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Microbial Origins of Life and Energy Conversions

Biol 251

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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…

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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

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Denitrification – ONLY by prokaryotes Microbial conversion of various nitrogen salts back to

atmospheric N2

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Nitrogen Cycle

www.textbookofbacteriology.net

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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

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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

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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

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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