Name: excavata protists and their adaptations
Uploaded: 2025-06-12T09:27:15.000Z
Duration: 1 min 15 s
Channel: JoVE

Excavata è un gruppo eterogeneo di protisti, che comprende sia specie chemioorganotrofe che fototrofiche. I gruppi chiave includono diplomonadi, parabasalidi ed euglenozoi. I diplomonadi come Giardia sono caratterizzati da due nuclei e mitosomi, che sono mitocondri sottosviluppati incapaci di produrre ATP. I parabasalidi sono specie anaerobi con un corpo parabasale. Trichomonas è un esempio importante, privo di mitocondri ma contenente idrogenosomi, che generano ATP con idrogeno gassoso come sottoprodotto. Gli euglenozoi sono specie mobili che hanno la capacità di funzionare sia come chemiotrofi che come fototrofi. La maggior parte delle specie ha tipicamente due flagelli, ma solo uno è visibile esternamente, emergendo da una tasca flagellare anteriore. L'esempio rappresentativo, Euglena, mostra mixotrofia. Prospera in presenza di luce, ma un lungo periodo di oscurità può fargli perdere i suoi cloroplasti e passare a una modalità di metabolismo chemioorganotrofico.

diversità dei protozoi

Diversity of Protists I

Excavata &#232; un gruppo eterogeneo di protisti che include specie chemioorganotrofe e fototrofe, alcune delle quali prosperano in ambienti anaerobici. Tra i gruppi principali all&#8217;interno degli Excavata figurano i diplomonadi e i parabasalidi, protisti flagellati privi di mitocondri e cloroplasti. Questi microrganismi abitano tipicamente ambienti anossici, come l&#8217;intestino degli animali, dove vivono in simbiosi o come parassiti, affidandosi alla fermentazione per la produzione di energia. Alcuni diplomonadi sono responsabili di gravi patologie nei pesci, negli animali domestici e negli esseri umani, mentre una specie di parabasalide &#232; l&#8217;agente eziologico di una delle principali malattie sessualmente trasmesse negli esseri umani. Entrambi i gruppi condividono un antenato comune relativamente recente prima di divergere in linee evolutive distinte.
I diplomonadi si caratterizzano per la presenza di due nuclei di uguali dimensioni e per il possesso di mitosomi, mitocondri altamente ridotti privi di proteine della catena di trasporto elettronico e degli enzimi associati al ciclo dell&#8217;acido citrico. Un membro rilevante di questo gruppo &#232; Giardia, che possiede un genoma relativamente piccolo e compatto, con pochi introni e privo di geni codificanti per vie metaboliche legate al ciclo dell&#8217;acido citrico. Tali caratteristiche genetiche riflettono il suo stile di vita parassita e anaerobico. La specie Giardia intestinalis, nota anche come Giardia lamblia, &#232; responsabile della giardiasi, una diffusa malattia diarroica trasmessa per via idrica negli Stati Uniti.
I parabasalidi possiedono una struttura distintiva chiamata corpo parabasalico, che fornisce supporto strutturale al complesso del Golgi. Questi protisti anaerobici sono privi di mitocondri ma contengono idrogenosomi, coinvolti nel metabolismo energetico. I parabasalidi si trovano comunemente nei tratti intestinali e urogenitali di vertebrati e invertebrati, dove vivono come parassiti o simbionti commensali. Un esempio prominente &#232; Trichomonas vaginalis, un organismo flagellato responsabile di un&#8217;infezione sessualmente trasmessa ampiamente diffusa negli esseri umani. I genomi dei parabasalidi presentano una struttura unica tra gli eucarioti, in quanto generalmente privi di introni. Il genoma di T. vaginalis &#232; particolarmente grande per un organismo parassita, con circa 160 megabasi. Mostra inoltre evidenze di trasferimento genico orizzontale da batteri ed &#232; altamente ripetitivo, con numerosi elementi trasponibili, rendendo l&#8217;analisi genomica complessa. Nonostante tali difficolt&#224;, si stima che T. vaginalis possieda circa 60.000 geni, quasi il doppio rispetto al genoma umano, risultando uno dei genomi eucariotici pi&#249; grandi conosciuti.
I kinetoplastidi rappresentano un altro gruppo rilevante all&#8217;interno di Excavata, cos&#236; denominati per il kinetoplasto, una massa densa di DNA situata all&#8217;interno dell&#8217;unico mitocondrio di grandi dimensioni. Questi organismi abitano prevalentemente ambienti acquatici, dove si nutrono di batteri, ma alcune specie sono parassiti responsabili di gravi malattie in esseri umani e animali. Il genere Trypanosoma include patogeni umani come Trypanosoma brucei, agente eziologico della malattia del sonno africana. Questa malattia cronica e generalmente letale si sviluppa dal sangue fino al sistema nervoso centrale, provocando infiammazioni cerebrali e midollari con sintomi neurologici. Il parassita &#232; trasmesso dalla mosca tse-tse (Glossina), un insetto ematofago diffuso in alcune regioni africane. Una volta all&#8217;interno della mosca, il parassita si moltiplica nel tratto intestinale e migra nelle ghiandole salivari, da cui pu&#242; essere trasmesso a un nuovo ospite umano tramite una puntura.
Altri parassiti kinetoplastidi umani includono Trypanosoma cruzi, causa della malattia di Chagas, e specie del genere Leishmania, responsabili della leishmaniosi. La malattia di Chagas &#232; trasmessa dalla puntura di un insetto ematofago noto come &#8220;cimice dei letti&#8221;. Sebbene spesso autolimitante, pu&#242; evolvere in una condizione cronica potenzialmente letale. La leishmaniosi, diffusa in regioni tropicali e subtropicali, &#232; trasmessa dal flebotomo e pu&#242; manifestarsi come infezione cutanea localizzata o come patologia sistemica che colpisce milza e fegato, risultando talvolta fatale.
Gli euglenidi, un altro gruppo ben studiato degli Excavata, si distinguono per la loro natura non patogena e la capacit&#224; di agire sia come chemotrofi che come fototrofi. Questi eucarioti microbici mobili, perlopi&#249; non patogeni, possono comportarsi sia da fototrofi che da eterotrofi. La maggior parte presenta due flagelli che emergono da una tasca flagellare anteriore, ma solo uno &#232; visibile esternamente, permettendo loro di muoversi tra ambienti illuminati e oscuri, a sostegno delle loro strategie nutrizionali flessibili. Gli euglenidi sono comunemente presenti in ambienti acquatici, sia d'acqua dolce che marina, e molte specie contengono cloroplasti che permettono la crescita fototrofa. Tuttavia, in assenza di luce, le cellule di Euglena, un rappresentante degli euglenidi, possono perdere i cloroplasti e passare a un metabolismo chemiorganotrofo. Inoltre, molti euglenidi sono capaci di fagocitosi, un processo mediante il quale inglobano cellule batteriche circondandole con la loro membrana citoplasmatica flessibile, consentendo la digestione interna.

Diversità dei Protozoi

Excavata &#232; un gruppo eterogeneo di protisti che include specie chemioorganotrofe e fototrofe, alcune delle quali prosperano in ambienti anaerobici. ...

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excavata protists and their adaptations

Excavata protists show many different ways of living, from photosynthesis to parasitism. This group includes organisms that can use chemical energy, light, or both. Some also live in low-oxygen, or anaerobic, environments where they survive without normal oxygen-based metabolism.

Among the main Excavata groups are diplomonads and parabasalids. These are flagellated protists, meaning they move with whip-like tails, and they lack mitochondria and chloroplasts. They often live in anoxic habitats, such as the intestines of animals, where they may be symbionts or parasites and depend on fermentation for energy. Some members cause major diseases in fish, domestic animals, and humans. A parabasalid species also causes a major sexually transmitted disease in humans. The two groups likely shared a recent common ancestor before they split into separate lineages.

Diplomonads are known for having two nuclei of equal size. They also have mitosomes, which are highly reduced mitochondria. Mitosomes do not contain electron-transport proteins or the enzymes used in the citric acid cycle. Giardia is a well-known diplomonad with a small, compact genome, very few introns, and no genes for citric acid cycle pathways. These features fit its parasitic, anaerobic lifestyle. Giardia intestinalis, also called Giardia lamblia, causes giardiasis, a common waterborne diarrheal disease in the United States.

Parabasalids have a structure called the parabasal body, which supports the Golgi complex. They lack mitochondria but contain hydrogenosomes, which help with energy metabolism. These protists are often found in the intestinal and urogenital tracts of vertebrates and invertebrates. In these places, they may live as parasites or as commensal symbionts. Trichomonas vaginalis is a flagellated parabasalid that causes a widespread sexually transmitted infection in humans.

Parabasalid genomes are unusual among eukaryotes because they generally lack introns. The genome of T. vaginalis is especially large for a parasite, at about 160 megabase pairs. It also shows signs of horizontal gene transfer from bacteria and contains many repetitive regions and transposable elements, which makes genomic study difficult. Even with these complications, T. vaginalis is estimated to have nearly 60,000 genes. That is nearly twice the number in the human genome and one of the largest known eukaryotic genomes.

Kinetoplastids are another major Excavata group. They are named for the kinetoplast, a dense mass of DNA found in their single, large mitochondrion. Most live in aquatic habitats and feed on bacteria, but some are parasites that cause serious disease in humans and animals. Trypanosoma brucei causes African sleeping sickness, a chronic and usually fatal disease that moves from the bloodstream to the central nervous system. It is spread by the tsetse fly, Glossina, a blood-feeding insect from parts of Africa. The parasite multiplies in the fly’s intestine and moves to the salivary glands, where it can infect a new human host through a bite. Other human-infecting kinetoplastids include Trypanosoma cruzi, which causes Chagas disease, and Leishmania species, which cause leishmaniasis. Chagas disease spreads through the bite of the blood-feeding kissing bug and can become chronic and fatal. Leishmaniasis is found in tropical and subtropical regions and is spread by sand flies. It can cause skin infections or a more serious systemic disease that affects the spleen and liver.

Euglenids are another well-studied Excavata group and are usually nonpathogenic. They can act as both chemotrophs and phototrophs, so they can get energy from chemicals or from light. Most have two flagella from an anterior flagellar pocket, but only one is visible outside the cell. This helps them move between bright and dark environments and supports their flexible feeding strategies. Euglenids are common in freshwater and marine habitats, and many contain chloroplasts for phototrophic growth. In low light, Euglena can lose its chloroplasts and switch to a chemoorganotrophic way of life. Many euglenids can also carry out phagocytosis, where they engulf bacterial cells with their flexible membrane and digest them inside the cell.

Excavata Protists and Their Adaptations

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diversity of protists i

Excavata protists show many different ways of living, from photosynthesis to parasitism. This group includes organisms that can use chemical energy, light, or both. Some also live in low-oxygen, or anaerobic, environments where they survive without normal oxygen-based metabolism. Among the main Excavata groups are diplomonads and parabasalids. These are flagellated protists, meaning they move with whip-like tails, and they lack mitochondria and chloroplasts. They often live in anoxic habitats,...

diversity of protists ii

Alveolates: Ciliates, Dinoflagellates, Apicomplexans

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diversity of protists iii

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...

diversity of protists iv

Amoebozoa: Movement and Life Cycles

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key terms and concepts

Explore Excavata protists, including diplomonads, parabasalids, kinetoplastids, and euglenids, and learn how their structures fit their habitats and lifestyles.

Excavata Protists and Their Adaptations | 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

Excavata Protists and Their Adaptations

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