Microbial Evolution A4.docx

Microbial Evolution Microbial Life on Early Earth Earth formed about 4.5 BYA based on analysis of slowly decaying radioactive isotopes. Intense heat and absence of liquid water made Earth devoid of life. Constant striking of comets and asteroids contributed to sterility of Earth. Earth was anoxic, 2...

Microbial Evolution Microbial Life on Early Earth Earth formed about 4.5 BYA based on analysis of slowly decaying radioactive isotopes. Intense heat and absence of liquid water made Earth devoid of life. Constant striking of comets and asteroids contributed to sterility of Earth. Earth was anoxic, 2.5 BYA, because there was no ozone layer. This was before the start of the great oxidation event. The oxidation rise happened during the start of the oxidation event. Liquid water is a requirement to life. Water on Earth originated from volcanic outgassing of the planet’s interior from the innumerable collisions with icy comets and asteroids. In the early years when Earth was incredibly hot, water was present as water vapor due to Earth’s high temperatures. Jack Hills Zircon (ZrSiO4) Oldest known mineral formed in the Hadean eon. Rocks undergo (4.4 billion years) metamorphosis. Analysis of ancient zircon crystals provide evidence for a solid crust and liquid water as early as 4.3 BYA. Zircons are resistant to chemical changes and are default standard for rock dating. Earth’s oldest surviving sedimentary rocks (formed underwater), found in south-western Greenland, dating 3.86 billion years old. Unusual chemical traces in these rocks may suggest that life existed when they formed. Origin of Cellular Life Subsurface Hypothesis life originated in the ocean floor. Mineral pores form the first biological compartments. Pores allow the coupling of energetic reactants to molecular replication. Liquid bilayers replace pores, forming the first true cells. Evidence: hydrothermal vents containing present nutrients. Submarine mounds and their possible link to the origin of life. Deep sea hydrothermal vents are likely a site for the origin of life on Earth. They are stable environments conducive to life and they provide a source of chemical energy, precursors required for the formation of biological molecules, and mineral pores that can serve as compartments to house pre-cellular biochemical reactions. (a) Precursor molecules form abiotically and accumulate within mineral pores. (b) Chemical gradients that form within mineral pores provide a source of energy that drives the replication of pre-cellular biological molecules. (c) Membranes eventually take over the role of mineral compartments, leading to the formation of the first cells. (d) Hot, mineral rich hydrothermal fluid mixes with cooler, more oxidized ocean water, forming precipitates of Fe and S compounds, clays, silicates, and carbonates. These mineral precipitates form pores that could have served as energy rich compartments to facilitate the evolution of precellular forms of life. Surface Hypothesis was not possible because the surface of the Earth was too hot for life. Gene-First/ RNA World Hypothesis Genes came first, followed by enzymes, and finally cells. Started with self-replicating RNA, followed by the appearance of enzymes a little later, creating a primitive form of the current genetic transcription mechanism with RNA, and finally, cells appeared to provide it all with the necessary physical cohesion. Suggest that life began with the emergence of RNA molecules that had two critical capacities: an ability for template-based self-replication and an ability to catalyze other reactions (ribozymes). Events hypothesized to precede the origin of cellular life. The earliest self-replicating biological systems may have been based on catalytic RNA. AT some point, RNA enzymes evolved the capability to synthesize proteins , and proteins became the main catalytic molecules. Conversion from RNA to DNA based genomes required the evolution of DNA and of RNA polymerase. The lipid bilayer is the site of electron transport, and the evolution of this structure was likely important for energy conservation, in addition to containing and protecting biomolecules. The last universal common ancestor which preceded the divergence of Bacteria and Archaea was a cellular organism that had a lipid bilayer and used DNA, RNA, and protein. Horizontal gene transfer may have allowed rapid transfer of beneficial genes among early forms of life. Metabolism-First Hypothesis The formation of cells came first, followed by enzymes, and finally genes. The environments on early Earth aided chemical reactions that resulted in the formation of more complex organic molecules from more simpler inorganic precursors. Proto-metabolisms in the form of organizations of molecular species emerged and evolved first without any involvement of self-replicating RNA. Classification of Organisms based on Metabolism. Early Life Forms Anaerobes Thermophilic chemolithotrophs 4H2 + CO2 -> 2H2O Hyperthermophiles grow at 80C – 113C with pH range of 0-9.0. Thermoproteus found in boiling muds near active volcanoes. Methanopyrus found in hot vent chimney rock. Archaeoglobus and Pyrococcus found in deep subterranean fluids under the North Sea. Last Universal Common Ancestor Likely existed at 3.7-3.8 BYA, the point at which Bacteria and Archaea diverged. 355 genes trace back LUCA. genes indicate that LUCA was an anaerobic organism. Lived in a high temperature environment ich in sulfur and iron and used H2 as a source of energy and CO2 as carbon source. LUCA could have lived in a hydrothermal environment. 3 Domain Classification based on the 16s RNA genome sequences Bacteria, Eukarya, Archaea Small subunit rRNA made the phylogenetic tree possible. Hyperthermophiles were the first to evolve. Great Oxidation Event Happened around 2.4 BYA. Anoxygenic bacterial photosynthesis involving purple and green bacteria where H2S was the electron source. Cyanobacteria with oxygenic photosynthesis produced the aerobic atmosphere called “The Great Oxidation Event”. Oxygenic Photosynthesis 6CO2 + 12H2O + light energy C6H12O6 + 6O2 + 6H2O Anoxygenic Photosynthesis CO2 + 2H2A + light energy [CH2O] + 2A + H2O The metabolism of cyanobacteria yielded O2 that oxidized reduced minerals containing Fe2+ to iron oxidizes containing Fe3+. Generation of Ozone (O3) – absorbs UV light and forms a protective layer in the atmosphere. Fossil Evidence Fossils - important for understanding the evolution of plants and animals; some microorganisms fossilize too. Stromatolites fossilized microbial formations. layered rocks: are formed when microbes cause the deposition of carbonate or silicate minerals that promote fossilization. Diverse and common on earth between 2.8 and 1 bya. Phototrophic bacteria, such as cyanobacteria, and anoxygenic phototrophs form modern stromatolites. Aggregates found in the Archean Apex chert of Western Australia revealed cell-like structures. Cyanobacterial trichomes reported to be 3.5 billion years old. 11 taxa of filamentous, dark brown to black carbonaceous microfossils have been identified in the deposit. The inability to demonstrate appropriate biomarkers in the microfossils has generated concern about the dating of these images. Biomarkers Presence of hopanes and steranes, derivatives of hopanoids and sterols, respectively found in the plasma membrane. Hopanoids: prokaryotes Sterols – eukaryotes; molecular O2 is required for the final enzyme steps. Detections of DNA sequences. Genome Fusion Horizontal Gene Transfer occurs through genome fusion between different species when two symbiotic organisms become endosymbiotic. Energy production and chemistry of lipids in cell membrane of eukaryotes more similar to Bacteria. Primitive Microbial Interactions Endosymbiotic Theory The organelles of eukaryotic cells arose from prokaryotic cells that had developed a symbiotic relationship with the eukaryote-to-be. Symbiosis is a relationship between two different kinds of organisms that live in close contact. IF one lives inside the other, the relationship is known as endosymbiosis. Serial endosymbiosis hypothesis Eukaryotic ancestor diverged from archaeal line and possessed a nucleus prior to endosymbiosis with bacterial ancestor of the mitochondrion. Another endosymbiosis with the cyanobacterial ancestor of the chloroplast gave rise to ancestors of plants and photosynthetic eukaryotes. Symbiogenesis hypothesis Eukaryotic cell evolved from a symbiotic relationship between Archaea and Bacteria Bacteria was engulfed by archaeal partner and evolved over time into the mitochondrion. The nucleus evolved after establishment of endosymbiosis. A later endosymbiosis with cyanobacteria ancestor of chloroplast gave rise to eukaryotic ancestor of plants. Genome Fusion HGT (Horizontal Gene Transfer) occurs through genome fusion between different species between two symbiotic organisms become endosymbiotic. Energy production and chemistry of lipids in cell membrane of eukaryotes more similar to Bacteria. Transcription and translation have characteristics of Archaea.