Introduction: Protists!

1. Introduction: Protists! • Domain Eukarya is the third domain on the tree of life, with Bacteria and Archaea. Eukaryotes range from single-celled organisms the size of bacteria to sequoia trees and blue whales. • Protists are a diverse group of organisms that includes all eukaryotes except the land plants, fungi, and animals.

2. Introduction • Protists do not make up a monophyletic group. Instead, they refer to a paraphyletic group—they represent some, but not all, of the descendants of a single common ancestor. No synapomorphies define the protists. There is no trait that is found in protists but no other organisms. • The common feature among protists is that they tend to live in environments where they are surrounded by water.

4. Impacts on Human Health and Welfare • Several types of protists can cause human disease, and some also cause disease in our crops. • For example, the most spectacular crop failure in history, the Irish potato famine of 1845, was caused by the protist Phytophthora infestans.

5. Malaria • Malaria, the world's most chronic public health problem, is caused by a parasitic protist called Plasmodium. • Plasmodium is transferred to humans from mosquitoes. • The cell types that make up each stage of the Plasmodium’s life cycle are each specialized for infecting a specific host cell.

6. Harmful Algal Blooms • Harmful algal blooms occur when dinoflagellates, toxin-producing protists, reach high densities in an aquatic environment. • Algal blooms of dinoflagellates are known as red tides. • Algal blooms can be harmful to humans because these toxins build up in clams and other shellfish. If a person eats contaminated shellfish, several types of poisoning can result.

7. Ecological Importance of Protists • Protists represent just 10 percent of the named eukaryotic species and have relatively low species diversity, but they are extraordinarily abundant.

8. Protists Play a Key Role in Aquatic Food Chains • Species that produce chemical energy by photosynthesis are called primary producers. • Production of organic molecules by protists living in the world’s oceans represents almost half of the total carbon dioxide that is fixed on Earth. • Diatoms and other small organisms that live near the surface of oceans or lakes and that drift along or swim only short distances are called plankton. • The organic compounds produced by phytoplankton (photosynthetic plankton) are the basis of food chains in freshwater and marine environments.

9. Protists Play a Key Role in Aquatic Food Chains • A food chain describes nutritional relationships among organisms. • Many of the species at the base of food chains in aquatic environments are protists. Without protists, most food chains in freshwater and marine habitats would collapse.

10. Could Protists Help Reduce Global Warming? • The movement of carbon atoms from carbon dioxide molecules in the atmosphere to organisms in the soil or the ocean and then back to the atmosphere is called the global carbon cycle. • Protists play a key role in the global carbon cycle and act as carbon sinks that could help reduce global warming. • A carbon sink is a long-lived carbon reservoir. Carbon sinks produced by protists can be either: • Sedimentary rocks. • Petroleum.

13. Study of protists through microscopy: Studying Cell Structure • Studies of cell structure found that protists can be grouped according to their overall form and/or distinctive organelles. • Researchers interpreted these types of distinctive morphological features as synapomorphies—shared, derived traits used to distinguish major monophyletic groups. • Microscopy allowed researchers to identify characteristics of specific lineages. • For example, Stramenopiles have unusual flagella covered with hollow hair-like structures.

14. Microscopy: Studying Cell Structure • Seven major groups of eukaryotes came to be identified on the basis of diagnostic morphological characteristics. • Of these, the group Plantae includes the green plants, and the group Opisthokonta includes the fungi and animals.

16. Evaluating Molecular Phylogenies • Scientists combined morphological data with DNA sequence data to create a phylogenetic tree of the seven eukaryotic lineages. • The Opisthokonta and Amoebozoa form the monophyletic group Unikonta. • The other five major lineages form the monophyletic group Bikonta. • The Alveolata and Stramenopila form the monophyletic group Chromalveolata. • The original split seems to have occurred between unikonts (one flagellum) and bikonts (two flagella). • As more data become available, our understanding of eukaryote phylogeny will continue to improve.

20. Themes in the Diversification of Protists • Because protists are a paraphyletic group, they do not share derived characteristics that set them apart from all other lineages on the tree of life. • But there have been novel morphological traits that occurred as they evolved. • These innovations triggered the evolution of species that live in a wide array of habitats and make a living in diverse ways.

21. What Morphological Innovations Evolved in Protists? • The earliest eukaryotes were probably single-celled organisms with • a nucleus and endomembrane system, • mitochondria, • a cytoskeleton, • but no cell wall. • These cells probably swam using a novel type of flagellum.

22. The Nuclear Envelope • The leading hypothesis for the origination of the nuclear envelope (and the endoplasmic reticulum) is that it is derived from the infoldings of the plasma membrane. • Nuclei diversified, leading to unique types of nuclei in the major protist lineages. The distinctive structure of the nucleus is a synapomorphy that allows us to recognize these lineages as distinct monophyletic groups.

23. Endosymbiosis and the Origin of the Mitochondrion • Mitochondria are organelles that generate ATP. • The endosymbiosis theory proposes that mitochondria originated when a bacterial cell took up residence inside a eukaryote about 2 billion years ago. • Symbiosis occurs when individuals of two different species live in physical contact. • Endosymbiosis occurs when an organism of one species lives inside an organism of another species • e.g., lactobacilli in humans

24. The Mitochondrion • The endosymbiosis theory proposed that mitochondria evolved through a series of steps: • eukaryotic cells started to use their cytoskeletal elements to engulf aerobic bacteria, which then lived symbiotically within their eukaryotic host. • The eukaryote supplied the bacterium with protection and carbon compounds, which the bacterium oxidized. Then the bacterium supplied the host cell with ATP. • The host cell, in contrast, is proposed to be a predator capable only of anaerobic fermentation.

25. The Mitochondrion • Observations consistent with the endosymbiosis theory include the following: • Mitochondria are about the size of an average bacterium and replicate by fission, as do bacteria. • Mitochondria have their own ribosomes to manufacture their own proteins. • Mitochondria have double membranes, consistent with the engulfing mechanism. • Mitochondria have their own genomes with genes that code for the enzymes needed to replicate and transcribe their own genomes.

26. The Mitochondrion • Phylogenetic data support the endosymbiosis theory. • Mitochondrial gene sequences are much more closely related to the sequences from α-proteobacteria (an endosymbiotic bacteria) than to sequences from the nuclear DNA of eukaryotes.

27. Protist Structures for Support and Protection • The basic structure of the cytoskeleton does not vary much among protists. • What does vary: the presence and nature of other structures that provide support and protection for the cell: • Some protists have cell walls / hard external structures called a shell / rigid structures inside the plasma membrane. • In many cases, the diversification of protists has been associated with the evolution of innovative structures for support and protection.

28. Synapomorphic Supportive and Protective Structures • Diatoms are surrounded by a glass-like cell wall. • Dinoflagellates have a cell wall made up of cellulose plates. • Some lineages within Foraminifera secrete an intricate, chambered calcium carbonate shell. Other Foraminifera and some amoebae cover themselves with tiny pebbles.

29. Synapomorphic Supportive and Protective Structures • The parabasalids have a unique internal support rod. • The euglenids have a collection of protein strips located just under the plasma membrane. • The alveolates have distinctive sac-like structures called alveoli that help stiffen the cell.

30. Multicellularity: key morphological innovation • Multicellularity arose independently in a wide array of eukaryotic lineages: the green plants, fungi, animals, brown algae, slime molds, and red algae.

31. How Do Protists Find Food? • One of the most important stories in the diversification of protists was the evolution of novel methods for finding food. • Many protists ingest their food—they eat bacteria, archaea, or even other protists whole. • When ingestive feeding occurs, an individual takes in packets of food much larger than individual molecules. Thus, protists feed by: • Ingesting packets of food. • Absorbing organic molecules directly from the environment. • Performing photosynthesis.

32. Ingestive Feeding • Many protists are large enough to surround and ingest bacteria and archaea; some protists are large enough to ingest other protists or microscopic animals. • Feeding by engulfing is possible in protists that lack a cell wall. • A flexible membrane and dynamic cytoskeleton give these species the ability to surround and swallow prey with long, fingerlike projections called pseudopodia. • Some protists also feed by sweeping food particles into their mouth with cilia.

34. Absorptive Feeding • nutrients are taken up directly from the environment, across the plasma membrane. • Some protists that live by absorptive feeding are decomposers, feeding on dead organic matter, or detritus. • Many other protists are parasites that live inside other organisms and absorb their nutrition directly from the environment inside their host, causing damage to the host.

35. Photosynthesis and the Endosymbiosis Theory • The endosymbiosis theory contends that • the chloroplast (photosynthesis) originated when a protist engulfed a cyanobacterium. Once inside the protist, the photosynthetic bacterium provided its eukaryotic host with oxygen and glucose in exchange for protection and access to light. • The evidence for an endosymbiotic origin for the chloroplast is even more persuasive than that for mitochondria.

36. Secondary Endosymbiosis • Secondaryendosymbiosis occurs when an organism engulfs a photosynthetic eukaryotic cell and retains its chloroplasts as intracellular symbionts. • The chloroplasts from secondary endosymbiosis are surrounded by four membranes instead of two.

37. Photosynthesis • Phylogenetic analysis shows that photosynthesis arose in protists by primary endosymbiosis, then spread among lineages via secondary endosymbiosis. • The major photosynthetic groups of protists are distinguished by the pigments they contain. • A particular photosynthetic pigment absorbs specific wavelengths of light. Different pigments indicate that different species absorb different wavelengths in the electromagnetic spectrum. • The diversity of pigments allowed eukaryotic species to harvest unique wavelengths of light and avoid competition.

39. How Do Protists Move? • Many protists actively move to find food or light. • Amoeboid motionis a sliding movement observed in some protists that is accomplished by streaming of pseudopodia. • The other major mode of locomotion involves swimming via flagella or cilia. • Flagella and cilia have identical structures, but flagella are long and are usually found alone or in pairs, whereas cilia are short and numerous.

42. How Do Protists Reproduce? • Sexual reproduction: offspring that are genetically different from their parents. • Asexual reproduction: offspring that are genetically identical to the parent. • Most protists undergo asexual reproduction routinely. Many protists undergo sexual reproduction only intermittently. • The evolution of sexual reproduction ranks among the most significant evolutionary innovations observed in eukaryotes.

43. Sexual versus Asexual Reproduction • The genotypes of many parasites and pathogens evolve very quickly, so natural selection favors host individuals with new genotypes that may be able to resist them. • Sexual reproduction provides individuals with many new combinations of genes. • Many biologists view sexual reproduction as an adaptation to fight disease.

44. Life Cycles—Haploid- versus Diploid-Dominated • A life cycle describes the sequence of events that occur as individuals grow, mature, and reproduce. • To analyze a life cycle, start with fertilization—the fusion of two gametes to form a diploid zygote. Then trace what happens to the zygote.

48. Life Cycles—Alternation of Generations • Many multicellular protists have one phase of their life cycle that is based on a haploid form and another that is based on a diploid form. This phenomenon is known as alternation of generations. • The multicellular haploid form is called a gametophyte, because specialized cells in this form produce gametes by mitosis. • The multicellular diploid form is a sporophyte, because it has specialized cells that undergo meiosis to produce haploid cells called spores.

49. Life Cycles—Alternation of Generations • A spore is a single cell that develops into an adult organism but is not a product of fusion by gametes. • When alternation of generations occurs, a spore divides by mitosis to form a haploid, multicellular gametophyte. • The haploid gametes produced by the gametophyte then fuse to form a diploid zygote that grows into the diploid, multicellular sporophyte.