L

Microbiology - Protists

Eukaryotic Origins

  • Eukaryotic cells feature a complex cytoskeleton and compartmentalization (nucleus and organelles). These features allow for greater internal complexity and functional specialization compared to simpler prokaryotic cells.

  • Eukaryotes are present in microfossils dating back approximately 1.5 billion years. The precise lineage and environmental conditions favoring their emergence are subjects of ongoing research, but genetic and fossil evidence points to a single common ancestor for all extant eukaryotes.

Endosymbiosis

  • Organelles such as mitochondria and chloroplasts originated through endosymbiosis, involving the engulfment of bacteria by ancestral eukaryotes. This theory suggests that a larger host cell engulfed a prokaryotic cell without digesting it, leading to a symbiotic relationship where both organisms benefited.

  • Mitochondria derive from aerobic bacteria, providing efficient energy production; chloroplasts from photosynthetic bacteria (cyanobacteria), enabling photosynthesis. Key evidence supporting endosymbiosis includes the presence of their own circular DNA (similar to bacterial DNA), ribosomes sensitive to bacterial antibiotics, and double membranes in mitochondria and chloroplasts.

  • Secondary endosymbiosis occurred when larger eukaryotes engulfed those with existing chloroplasts, leading to chloroplasts with more than two membranes (e.g., in some protists).

Characteristics of Protists

  • Protists are a diverse group not classified as fungi, plants, or animals (paraphyletic), meaning they do not share a single common ancestor exclusively among themselves. They inhabit nearly every environment, from aquatic habitats (freshwater and marine) to moist terrestrial soils and even within other organisms as parasites or symbionts.

  • They vary in cellular organization:

    • Unicellular, colonial, and multicellular.

    • Utilize diverse nutritional modes: autotrophic (photosynthetic, chemoautotrophic) and heterotrophic (phagotrophs, osmotrophs, mixotrophs). Chemoautotrophic protists are less common, often found in unique environments. Heterotrophic phagotrophs engulf food particles, while osmotrophs absorb dissolved organic molecules. Mixotrophs (e.g., Euglena) can switch between photosynthetic and heterotrophic modes depending on resource availability.

  • Locomotion methods include flagella, cilia, and pseudopodia. These structures are vital for movement, feeding (e.g., creating water currents for filter feeding), and sensing the environment. Pseudopodia ('false feet') are dynamic extensions of the cell, allowing amoeboid movement.

Reproduction in Protists

  • Primarily asexual via mitosis; some utilize budding and schizogony. Budding involves the formation of a new organism from an outgrowth or bud, while schizogony (multiple fission) is characteristic of some parasitic protists, where the nucleus divides multiple times before the cell itself divides into many daughter cells.

  • Sexual reproduction occurs through meiosis, facilitating genetic recombination. While generally less common than asexual reproduction, sexual reproduction often occurs in response to environmental stress, providing genetic variation that can aid survival and adaptation.

Protists and Multicellularity

  • Multicellularity has evolved in protists multiple times (convergent evolution), important for organism specialization. This evolution allowed for cellular specialization, where different cells perform distinct functions, leading to more complex organismal structures and efficiencies, a critical step towards the evolution of plants, animals, and fungi.

Major Protist Groups

  • Excavata: Includes diplomonads (e.g., Giardia, known for intestinal infections), parabasalids (e.g., Trichomonas, a sexually transmitted parasite), and euglenozoans (euglenids and kinetoplastids like Trypanosoma, which causes sleeping sickness). Often characterized by a 'feeding groove' and unusual flagella.

  • SAR: A supergroup defined by genetic similarities; includes highly diverse organisms. It contains stramenopiles (brown algae, diatoms, oomycetes, often possess two flagella of unequal length, one hairy and one smooth), alveolata (dinoflagellates, apicomplexans, ciliates, characterized by sac-like structures (alveoli) beneath the plasma membrane), and rhizarians (radiolarians, foraminifera, cercozoa, amoeboid protists often with slender pseudopodia).

  • Archaeplastida: Includes red and green algae, with green algae leading to land plants. United by primary endosymbiosis of cyanobacteria, giving rise to chloroplasts.

  • Amoebozoa: Features amoebas and slime molds. Move using blunt, lobe-shaped pseudopods.

  • Opisthokonta: Comprises choanoflagellates, fungi, and animals. Characterized by a single posterior flagellum in motile cells (if present).

Specific Organism Features

  • Diatoms: Unicellular with intricate, silica-based shells composed of two fitting halves. They are photosynthetic and form a significant portion of phytoplankton, vital for global carbon fixation; their shells contribute to diatomaceous earth when they die.

  • Brown Algae: Large, complex multicellular seaweeds (e.g., kelp) with specialized tissues for anchorage, buoyancy, and photosynthesis. They have a haplodiplontic life cycle involving alternation of multicellular sporophyte and gametophyte generations.

  • Ciliates: Possess numerous cilia, which are short, hair-like structures used for locomotion and feeding. They also have two types of nuclei: a micronucleus (involved in genetic recombination) and a macronucleus (controls daily cellular functions, typically polyploid).

  • Apicomplexans: Spore-forming obligate intracellular parasites (e.g., Plasmodium, which causes malaria). They possess an apical complex of organelles for penetrating host cells, and their complex life cycles often involve multiple hosts (e.g., mosquito and human for Plasmodium).

  • Foraminifera: Marine protists that produce elaborate, multi-chambered calcium carbonate shells (tests) through which slender pseudopodia extend for feeding and movement. Their fossilized remains are important geological indicators.

  • Rhodophyta: Red algae. They lack flagella and contain accessory pigments like phycoerythrin, which allows them to absorb blue-green light at greater depths where other wavelengths cannot penetrate, enabling photosynthesis in deeper waters.

  • Green Algae: Includes chlorophytes (e.g., colonial Volvox and filamentous Spirogyra) and charophytes; the latter are closely related to land plants