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Prokaryote diversity Level 1 Biological Diversity Jim Provan Campbell: Chapter 27 Prokaryotes are (almost) everywhere Prokaryotes dominate the biosphere: Collective mass of prokaryotes outweighs eukaryotes tenfold More prokaryotes in the average human mouth than all the humans that ever lived Prokaryote diversity: About 5,000 species known Actual numbers believed to be 400,000 to 4 million The three domains of life Domain Bacteria Domain Archaea Domain Eukarya Diversity of prokaryotic cell shapes Prokaryote cell walls Prokaryote cell walls Present in nearly all prokaryotes: Provides physical protection Prevents cells from bursting in hypotonic environment Reason prokaryotes were sometimes grouped with plants in early classification systems Cell wall contains peptidoglycan, rather than cellulose: Gram-negative bacteria generally more pathogenic than gram-positive bacteria: – Lipopolysaccharides can be toxic – Outer membrane less permeable to antibiotics Antibiotics inhibit cross-linking of peptidoglycan Prokaryote organisation Prokaryotes have smaller, simpler genomes than eukaryotes: On average, about one-thousandth as much DNA Concentrated in twisted nucleoid region Single, circular chromosome Also have “accessory” DNA – plasmids General mechanism of DNA replication and protein translation is largely the same as in eukaryotes No organelles, but some have specialised membranes which carry out similar functions Prokaryote organisation Prokaryote reproduction Prokaryotes reproduce only asexually by binary fission: DNA synthesis is almost continuous No mitosis or meiosis Gene transfer is possible: Transformation – take up of genes from surroundings Conjugation – direct transfer of genes between cells Transduction – gene transfer by viruses Mutation is the major source of genetic variation “Growth” is actually increase in numbers, which effectively proceeds geometrically Prokaryote metabolic diversity Mode Energy source Carbon source Example Photoautotroph Light CO2 Cyanobacteria, plants, protists Chemoautotroph Inorganic compounds CO2 Some prokaryotes (Sulfolobus) Photoheterotroph Light Some prokaryotes Chemoheterotroph Organic compounds Organic compounds Organic compounds Prokaryotes, protists, fungi, animals, parasitic plants Proteobacteria a b g d e DOMAIN ARCHAEA DOMAIN EUKARYA Crenarchaeota DOMAIN BACTERIA Euryarchaeota Cyanobacteria Gram positive bacteria Spirochetes Chlamydias A survey of prokaryote diversity The three domains of life DOMAIN CHARACTERISTIC Bacteria Archaea Eukarya Nuclear envelope x x Organelles x x Peptidoglycan cell wall x x Membrane lipids Unbranched Some branched Unbranched RNA polymerase One Several Several Initiator amino acid f-Met Met Met x Inhibited Not inhibited Not inhibited x Introns Antibiotic response Histones Domain Archaea Inhabit most extreme environments: Methanogens - strict anaerobes which use H2 to reduce CO2 to methane (CH4) Extreme halophiles - live in saline environments five to ten times saltier than sea water Extreme thermophiles - live in very hot environments such as sulphur springs of deep-sea hydrothermal vents Domain Bacteria: Proteobacteria Domain Bacteria: others Ecological impact of prokaryotes Prokaryotes are crucial components of ecosystems: Decomposition of waste products Recycling of elements from inorganic sources (soil, air etc.) Many prokaryotes are symbiotic: Rhizobium – N2 fixation Bioluminescence Some prokaryotes are pathogenic Pathogenic prokaryotes cause half of all human disease Some pathogens are opportunistic: they cause illness when the host’s defences are weakened Mostly produce toxins: Exotoxins secreted by prokaryotes e.g. in food or water Endotoxins are components of bacterial membranes Humans use prokaryotes Used as model systems to study genetics e.g. Escherichia coli Used in bioremediation: Sewage treatment Decomposition of oil spills by soil bacteria Metabolic “factories”: Organic solvents Antibiotic production Food industry