization for pseudopod protrusion, coupled with II-mediated contractility, but are finely tuned by for weak substrate interactions and like CCL19/CCR7 for directional guidance within animal tissues. migration speeds typically range from 1 to 5 μm/min for fibroblasts and up to 20-30 μm/min for leukocytes, reflecting adaptation to tissue density and signaling. Illustrative cases include coelomocytes, petaloid that extend pseudopods to clear cellular debris and foreign particles from the coelomic cavity, mirroring immune functions in vertebrates. In humans, dendritic cells within s utilize amoeboid migration to survey antigens, with studies from the 2020s revealing how enhances their migration speed, persistence, and mechanical resilience in confined environments, aiding lymph node homing and pathogen detection.

Amoeboid Structures in Plants and Fungi

In , cytoplasmic facilitates the movement of nutrients and organelles within cells, with amoeboid streaming patterns observed in certain contexts that resemble dynamic cytoplasmic flow akin to pseudopodial extension. This process is mediated by plasmodesmata, which are cytoplasmic channels connecting adjacent cells, enabling symplastic transport in tissues such as sieve tubes of the . In , streaming in companion cells drives the bulk flow of sap, supporting long-distance transport of photosynthates like , where velocities can reach up to 100 µm/s in related algal models, though slower in higher . This amoeboid-like flow contributes to efficient phloem loading and unloading without requiring external pressure gradients alone. Root hairs in exhibit tip growth through amoeboid extension, where the cell tip elongates via actin-myosin interactions rather than solely . This mechanism involves cycling of the , forming a molecular that propels the plasma forward while maintaining a rigid tube. from diatoms and plant setae supports this amoeboid model, showing inward deformations during extension that indicate cytoskeletal forces dominate over hydrostatic pressure. In root hairs, this results in growth rates of 0.6–2.4 µm/min, allowing enhanced soil nutrient absorption. In fungi, hyphal tip growth relies on the Spitzenkörper, a vesicle-organizing center at the apex that directs for extension, mirroring pseudopod dynamics in amoebae through polarized vesicle trafficking. The Spitzenkörper, composed of micro- and macrovesicles surrounded by F-actin, moves forward to dictate elongation rates and direction, with fungal displaying amoeboid in wall-less mutants that form pseudopodia-like protrusions. This enables rapid , with tip extension rates varying by species but often exceeding 1 µm/min in filamentous fungi like . Chytrid fungi produce amoeboid zoospores that transition from flagellar to crawling for substrate exploration and attachment. These zoospores use actin-driven for amoeboid locomotion post-encystment, facilitating dispersal in aquatic environments. In like Rhizophydium, an amoeboid stage follows , allowing the zoospore to crawl and invade hosts via directed cytoplasmic extensions. This dual motility enhances survival in heterogeneous habitats. Amoeboid structures in and fungi primarily serve absorption functions, as seen in fungal haustoria that invade host cells to extract sugars and . Haustoria in biotrophic pathogens like rust fungi form specialized interfaces with the host plasma membrane, using transporters such as HXT1 for hexoses and AAT1-3 for , powered by proton gradients, while secreting effectors to maintain host viability and prevent . This allows sustained uptake from living tissues, with the extrahaustorial matrix acting as a selective barrier. In , analogous amoeboid extensions in hairs increase surface area for soil uptake by up to 10-fold. These amoeboid features in and fungi demonstrate evolutionary convergence with amoebae, arising from co-option of shared eukaryotic machinery like the for and extension. Actin-based pseudopod formation and vesicle trafficking evolved early in eukaryotes, enabling diverse crawling and tip-growth modes across kingdoms without common ancestry in the clade. Fungal genomic studies from the late highlight expansions in cytoskeletal regulators, such as formins and myosins, that underpin hyphal multicellularity and amoeboid-like polarity, as seen in comparative analyses of 72 fungal genomes revealing correlated changes in 414 gene families. A representative example is growth in , where amoeboid elongation at the tip drives fertilization, with rates reaching 0.1–0.5 µm/s under optimal conditions, powered by oscillatory calcium gradients and actin remodeling. This primitive amoeboid motion ensures targeted delivery of sperm cells, distinct from broader .

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