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Chapter 19

Microbial Taxonomy, Evolution and Diversity

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Microbial Evolution

• Planet earth is estimated to be 4.5 – 4.6 billion years old

• First direct evidence of cellular life discovered in 1977 in the Swartkoppie chert.– microbial fossils ~3.4 billion years old

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Figure 19.1

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The First Self Replicating Entity: The RNA World

• Pre-cellular life may have been an RNA world because of the capacity of RNA to both replicate and catalyze chemical reactions ((ribozymes)

• RNA could have given rise to structurally similar double stranded DNA

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More Evidence supporting the RNA World

• The energy currency of the cell is ATP, a ribonucleotide

• RNA can play a role in gene expression

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Evidence used against the RNA World hypothesis

• The early hot, anoxic atmosphere on earth would prevent the stable formation of RNA precursors

• RNA is not a stable molecule

• Ribozyme involved in RNA replication has not been found

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Early Cellular Life

• FeS-based metabolism used by some archaea may be remnant of early form of chemiosmosis

• Photosynthesis also thought to have evolved early in Earth’s history

– fossil evidence places evolution of cyanobacteria and oxygenic photosynthesis to ~3 billion years ago

– what appear to be fossilized remains found in stromatolites and sedimentary rocks

• stromatolites – layered rocks formed by incorporation of mineral sediments into microbial mat

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Figure 19.2

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The Three Domains of Life

• Carl Woese and George Fox using the nucleotide sequence of the small subuit ribosomal RNAs (rRNAs) determined that all living organisms belong to one of three domains– Archaea– Bacteria– Eucarya

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Table 19.1

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The Evolutionary History of Microbes

• The universal phylogenetic tree– based on the rRNA sequence from three

domains of life– evolutionary relationships based on rRNA

sequence comparisons– the root of the tree suggests that the three

domains have a single common ancestor, but Archaea and Eucarya evolved independently of the Bacteria

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Figure 19.3

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Genome Fusion Hypothesis

• Attempts to explain evolution of the nucleus

• Claims the combining of certain archaeal and bacterial genes resulted in the formation of a single eucaryotic genome

• Origin of nucleus is still unresolved

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The Endosymbiosis Hypothesis

• Claims that endosymbiosis was responsible for the origin of mitochondria and chloroplasts

– both organelles have bacteria-like ribosomes

– most have a circular chromosome

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Mitochondria

• Believed to be descended from an proteobacterium– became engulfed in a precursor cell– provided essential function for host

• engulfed organism thought to be aerobic, thereby eliminating oxygen toxicity to the host cell

• host provided nutrients and a safe environment for engulfed organism

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More About Mitochondria and Chloroplasts

• Engulfed organisms– endosymbionts which evolved into

mitochrondria

• Chloroplasts are also thought to have evolved from endosymbionts in a similar process

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Hydrogen Hypothesis

Another endosymbiosis Theory

• Asserts that the -proteobacterium endosymbiont was an anaerobic bacterium that produced H2 and CO2 as fermentation end products

– hosts lacking external H2 source became dependent on endosymbiont which made ATP by substrate level phosphorylation

– symbiont ultimately evolved into a mitochondrion or a hydrogenosome (organelle found in protists that produce ATP by fermentation)

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Hydrogenosomes of Trichomonas vaginalis

Figure 19.4

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Serial Endosymbiotic Theory (SET)

• Put forth by Lynn Margulis and colleagues

• Suggests eucaryotes evolved in a series of discrete endosymbiotic steps starting with motility and followed by nuclei and mitochondria

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Evolutionary Processes

• Anagenesis (microevolution)– small, random genetic changes over

generations which slowly drive either speciation or extinction, both of which are forms of macroevolution

• Punctuate equilibria– a phenomenon caused by the slow, steady

pace of evolution being periodically interrupted by rapid bursts of speciation due to abrupt environmental changes

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Figure 19.5