Chapter 27- Prokaryotes and the Origin of Metabolism  Key Points: 1) The evolution of Prokaryotic metabolism was both cause and effect of changing environments on Earth 2) Molecular Systematics is leading to phylogenetic classification of prokaryotes 3) Prokaryotes are indispensable links in the recycling of chemical elements in ecosystems 4) Many Prokaryotes are symbiotic 5) Humans use Prokaryotes in research and technology

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.

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

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

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

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

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

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

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

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

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

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.

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.

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.

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.

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

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”

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.

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.

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

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.

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

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

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

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