The Origin and Evolution of Microbial Life: Prokaryotes and Protists Chapter.16 Introduction: How Ancient Bacteria Changed the World • Virtually all metabolic pathways on Earth evolved in prokaryotic cells, before the evolution of eukaryotes • The products generated by prokaryotic metabolism changed the Earth’s atmosphere and rocks • Fossilized stromatolites from 3 billion years ago contain the fossils of photosynthetic cyanobacteria – These bacteria produced O2 and made Earth’s atmosphere aerobic Copyright © 2009 Pearson Education, Inc. PROKARYOTES Prokaryotes are diverse and widespread • Prokaryotes lived alone on Earth for over 1 billion years – They remain the most numerous and widespread organisms on Earth – The total biomass of prokaryotes is ten times that of eukaryotes – Most prokaryotes are 1–5 µm in diameter (vs. 10–100 µm for eukaryotic cells) – More prokaryotes live in your mouth than the total number of humans that have ever lived – There are ten times as many prokaryotes living in and on your body as the number of cells in your body Copyright © 2009 Pearson Education, Inc. Prokaryotes are diverse and widespread • Prokaryotes live in cold, hot, salty, acidic, and alkaline habitats • Although some bacteria are pathogenic (pathogen) and cause disease, most bacteria on our bodies are benign or beneficial ! Several hundred species of bacteria live in and on our bodies, • decomposing dead skin cells, • supplying essential vitamins, and • guarding against pathogenic organisms. • Prokaryotes in soil decompose dead organisms, sustaining chemical cycles Copyright © 2009 Pearson Education, Inc. Bacteria and archaea • The two prokaryotic domains, Bacteria and Archaea, diverged soon after life on Earth arose • Present day Archaea and Eukarya evolved from a common ancestor, complicated by gene transfer between prokaryotic lineages • Some genes of Archaea are similar to bacterial genes, some are similar to eukaryotic genes, and some are unique to Archaea External features ! Prokaryotic cells have three common cell shapes. • Cocci are spherical prokaryotic cells. They sometimes occur in chains that are called streptococci. • Bacilli are rod-shaped prokaryotes. Bacilli may also be threadlike, or filamentous. • Spiral prokaryotes are like a corkscrew. – Short and rigid prokaryotes are called spirilla. – Longer, more flexible cells are called spirochetes. 7 Cocci Bacilli Spirochete 8 External features ! Nearly all prokaryotes have a cell wall. Cell walls • provide physical protection and • prevent the cell from bursting in a hypotonic environment. ! When stained with Gram stain, cell walls of bacteria are either • Gram-positive, with simpler cell walls containing peptidoglycan – Penicillin class antibiotics for Gram positive • Gram-negative, with less peptidoglycan, and more complex and more likely to cause disease. • More toxic due to lipid molecule functioning as a toxin ! The cell wall of many prokaryotes is covered by a capsule, a sticky layer of polysaccharides or protein. ! The capsule • enables prokaryotes to adhere to their substrate or to other individuals in a colony and • shields pathogenic prokaryotes from attacks by a hosts immune system. 9 LPS : endotoxin External features ! Some prokaryotes have external structures that extend beyond the cell wall. • Flagella help prokaryotes move in their environment. • Hairlike projections called fimbriae enable prokaryotes to stick to their substrate or each other. 11 Rapid adaptation in the environment ! Prokaryote population growth • occurs by binary fission, • can rapidly produce a new generation within hours, and • can generate a great deal of genetic variation – by spontaneous mutations, – increasing the likelihood that some members of the population will survive changes in the environment. ! The genome of a prokaryote typically • has about one-thousandth as much DNA as a eukaryotic genome and • is one long, circular chromosome packed into a distinct region of the cell. ! Many prokaryotes also have additional small, circular DNA molecules called plasmids, which replicate independently of the chromosome. 12 13 15 Various structural features contribute to the success of prokaryotes • Some prokaryotes stick to the substrate or each other with hair-like appendages called pili – Sex pili join prokaryotes during conjugation • Transfer DNA • The flagella of Bacteria and Archaea allow them to move in response to chemical and physical signals in their environment • The prokaryotic flagellum is a naked protein without microtubules – The flagellum rotates like a propeller • Are prokaryotic flagella homologous or analogous to eukaryotic flagella? • Fast cell division : 2n – A single prokaryote could give rise to a colony outweighing Earth in only three days! Flagellum Plasma membrane Cell wall Rotary movement of each flagellum Various structural features contribute to the success of prokaryotes • Some prokaryotes can withstand harsh conditions by forming endospores within an outer cell – The endospore has a thick protective coat – It can dehydrate and is tolerant of extreme heat or cold – Hard to kill the most of these resistant cells by boiling – Autoclave : 121C with high pressure steam – High gamma radiation (44kGy) for space food • When conditions improve, the endospore absorbs water and resumes growth, sometimes after centuries Copyright © 2009 Pearson Education, Inc. Endospore 20 21 22 23 Various structural features contribute to the success of prokaryotes • Some prokaryotic cells have specialized membranes that perform metabolic functions – Aerobic prokaryotes carry out cellular respiration on infoldings of the plasma membrane – Cyanobacteria carry out photosynthesis on infolded thylakoid membranes • Prokaryotic DNA forms a circular chromosome – Smaller rings of DNA called plasmids carry genes that may provide resistance to antibiotics or metabolize rare nutrients, among other metabolic activities • Many prokaryotes can transfer genes, such as antibiotic resistance genes, within or between species Copyright © 2009 Pearson Education, Inc. Respiratory membrane Thylakoid membrane Prokaryotes obtain nourishment in a variety of ways • Nutrition diversity of prokaryote :diverse Energy and carbon source – Source of Energy • Sunlight: phototrophic • Chemicals (inorganic or organics): chemotrophic ! Two sources of carbon are used by prokaryotes. • Autotrophs obtain carbon atoms from carbon dioxide. • Heterotrophs obtain their carbon atoms from the organic compounds present in other organisms. • Mode of nutrition – Photo-autotrophs, Photo-heterotrophs, Chemoautotrophs, Chemo-heterotrophs ENERGY SOURCE Sunlight Chemicals Photoautotrophs Chemoautotrophs 2 CO Oscilliatoria Unidentified rock-eating bacteria Photoheterotrophs Chemoheterotrophs CARBONSOURCE Rhodopseudomonas A Bdellovibrio attacking a Organic compounds larger cell Biofilm • In some prokaryotes, metabolic cooperation occurs in surface-coating colonies called biofilms – Biofilms form when cells in a colony secrete signaling molecules that recruit nearby cells – Channels in the biofilm allow nutrients and wastes to move inside and outside the biofilm • Biofilms cause ear and urinary tract infections and the dental plaque that produces tooth decay (ex. Dental plaque) ! Prokaryotes attach to surfaces and form biofilm communities that • are difficult to eradicate and • may cause medical and environmental problems. ! Biofilms are large and complex “cities” of microbes that • communicate by chemical signals, • coordinate a division of labor and defense against invaders, and • use channels to distribute nutrients and collect wastes. ! Biofilms • clog and corrode pipes, • gum up filters and drains, and • Coat the hulls of ships. 28 29 30 Prokaryotes help clean up the environment ! Prokaryotes are useful for cleaning up contaminants in the environment because prokaryotes • have great nutritional diversity, • are quickly adaptable, and • can form biofilms. ! Bioremediation is the use of organisms to remove pollutants from soil, air, or water. ! Prokaryotic decomposers are the mainstays of sewage treatment facilities. • Raw sewage is first passed through a series of screens and shredders. • Solid matter then settles out from the liquid waste, forming sludge. • Sludge is gradually added to a culture of anaerobic prokaryotes, including bacteria and archaea. • The microbes decompose the organic matter into material that can be placed in a landfill or used as fertilizer. 31 Biomediation • Bioremediation is becoming an important tool for cleaning up toxic chemicals released into the soil and water by industrial processes. • Environmental engineers change the natural environment to accelerate the activity of naturally occurring prokaryotes capable of metabolizing pollutants. 32 33 Archeae ! New studies of representative genomes of prokaryotes and eukaryotes strongly support the three-domain view of life. • Prokaryotes are now classified into two domains: – Bacteria and – Archaea. • Archaea have at least as much in common with eukaryotes as they do with bacteria. 34 35 Archaea thrive in extreme environments—and in other habitats • Archaea are among the most abundant cells on Earth • They are a major life-form in the oceans – Extreme halophiles • Salt-loving archaes, ex. Halobacterium halobium – Extreme theromophiles • Heat-loving archaes, ex. Sulfolobus – Taq polymerase (from thermophilic bacterium, Themus aquaticus) vs Pfu DNA polymerase (from hyperthermophilc archaeon, Pyrococcus furious) – Methanogens • Live in anaerobic environments and