Chapter 27- Prokaryotes and the Origin of Metabolism

1. Chapter 27- Prokaryotes and the Origin of Metabolism • Key Points: • The evolution of Prokaryotic metabolism was both cause and effect of changing environments on Earth • Molecular Systematics is leading to phylogenetic classification of prokaryotes • Prokaryotes are indispensable links in the recycling of chemical elements in ecosystems • Many Prokaryotes are symbiotic • Humans use Prokaryotes in research and technology

2. Evolving metabolism is a cause and effect of changing environments • Nutrition and Metabolic Pathways evolved before Prokaryotes • Met with constantly changing physical and biological environments • The evolved metabolic characteristics, in response to the change, in turn effects the environment. • Studied through molecular systematics, comparisons to other prokaryotes, and geological evidence.

3. Origins of Metabolism • Similar metabolic pathways in prokaryotes indicate that metabolic processes developed early from an ancient ancestor • Glycolysis and ATP The First Prokaryotes that originated 3.5 to 4.0 billion years ago were anaerobes

4. Traditional Evolutionary Hypothesis • Earliest cells were chemoheterotrophs • Absorbed free organic compounds such as ATP • This depleted supply of free ATP • Natural Selection prefers those organism that produce their own ATP • Led to evolution of glycolysis and generation of ATP by substrate phosphorylation

5. Modern Hypothesis • Find it unlikely that early Earth produced as much free ATP to support the chemoheterotrophs • Chemoautotrophs instead of chemoheterotrophs • May have made energy through reacting compounds of iron and hydrogen sulfide

6. FeS + H2S = FeS2 + H2 + free energy • Early source of energy • The free energy was used to split H2 into protons and electrons to establish proton gradient • This gradient must have drive synthesis of ATP • Natural Selection prefers those cells that can manipulate hydrogen and establish electron transport chains

7. Origin of Photosynthesis • In early prokaryotes, light-absorbing pigments may have absorbed excess energy and coupled with membrane proteins involved in ATP synthesis. • Best seen in modern archaea known as extreme halophiles

8. Halophiles • Contain a pigment known as bacteriorhodopsin that absorbs light. • It uses this energy to pump H+ ions across the membrane and generate gradient • Drives the synthesis of ATP • Simplest known form of photophosphorylation

9. Cont. Origins of Photosynthesis • In some prokaryotes, pigments and photosystems evolved to use light to move electrons from H2S to NADP+. • This allows the potential fixation of CO2 • Don’t necessarily produce O2 yet

10. Cyanobacteria • bacteria that could use H20 instead of H2S as a source of electrons. • Also known as blue-green Algae • Their ability to make organic compounds from H20 allows for O2 to be released • Changed our world

11. Cyanobacteria and Oxygen Revolution • Evolved 2.5 and 3.4 years ago • Lived in marine ecosystems • Geology- marine sediments 2.5 billion years old are iron oxide • Thus, infer that O2 only entered the atmosphere when all the dissolved iron was percipitated

13. Origin of Cellular Respiration • Increasing oxygen levels caused extinction of many prokaryotes. • Some prokaryotes evolved into organisms that could tolerate oxygen • Some even used the oxidizing property of O2 to pull electrons down electron chains. • Some bacteria gave up photosynthesis to become solely chemoheterotrophs.

14. Molecular Phylogeny related to the classification of prokaryotes • Researchers first realized that the domains, Archaea and Eukarya are different because each prokaryotic domain has unique signature sequences • Signature sequences are taxon-specific base sequences at comparable locations on the ribosomal RNA or other nucleic acids.

16. Domain Archaea • They inhabit the more extreme environments of Earth • This would require them to adapt to the new environmental situations, and thus may have unique energy metabolism . • Researchers classify Archaea into three groups: methanogens, extreme halophiles, and extreme thermophiles.

17. Archaea- methanogens • Have unique form of metabolism, because they use H2 to reduce CO2 into methane (CH4). • Are Anaerobes that are poisoned by oxygen • Live in swamps and marshes, where microbes have already used up all the oxygen • Important decomposers in sewage treatment • Farmers found that they could use methanogens to convert garbage and dung into methane, which could serve as a valuable fuel. • Some reside in the guts of certain animals, whose diet mainly relies upon cellulose.

18. Archaea- extreme halophiles • Live in saline places such as Great Salt Lake and the Dead Sea. • Some species are tolerant to salinity, while other species actually require high salinity to undergo metabolism • Form a purple scum, a resultant of bacteriorhodopsin

20. Archaea- Extreme thermophiles • Live in hot environments • Temperature in environments is ideally 60 to 80 degrees Celsius • Sulfolobus is an extreme thermophile that thrives in the hot sulfur springs of Yellowstone • Others live at deep-sea hydrothermal vents where water temperature is around 105 degrees Celsius • James Lake of UCLA has a theory that extreme thermophiles are prokaryotes that are most closely related to eukaryotes. • He calls them eocytes, or “dawn cells”

22. Domain Bacteria • Majority of all Prokaryotes • Since Bacteria have diversified very long ago, evolutionary connections between different phylogenetic groups of Bacteria have been hard to distinguish • Through molecular systematics, scientists have studied gene sequences to further subdivide the Bacteria domain • Recently, Domain Bacteria was subdivided into Proteobacteria, Gram-Positive Bacteria, Gram Positive Bacteria, Cyanobacteria, and Spirochetes.

24. Ecological Impact of Prokaryotes- recycling of chemical elements • Basic Principle- atoms in our bodies were parts of the inorganic molecules such as soil, air and water • Therefore, to continue life on Earth, these chemical elements need to be recycled between biological (organic) and chemical (inorganic) parts of ecosystem • Thus, Bacteria such as decomposers are required to convert carbon, nitrogen and other elements essential to life between biological and physical systems.

26. Many prokaryotes are symbiotic • Prokaryotes interact in groups, and rarely by themselves • Symbiosis is the ecological relationship between different species that are in direct contact • Sybionts ae the organisms involved in symbiosis • There are three categories of symbiosis: mutualism, commensalism, and parasitism

27. Evolutionary context of symbiosis • Symbiosis played a major role in prokaryotic evolution and the origin of early eukaryotes • Mitochondria have their own DNA in eukaryotic cells, so scientists believe mitochondria were early prokaryotes with a symbiotic relationship with the cell.

28. Prokaryotes and Disease • For pathogens to affect the body, they must resist all internal defenses of body and harm the host • Opportunistic pathogens are parasitic prokaryotes that normally live inside the host, but cause illness when the host’s immune system is altered • German physician, Robert Koch, proposed the Koch Postulates, which lay the guideless for modern medical microbiology

29. Koch Postulates • To propose that a certain pathogen is related to a certain disease: • Researcher must find the same pathogen in each individual with the same disease • Isolate pathogen from the diseased subject and grow the microbe in a pure culture • Induce disease in Experimental animals by transferring pathogen from the culture to the animal • Isolate the same pathogen from experimental animals after disease/symptoms develops There are certain circumstances where Koch Postulates don’t work

30. Disease- Exotoxins • Exotoxins are proteins secreted by prokaryotes • Produce really strong poisons, such as botulism toxin, where a gram of this sample can kill million people • Linked to Botulism, Cholera, and traveler's diarrhea

32. Disease- Endotoxins • Endotoxins are certain components of the outer membranes of certain bacteria that cause disease • Related to Salmonella.