Sulfur Cycle

• Number of oxidation states
  • -2(sulfhydryl, R-SH, and sulfide, HS^–), 0(elemental sulfur, S⁰), and +6(sulfate,SO₄^2-) are significant in nature
  • Primarily gypsum (CaSO₄) and sulfide minerals (pyrite, FeS₂) constitute bulk of Earth’s S
  • Significant amount of SO₂ gas enters the sulfur cycle from human activities
    • Primarily burning fossil fuels

Hydrogen Sulfide and Sulfate Reaction

• H₂S major volatile sulfur gas
• SO₄^2-+ 4H₂ → H₂S + 2H₂O + 2OH^–
• H₂S exist below pH-7
• Nonvolatile HS^– and  S^2- exist above pH-7
• In marine sediments rate of sulfate reduction is carbon limited and can be greatly increased by an influx of organic matter
• Hydrogen sulfide is toxic to plants and animals
• Sulfur is detoxified in nature by binding with iron for FeS.

Sulfide and Elemental Sulfur Oxidation-Reduction

• Under oxic conditions sulfide oxidizes at neutral pH
• Sulfur-oxidizing chemolithotrophic bacteria(aerobes) can catalyze oxidation of sulfide
• Microbial sulfide oxidation is significant only in areas where H₂S emerges from anoxic environments and meets air
• Elemental Sulfur is chemically stable but is readily oxidized by sulfur-oxidizing chemolithotrophic bacteria.
• S⁰ is insoluble
  • Bacteria that oxidize must attach to S⁰ crystals to obtain their substrate
  • S⁰ forms sulfuric acid and thus S⁰ oxidation characteristically lowers the pH in the environment
• Reduction of S⁰ to sulfide(anaerobic respiration) is major ecological process
• Organic forms of sulfur
  • Dimethyl sulfide

Nitrogen Cycle

• Nitrogen Fixation
  • N₂ is most stable form of Nitrogen
  • N₂ + 8H→ 2NH₃ +H₂
  • Most Nitrogen on earth is “fixed”
    • Combined with other elements NH₃ or NO₃^–

Dentrification

• Reduction of NO₃^– to gaseous nitrogen compounds
  • How N₂ and N₂O are formed
• Can be detrimental
• Aids in wastewater treatment
• Can also contribute to global warming
  • O₃ destruction- increases passage of UV radiation to the surface of Earth

Ammonification

• Ammonia released during decomposition of organic nitrogen compounds
  • Amino acids and nucleotides
• Reduction of NO₃^– to NH₃ (dissimilative reduction of nitrate to ammonia
• At neutral pH NH₃ exists as NH₄⁺
• NH₃ is volatile
  • Can be lost from alkaline soils by vaporization and there are major losses of NH₃ to the atmosphere in areas with dense animal populations

Nitrification and Anammox

• Nitrification major process in well drained oxic soils at neutral pH
  • Nitrification of prokaryotes
• Two step process
  • NH₃ to NO₂^–
  • NO₂^– to NO₃^–
• NO₃^– is readily assimilated by plants
  • Readily soluble
  • Nitrification is not beneficial for plant agriculture
• Ammonium is positively charged and strongly adsorbed to negatively charged soils
• Ammonia can be oxidized under anoxic conditions by Brocadia(Bacterium) by a process called anammox
• NH₃ is oxidized anaerobically with NO₂^–as the electron acceptor
  • N₂ is final product
  • Anammox is a major process in sewage and in anoxic marine basins and sediments, but significant in well-drained soils

Predatory Bacteria

• Proteobacteria and Bacteroidetes
• Epibiotic Predators
  • Attach to surface of prey and acquire nutrients form its cytoplasm/periplasm
  • Alphaproteobacteria
• Cytoplasmic Predators
  • Invade host cells and replicate in the cytoplasm
  • Epsilopnproteobacteria
• Periplasmic Predators
  • Invade and replicate within the periplasmic space of their prey cells
  • Bdellovibrio
• Social Predators
  • Use swarming behavior to find prey, which they lyse and feed upon
  • Gammoproteobacteria and Deltaproteobacteria

Bdelllovibrio

• Small, highly mobile and curved bacteria
• After attachment to prey it penetrates the cell wall of prey and replicates in periplasmic space forming a spherical structure, bdelloplast
• Obligate anaerobe
  • Obtains energy from oxidation of amino acids and acetate
• Assimilates nucleotides, fatty acids, peptides, and even some intact proteins directly form host without hydrolyzing first
• Deltaproteobacteria are widespread in aquatic habitats

Myxobacteria

• Most complex behavioral pattern
• Formation of fruiting bodies
• Vegetative cells
  • Simple, nonflagellated, gram-negative rods that glide across surfaces and obtain their nutrients by using extracellular enzymes to lyse other bacteria and use the released nutrients
  • Excretes slime, leaving trail
  • Form swarm that exhibits self-organizing behavior
    • Allows them to behave as a single coordinated entity in response to environmental cues
  • Exhaustion
    • Vegetative cells migrate toward each other
    • Aggregation is mediated by chemotactic or quorum-sensing
    • Myxospores
      • Resistant to drying, UV radiation, heat(endospore more resistant)
    • Fruiting bodies
      • Masses of myxospores embedded in slime, or complex, consisting of stalk and heads
      • Stalk composed of slime
      • Majority of cells migrate to head where they undergo differentiation into myxospores