7.34: Diversity of Protists I

Excavata is a diverse group of protists that includes both chemoorganotrophic and phototrophic species, with some thriving in anaerobic environments. Among the key groups within Excavata are diplomonads and parabasalids, which are flagellated protists that lack mitochondria and chloroplasts. These microorganisms typically inhabit anoxic environments, such as the intestines of animals, where they exist either symbiotically or as parasites, relying on fermentation for energy production. Some diplomonads are responsible for significant diseases in fish, domestic animals, and humans, while a particular parabasalid species is the causative agent of a major sexually transmitted disease in humans. Both groups share a relatively recent common ancestor before diverging into separate evolutionary lineages.

Diplomonads are characterized by the presence of two nuclei of equal size and the possession of mitosomes, which are highly reduced mitochondria that lack electron-transport proteins and enzymes associated with the citric acid cycle. One notable member of this group, Giardia, has a relatively small and compact genome, with minimal introns and an absence of genes encoding metabolic pathways related to the citric acid cycle. These genetic characteristics align with its parasitic and anaerobic lifestyle. The species Giardia intestinalis, also referred to as Giardia lamblia, is known to cause giardiasis, a prevalent waterborne diarrheal disease in the United States.

Parabasalids possess a distinct structure called the parabasal body, which provides structural support to the Golgi complex. These anaerobic protists lack mitochondria but contain hydrogenosomes, which play a role in energy metabolism. Parabasalids are commonly found in the intestinal and urogenital tracts of vertebrates and invertebrates, where they exist as parasites or commensal symbionts. A prominent example, Trichomonas vaginalis, is a flagellated organism that causes a widespread sexually transmitted infection in humans. The genomes of parabasalids exhibit a unique structure among eukaryotes, as they generally lack introns. The genome of T. vaginalis is particularly large for a parasitic organism, containing approximately 160 megabase pairs. It also shows evidence of horizontal gene transfer from bacteria and is highly repetitive, with numerous transposable elements, making genomic analysis challenging. Despite these complexities, T. vaginalis is estimated to have nearly 60,000 genes, nearly double the number found in the human genome and one of the largest known eukaryotic genomes.

Kinetoplastids represent another significant group within Excavata, named for the kinetoplast—a dense mass of DNA located within their single, large mitochondrion. These organisms primarily inhabit aquatic environments, where they consume bacteria, but some species are parasitic and cause severe diseases in humans and animals. The genus Trypanosoma includes pathogens that infect humans, such as Trypanosoma brucei, which causes African sleeping sickness. This chronic and typically fatal disease progresses from the bloodstream to the central nervous system, leading to brain and spinal cord inflammation that results in neurological symptoms. The parasite is transmitted by the tsetse fly (Glossina), a blood-feeding insect native to certain regions of Africa. Once inside the fly, the parasite proliferates in the intestinal tract and migrates to the salivary glands, from which it can be transmitted to a new human host through a bite.

Other kinetoplastid parasites that affect humans include Trypanosoma cruzi, which causes Chagas disease, and species of Leishmania, which are responsible for leishmaniasis. Chagas disease is transmitted by the bite of a blood-feeding insect known as the "kissing bug." While often self-limiting, the disease can develop into a chronic and potentially fatal condition. Leishmaniasis, found in tropical and subtropical regions, is spread by the sand fly and can manifest either as localized skin infections or as a systemic disease affecting the spleen and liver, which can be life-threatening.

Euglenids, another well-studied group within Excavata, are distinguished by their nonpathogenic nature and their ability to function as both chemotrophs and phototrophs. These motile microbial eukaryotes, mostly nonpathogenic protists can function as phototrophs and heterotrophs. Most have two flagella emerging from an anterior flagellar pocket, but only one is externally visible, which allows them to navigate between illuminated and dark environments, supporting their flexible nutritional strategies. Euglenids are commonly found in aquatic habitats, both freshwater and marine, and many species contain chloroplasts that enable phototrophic growth. However, in the absence of light, cells of Euglena, a representative euglenid, can lose their chloroplasts and switch to a chemoorganotrophic mode of metabolism. Additionally, many euglenids are capable of phagocytosis, a process in which they engulf bacterial cells by surrounding them with their flexible cytoplasmic membrane, allowing internal digestion.