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1
Microbial Origins of Life and Energy Conversions
Biol 251
2
Terms to Know for this Lecture
Science – Questioning
Religion - Believing
Fact – What most experts agree on…often becomes dogma (essentially the
truth)
Truth – What is… does anyone really know what truth is? Inherent bias…
3
The universe was created sometime about 13.5 billion years ago from a cosmic explosion that hurled matter and in all directions (the “big bang”)
The Earth is thought to have
formed about 4.5-4.6 billion
years ago
The “Big Bang” and Earth
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Geologic Time….Oldest sedimentary rocks, Greenland 3.8 bya
3.5 bya anaerobic prokaryoteslithotrophic and orfermentative
2.4 bya origin of eukaryotic cells
2.3 bya photosynthetic cyanobacteria
CH4 dominated environment
O2 accumulatesrapidly
Atmosphere is warm[CH4] [CO2] [H2O]
Atmosphere is cold
1.7 byaWestern AustraliaSouth Africa
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Origin of Early Life 3.8 byaThe primitive Earth was hot (>100°C), anaerobic with warm oceansSimple organic molecules formed from atmospheric gases (CO2, NH3, H2S, CH4, HCN and CO) and dissolved in the oceansLightning, heat & UV light - energySimple macromolecules:
sugars, amino acids, nucleotides, lipids
How do simple organic molecules form a protocell?Spontaneous generation?
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Experimental Results
Laboratory experiments that attempt to address how cells developed
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Primordial Soup Experiment 1953Replicate environmental conditions
of prebiotic timesAtmosphere
H2O, H2, CH4 & NH3
Organic compounds Amino acids
This experiment has been modified over the last 50 years and has yielded all 20 amino acids, nucleotides, lipids, sugars and ATP
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Protocells All cells have a outer plasma membrane Protocells
3.8 bya Simple membrane bound sacs
Created simple membranes under laboratory conditions Fatty acids & alcohols Bubble hypothesis Proteinoids
“protein-like” molecules that are produced when amino acid solutions are heated
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What was hereditary material of early organisms? - RNA
Genetic & enzymatic components of early cells were probably RNA
Lab experiments have producedRNARibozymes
Enzymatic RNA molecule that catalyzes reactions during RNA splicing
Clay can concentrate charged moleculesCatalysis of polymers
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Classification and naming of bacteria by how they derive energy and carbon
4 parts to name
1. How they get energy (chemo- versus photo-)
2. Where they get electrons from? Organic versus inorganic molecules (organo- versus litho- (rock eater)
3. Where do they get their carbon from? Auto- (CO2) versus hetero- (organic carbon source- e.g., glucose)
4. Add troph…
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What were earliest organisms (bacteria?) like metabolically?
Aerobic versus anaerobic?
Photo- versus chemotrophic?
Litho- versus organotrophic?
Auto- versus heterotrophic?
Optimal growth temperature…Psychrophile: <15° CMesophile: From 20 to 40° CModerate Thermophiles: 40 to 80° CHyperthermophiles: > 80°C
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Stromatolites Banded domes of sedimentary rock similar to layered mats of heterotrophic bacteria & cyanobacteria
Stromatolites in western Australia 3.5 billion years oldmicroscopic resemblance to photosynthetic organisms
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The Origin of Prokaryotes
Fossils of microbes dating from 950 mya Palaeolyngbya from Shale in Siberia
Divisions reminiscent of membranes or cell walls
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When did eukaryotes arise?
Sterols, including cholesterol have been found in oil droplets within quartz crystals
Sterols are produced almost exclusively by eukaryotes
Quartz is dated at 2.4 byaPredates the “Great Oxidation Event”
O2 production by cyanobacteria
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The oldest eukaryotic fossils are 2.1 bya
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How did organelles develop?
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Theory of Endosymbiosis page 125
Symbiotic relationship between two microorganisms, in which one is living inside the otherChloroplast
Cyanobacterium engulfed by larger organismPhotosynthesis provided carbohydrates &
produced O2
Protected habitat for the cyanobacteriumBoth organisms benefit - mutualismRelationship became obligatory
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Evidence for Endosymbiosis
Modern chloroplastsCircular chromosome with prokaryotic-
like genes Independent divisionProkaryotic ribosomesHas 16s rRNA gene in genome and 16s
rRNA molecule in ribosome
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Endosymbiosis
MitochondriaCircular chromosome with prokaryotic
like genes Independent divisionProkaryotic like ribosomesProkaryotic like membranesHas 16s rRNA gene in genome and 16s
rRNA molecule in ribosome
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Endosymbiosis
EukaryotesFusion of bacterial & archaeal cellsGenomes fused
Eukaryotic flagella & ciliaConsequence of a spiral bacterium & a
eukaryotic cell
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Relationships that support the Theory of Endosymbiosis
Many protozoans are infected with bacteria in an Endosymbiotic relationship
Many symbiotic relationships between microorganisms in natureLichens
Cyanobacterium or alga with a fungusCyanobacteria are endosymbionts of
plants, various protists and sponges
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Bacteria Archaea Eukarya
Node - LUCALast Universal Common Ancestor
HyperthermophileAnaerobeArchaea or Bacteria?
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Thermotoga maritimaA model for LUCA
Deep sea thermal vents Grows at 90ºC so hyperthermophile (Domain
Bacteria) Anaerobic Heterotroph
Must consume carbon compounds Contains genes that can be classified as…
Bacterial Archaeal
¼ of the genes Eukaryotic
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Deep Sea Hydrothermal Vents
Minerals precipitate out of sea water“Black Smoker” … smoke is precipitate of metal sulfides from H2S
Water temperatures >350°C
Tremendous diversity of marine organisms surrounding thermal vents
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Global Energy Conversions –Microbes Rule the Earth!!
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Microbes comprise nearly half of all biomass on Earth
All habitats that support plants and animals have abundant populations of microorganims.
Microorganisms also exist in habitats too extreme for plants and animals.
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Prokaryotes are the most abundant form of life on Earth
Greatest amount of biomass and total numbers of species
Prokaryotes compose 90 % of the total combined weight of all organisms in the oceans
> 109 bacterial cells are present in 1.0 g of agricultural soil
Outnumber all eukaryotic cells by 10,000 : 1
3,000 species of Bacteria and Archaea are currently recognized
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The main role of microorganisms in the biosphere is to act as catalysts of biogeochemical cycles.
Microorganisms catalyze reactions that cycle C, N, O, P and many other elements.
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BIOGEOCHEMICAL CYCLES
• Elements required for cells are constantly progressing through a cycle involving microorganisms
• Leaf falls from tree• Decomposes• Elements making leaf used by microbes
• Four key elements constitute four primary cycles • CARBON• NITROGEN• SULFUR• PHOSPHORUS
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Carbon Cycle
www.textbookofbacteriology.net
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The Carbon Cycle• Carbon is fixed when photosynthetic organisms fix CO2 into organic compounds• Herbivores consume plants• Carbon from herbivores recycled by four mechanisms
• Exhaled CO2 is used by photosynthetic organisms
• Feces utilized by soil microbes
• Prey for carnivores
• Dead animals are decomposed by soil microbes
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The Carbon Cycle
• Prehistoric decomposed matter was converted into fossil fuels• Burning of fossil fuels generates CO2
• CO2 reenters cycle through photosynthetic plants• Methanogens reduce CO2
anaerobically and give off CH4
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Nitrogen Cycle
N2 gas is the most abundant (~80%) gas in Earth’s atmosphere
Involves several types of microbes 4 types of reactions
Nitrogen fixation – ONLY by prokaryotes N2 gas is converted to NO2
- (nitrite) , NO3- (nitrate) , or
NH3 (ammonium salts) Ammonification
Bacteria degrade of organic compounds to ammonia Nitrification
Convert NH3 to NO2- and NO3
-
Denitrification – ONLY by prokaryotes Microbial conversion of various nitrogen salts back to
atmospheric N2
34
Nitrogen Cycle
www.textbookofbacteriology.net
35
36
Free Living Nitrogen-Fixing Organisms
•NITROGENASE complex – ONLY in prokaryotes!!!!!• Enzyme of nitrogen-fixation• Two protein subunits that work together• Destroyed by O2
• Nitrogenase must be maintained in an anaerobic environment
• Cyanobacteria - fix nitrogen in specialized cells HETEROCYSTS• Provide anaerobic environment required for nitrogenase
• Plant-associated bacteria – many produce nodules• In nodules, plant produces a unique form of hemoglobin called leghemoglobin• This protein binds O2 and “protects” nitrogenase
37
Symbiotic Nitrogen Fixing Organisms
• Rhizobium species - infect roots of legumes (Pea Family of Plants)
• Alfalfa, peas, beans, clover, soybeans, & peanuts
• Attach to root hair, “infection thread” forms
• Bacteria enter through thread and penetrate root cells
• Bacteria differentiate into BACTEROIDS
•Thicker cell walls
•Combination of plant & bacterial cell wall
• Dense cytoplasm
• Do not divide
38
Consume carbohydrates from the plant and fix N2
Synthesize amino acids Causes enlargement of root cells Results in formation of a root
NODULE Bacteria within nodule fix nitrogen
Plant use amino acids
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Rhizobium & BradyrhizobiumSymbiotic association with legume rootsAlfalfaBeansCloverPeas
Nodules
N2 + 8H+ + 8e- + 16 ATP 2NH3 + H2 + 16 ADP + 16 Pi
40
Deamination• After N2 is fixed
• Converted to biologically relevant molecules• Majority of atmospheric nitrogen is incorporated into amino acids
• Plant is consumed and amino acids incorporated into herbivore
• Herbivore excretes waste• Microbes break down proteins into amino acids
• A second set of microbes break amino acids down into ammonia
• DEAMINATION
• Often times NH3 is released into soil • AMMONIFICATION• Highly soluble in moist soil• Available to plants and other microbes
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Nitrification – ONLY in prokaryotes
• Not all NH3 is used by plants• Some moves to next step in cycle
• Some organisms oxidize NH3 to produce nitrite (NO2-)
• NITRIFICATION• Nitrosomonas
• Nitrite is further oxidized to nitrate (NO3-)
• Easily moves through soil via diffusion• Nitrobacter
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Denitrification – ONLY in prokaryotes
• NO3- (nitrate) can be used as terminal electron acceptor
• under anaerobic conditions
• Results in conversion of NO3- to atmospheric nitrogen N2
• Three reactions involved in process
• Completes nitrogen cycleNO3- NO2
- N2O N2