Name: rhizaria: pseudopods, tests, and skeletons
Uploaded: 2025-06-12T06:30:33.000Z
Duration: 1 min 27 s
Channel: JoVE

I rizzari rappresentano un gruppo eterogeneo di protisti che possiedono pseudopodi filiformi. Ci sono tre sottogruppi principali: Chlorarachniophyta, Foraminiferi e Radiolaria. Le Chlorarachniophyta sono protisti fototrofi che si trovano sia in ambienti d'acqua dolce che marini. Hanno cloroplasti con quattro membrane a causa di endosimbionti algali che sono stati acquisiti durante l'endosimbiosi secondaria. I foraminiferi sono rigorosamente protisti marini che costruiscono strutture simili a conchiglie chiamate test, composte da carbonato di calcio e materiale organico. La maggior parte dei foraminiferi si estende verso l'esterno dai loro test per nutrirsi di materia organica in decomposizione, batteri o altri piccoli protisti sul fondo dell'oceano. Alcuni foraminiferi sono planctonici e ospitano alghe endosimbiotiche per i nutrienti. I radiolari sono chemioorganotrofi planctonici marini. Possiedono test composti da silice. I radiolari abitano i 100 m superiori dell'oceano, dove rimangono sospesi da goccioline lipidiche, grandi vacuoli citoplasmatici e pseudopodi aghiformi. Si nutrono di batteri e materia organica. Al contrario, alcune altre specie stabiliscono relazioni simbiotiche, ma non endosimbiotiche, con le alghe.

diversità_dei_protisti-iii

Diversity of Protists III

I Rhizaria sono un gruppo eterogeneo di protisti unicellulari caratterizzati da sottili estensioni citoplasmatiche filamentose, note come pseudopodi. Queste strutture facilitano sia la locomozione sia l&#8217;alimentazione, conferendo ai Rhizaria un aspetto ameboide. Tale morfologia ha in passato generato confusione nella classificazione tassonomica, ma la filogenetica molecolare ha chiarito la loro collocazione evolutiva, evidenziando l&#8217;uso condiviso degli pseudopodi, nonostante le linee evolutive divergenti.
Questo clade comprende lignaggi molto diversi come Chlorarachniophyta, Radiolaria e Foraminifera, tutti adattatisi a nicchie ecologiche distinte e caratterizzati da una variet&#224; di strategie metaboliche.
Chlorarachniophyta
I Chlorarachniophyta sono protisti unicellulari fototrofici presenti sia in ambienti marini sia in ambienti di acqua dolce. Rappresentano un esempio di endosimbiosi secondaria, nella quale un ospite eucariotico non fotosintetico ha inglobato un eucariote algale fotosintetico. A differenza dei plastidi primari, che in genere possiedono due membrane, i cloroplasti dei chlorarachniophyta sono circondati da quattro membrane e conservano un nucleomorfo&#8212;un nucleo vestigiale derivante dall&#8217;alga inglobata. Questo nucleomorfo rappresenta uno stato intermedio raro nell&#8217;integrazione plastidica, conservando prove genomiche e strutturali dell&#8217;evento endosimbiotico.
L&#8217;architettura genetica dei chlorarachniophyta &#232; straordinariamente complessa. Sono presenti almeno cinque genomi: (1) il genoma nucleare dell&#8217;ospite, (2) il genoma mitocondriale dell&#8217;ospite, (3) il genoma del nucleomorfo, (4) il genoma mitocondriale algale e (5) il genoma plastidico. Nel corso dell&#8217;evoluzione, molti geni dell&#8217;endosimbionte si sono trasferiti al nucleo dell&#8217;ospite, determinando una riduzione genomica nel nucleomorfo e nel cloroplasto. Questa integrazione genomica riflette un&#8217;ottimizzazione evolutiva, in cui le funzioni ridondanti vengono eliminate o riassegnate.
Foraminifera
I foraminiferi sono protisti marini ameboidi che costruiscono test elaborati, spesso composti da carbonato di calcio. Questi test non fanno parte della cellula vivente, ma offrono protezione strutturale e supporto per l&#8217;alimentazione tramite pseudopodi. I foraminiferi bentonici vivono sul fondo oceanico, alimentandosi di materia organica particolata e stabilendo simbiosi trofiche con alghe. Le specie planctoniche ospitano endosimbionti fototrofici e abitano gli strati superiori dell&#8217;oceano, dove la luce solare &#232; disponibile.
Il potenziale di fossilizzazione dei test foraminiferi li rende indispensabili nella biostratigrafia e nella geologia petrolifera. La loro distribuzione negli strati geologici consente di inferire l&#8217;et&#224; relativa dei depositi sedimentari e valutare la potenzialit&#224; di giacimenti petroliferi.
Radiolaria
I radiolari sono principalmente protisti marini planctonici che abitano le acque superficiali. I loro scheletri simmetrici, costituiti da silice, sono altamente ornamentali e persistono dopo la morte, contribuendo alla formazione di fanghi silicei sul fondale oceanico. I radiolari si nutrono di detriti organici e batteri e talvolta stabiliscono relazioni simbiotiche con alghe. A differenza dei foraminiferi, i radiolari non praticano l&#8217;endosimbiosi. La loro galleggiabilit&#224; &#232; mantenuta mediante inclusioni lipidiche, vacuoli e pseudopodi rigidi che contrastano l&#8217;affondamento gravitazionale.

Diversità_dei_Protisti-III

I Rhizaria sono un gruppo eterogeneo di protisti unicellulari caratterizzati da sottili estensioni citoplasmatiche filamentose, note come pseudopodi. ...

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rhizaria: pseudopods, tests, and skeletons

Rhizaria are unicellular protists with threadlike pseudopodia, which are thin cytoplasmic extensions used for movement and feeding. Their amoeboid shape once caused taxonomic confusion, but molecular phylogenetics has since shown where they fit on the evolutionary tree. This group includes Chlorarachniophyta, Radiolaria, and Foraminifera, and each lineage has adapted to a different ecological niche and metabolic strategy.

Chlorarachniophyta are unicellular phototrophic protists found in marine and freshwater environments. They are a clear example of secondary endosymbiosis, where a non-photosynthetic eukaryotic host engulfed a photosynthetic algal eukaryote. Their chloroplasts have four membranes, unlike the two membranes seen in primary plastids, and they also keep a nucleomorph, which is the vestigial nucleus of the engulfed alga.

Chlorarachniophytes also have unusually complex genetics. At least five genomes are present: the host nuclear genome, the host mitochondrial genome, the nucleomorph genome, the algal mitochondrial genome, and the plastid genome. Over time, many genes moved from the endosymbiont to the host nucleus. This left the nucleomorph and chloroplast reduced and shows how cells can streamline after endosymbiosis.

Foraminifera are marine amoeboid protists that build tests, which are protective shells often made of calcium carbonate. The test is not part of the living cell, but it supports the cell and helps with feeding through pseudopodia. Benthic foraminiferans live on the ocean floor and feed on particulate organic matter, and some also form nutrient-exchange symbioses with algae.

Other foraminiferans are planktonic and live in the upper ocean where sunlight is available. These species can host phototrophic endosymbionts. Their tests fossilize well, which makes foraminifera useful in biostratigraphy and petroleum geology. Scientists use their fossils to compare the relative age of sediment layers and to help judge where oil deposits may be found.

Radiolaria are mostly planktonic marine protists found in surface waters. They have ornate, symmetrical skeletons made of silica, and these structures remain after death to form siliceous ooze on the seafloor. Radiolarians feed on organic detritus and bacteria, and some live with algae in symbiosis. Unlike foraminifera, they do not use endosymbiosis. Their buoyancy comes from lipid inclusions, vacuoles, and stiff pseudopodia that help keep them from sinking.

Rhizaria: Pseudopods, Tests, and Skeletons

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diversity of protists iii

Rhizaria are unicellular protists with threadlike pseudopodia, which are thin cytoplasmic extensions used for movement and feeding. Their amoeboid shape once caused taxonomic confusion, but molecular phylogenetics has since shown where they fit on the evolutionary tree. This group includes Chlorarachniophyta, Radiolaria, and Foraminifera, and each lineage has adapted to a different ecological niche and metabolic strategy. Chlorarachniophyta are unicellular phototrophic protists found in marine...

diversity of protists iv

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key terms and concepts

Explore Rhizaria in high school biology, including pseudopodia, chlorarachniophytes, foraminifera, and radiolarians with their unique structures and roles.

Rhizaria: Pseudopods, Tests, and Skeletons | JoVE K12

JoVE Core Microbiology provides an engaging introduction to the microbial world and its influence on health, industry, and the environment. This comprehensive collection covers key concepts such as microbial structure, metabolism, genetics, and ecology. It introduces essential laboratory techniques and microscopy methods through visually engaging animations. Corresponding scientist-in-action videos bridge classroom concepts with real-world research and applications.

microbiology

Rhizaria: Pseudopods, Tests, and Skeletons

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