Chapter 24: Protists – Eukaryotic Microbes & Their Roles

Structurally complex despite lacking tissue organization, these single cells contain specialized organelles, including the plasmalemma (cell membrane), various vacuoles (contractile for osmoregulation, phagocytic for digestion), and sometimes a supportive pellicle. Protists exhibit varied nutritional strategies—chemoorganotrophs (protozoa) often use osmotrophy or phagocytosis, while photoautotrophs (algae) perform oxygenic photosynthesis; many are mixotrophs. Energy conservation occurs via mitochondria, hydrogenosomes, or primitive mitosomes. Motility relies on flagella, cilia, or pseudopodia (lobopodia, filopodia, reticulopodia). Reproduction is complex, involving asexual methods like binary fission, and sexual processes such as syngamy (fusion of gametes) or conjugation (exchange of nuclei), with ciliates notably possessing both macro- and micronuclei. The chapter organizes this vast diversity into phylogenetic supergroups. Excavata includes primitive eukaryotes and pathogens such as Giardia intestinalis and Trypanosomes, which cause African sleeping sickness. Amoebozoa utilizes pseudopodia for movement and feeding, encompassing pathogens like Entamoeba histolytica and Dictyostelium discoideum, a cellular slime mold model organism known for its unique cellular cooperation and primitive farming behavior. The globally significant SAR supergroup includes Rhizaria (such as Foraminifera with calcareous tests, providing crucial geological markers), Alveolata (Dinoflagellates, often linked to red tides; Ciliates; and Apicomplexans, which are all parasites distinguished by an apical complex and containing the essential apicoplast, exemplified by Plasmodium spp., the cause of malaria), and Stramenopila (featuring Diatoms, major oceanic carbon fixers characterized by silica frustules, and destructive plant pathogens like Phytophthora infestans). Finally, Archaeplastida includes the photosynthetic Chloroplastida, or green algae, such as the model organism Chlamydomonas.