Chapter 27 : Prokaryotes and Metabolic Diversity

 

 

 

Kingdom Monera: 3.5 billion years of evolutionary success!

 

 

Fig. 26.1  Important Dates in Moneran Evolution--recap

 

4.6 billion years ago              Origin of the earth

 

3.5 billion years old               First confirmed fossils, prokaryotic endospores (resistant

                                               cell formed under adverse conditions)

 

4.0 - 3.5 b.y.a                               Stromatolites resemble dense mats of cyanobacteria

                                               (photosyn. prokaryotes) forming rock layers (Fig. 26.4)

 

3.4 - 2.5 b.y.a                        cyanobacteria begin evolving oxygen, initially absorbed

                                               by dissolved iron ions in oceans and deposited as iron oxide

                                               to form banded iron deposits (Fig. 26.5)

 

2.5 b.y.a.                               Oxygen begins accumulating in atmosphere

 

2.1 b.y.a.                               Oldest protist (eukaryote) fossils found ("acritarchs")

 

 

somewhere in this first billion years, “life” evolved relatively fast

 

Theories still controversial

1.   Panspermia-theory of meteor bombardment (organics found in actual samples; amino acids which form vesicles with water

2.   Molecular evolution (p. 518)

1.   maybe both!

 

Characteristics of Life:

·     reproduces (self-replicating)-excludes viruses, prions

·     consumes energy, metabolizing organic molecules

·     responds to external stimuli (excitability, tropisms)

 

 

unique features of Monerans

·     first form of life on earth, found everywhere (rarely noticed, but huge impact)

·     may be unicellular, aggregates, colonies, even multicellular

·     relatively small (1-5 um per cell vs. 10-100 um for Eukaryotes, mycoplasmas even smaller at 0.1-0.25 um)

·     lack membrane-enclosed organelles (theorized to be precursors to organelles in euks)

·     specialized regions of plasmalemma for respiration and photosynthesis

·     ribosomes smaller and different enough to allow antibiotics (streptomycin, tetracycline) to inhibit prokaryotic but not eukaryotic protein synthesis

·     different genetic replication and asexual binary fission vs. mitosis

new DNA synthesized continuously under favorable conditions

geometric growth, generations 0.5-3 hours apart

 

·     simpler, smaller genomes ( app. 1/1000 the size of eukaryote cells)

nucleoid region vs. nucleus

genophore (one DNA ring) vs. chromosomes

small separate plasmids carry genes for specific traits eg. antibiotic resistance, metabolic abilities; replicate independently of genophore

·     almost all have cell walls (only Mycoplasmas lack cell walls)

 

functions of cell wall:

·     maintains shape

·     affords physical protection

·     prevents bursting  in hypoosmotic env. (may still plasmolyze and die in hypertonic sol’n)

·     cell walls differ in composition and construction from algae and plants, with which they were once grouped

 

Gram Stain (Fig. 27.5) separates two distinct groups of Eubacteria based on amount and location of peptidoglycan in cell wall (polymer of modified sugars X-linked by polypeptides)

 

 a. Gram positives eg. Bacillus, Clostridium, Mycoplasma, Streptomyces

peptidoglycan in outer layer, binds violet stain of Crystal violet

 

b. Gram negatives eg. enterics E. coli and Salmonella

peptidoglycan sandwiched between lipopolysaccharide bilayer, does not bind stain (pink color due to Safranin stain)

 

Penicillin first modern antibiotic

·     product of the fungus Penicillium

·     inhibits x-linking of peptidoglycan (so, more active against Gram + species) see movie clip in “Cells Alive!”

·     prevents encroachment of bacteria on fungus’ food supply

 

actinomyctes (soil bacteria) such as Streptomyces a source of other commercially important antibiotics-take action against ribosomes of competing species in the wild, inhibiting protein synthesis (streptomycin, neomycin, tetracycline, erythromycin)

 

Monerans classified by:

·     cell shape differences--FIG. 27.3) (rods = bacilli; spheres = cocci; spirals = spirilli, spirochetes)

·     colony shape, border, appearance (FIG. 27.9)

·     cell surface properties (see Gram Stain above)

·     physiological tests--metabolism of different substrates; different pH, temp. optima

·     molecular systematics--eg. signature sequences of rRNA, other NA’s

 

motility--about 1/2 capable of directed movement (taxis)-- 3 modes:

1. flagella--most common; not covered by plasma membrane (Fig. 27.7)

2. corkscrew motion (some spirochetes)--effective in viscous media

3. gliding on secreted slime

 

 

sources for genetic variation:

1.   mutation (favored by short life cycles)

2.   transformation--uptake of foreign DNA from env.(especially plasmids)

3.   conjugation--transfer from one bacterium to another (via hairlike pilli)

4.   transduction--viral transmission

 

metabolic diversity: only kingdom exhibiting all four types  (Table, Link 27.1)

 

Mode of Metabolism               Carbon source                     Energy Source

Photoautotrophic                     carbon dioxide                   sunlight

Photoheterotrophic                  organic molecules                 sunlight

Chemoautotrophic                   carbon dioxide               inorganic chems eg. H2S, NH3, Fe

Chemoheterotrophic                organic molecules                 organic molecules

 

majority are chemoheterotrophs:

 

a. saprobes (absorptive decomposers)

b. parasites (absorb nutrients from hosts’ fluids)

c. phagotrophs (phagocytize food particles)

 

Five Kingdom system places all prokaryotes in Kingdom Monera Fig. 26.16)

 

recent evidence supporting separation into Domains at a level above the kingdom (Fig. 27.2)

·     Domain Bacteria = the eubacteria or “true” bacteria

·     Domain Archaea = archaebacteria or “ancient” bacteria

·     Domain Eukarya = all eukaryotes

 

Archaebacteria represented by very few extant genera, found in extreme environments similar to those of early earth  (Table 27.2)

·     molecular systematics supports fundamental difference between Archaea and Eubacteria, closer link to Eukarya -RNA polymerase;  also initiator amino acid for start of protein synthesis; presense of introns in genes; lack of sensitivity to bacterial antibiotics eg. Streptomycin, chloramphenicol ; lack peptidoglycan

·     similarities to Eubacteria--no nuclear envelope, true organelles

 

Archaea  are all extremophiles, divided into three groups  based on their ecology:

1.   methanogens--anaerobic, inhabit swamps, marshes, guts of ruminants and termites convert CO2 + H2 to methane gas (CH4)

2.   extreme halophiles--photosynthetic; bacteriorhodopsin as red PSN pigment

3.   thermoacidophiles--extreme heat (60-80 degrees C) and acidity (pH 2-4) eg. deep sea vents

 

PHYLOGENETICALLY, ARCHEA ARE PLACED IN 2 GROUPS:

  1. EURYARCHEOTA—ALL METHANOGENS AND HALOPHILES
  2. CRENARCHEOTA—MOST THERMOPHILES

 

importance of bacteria

·     most are free-living, nonpathogenic

·     vital decomposers in food webs, returning C and N back to producers, consumers

fix S,N, Fe into organic forms, enriching soils (no eukaryotes can do this!)

 

eg. Anabaena: heterocysts (specialized cells for N fixation)

eg. Rhizobium: N-fixing bacterium, lives inside root nodules of legumes, convert atmospheric N2 to NH3; example of both:

1.   mutualism--(both benefit) share fixed nitrogen, carbohydrates

2.   coevolution--cooperative synthesis of leghemoglobin molecule which binds O2 (genetic info from bacterium)

 

·     first photosynthetic organisms (fix CO2, produce O2 as byproduct) using H2O vs. H2S as electron source

·     cyanobacteria changed earth’s atmosphere from reducing to oxidizing through oxygen production, allowed evolution of aerobic life forms

·     banded iron formations mark period of rapid oxygen evolution (app 2.5 bya)

 

·     important agents of disease--cause app. 1/2 known human diseases

 

a. opportunistic organisms--already present, only cause disease if host’s resistance is low  eg. Streptococcus pneumoniae

b. exotic ie. foreign bacteria

 

modes of disease induction

·     disrupt host physiology--actual growth and invasion of host tissue eg.rickettsias (Rocky Mountain Spotted Fever; typhus);  actinomycetes causing leprosy, tuberculosis (fungus-like, colonial growth)

·     producing toxins:

a.    exotoxins--proteins secreted by cells--some of most potent poisons known eg. Clostridium botulenium (botulism); Vibrio cholerae (cholera); E. coli (“traveler’s diarrhea” from foreign strains)

b. endotoxins--components of outer membranes of gram - bacteria

eg. Salmonella  spp.--typhoid fever, food poisoning (flulike symptoms; fever and aches)