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Biogeochemical Cycles

Biogeochemical Cycles. Objective to understand the principles and importance of biological systems in the cycling of key elements and nutrients Reference Butcher S.S. et al Global Biogeochemical Cycles. Carbon Cycling. The Major Carbon Reservoirs CO 2 in Air 750 Pg

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Biogeochemical Cycles

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  1. Biogeochemical Cycles • Objective • to understand the principles and importance of biological systems in the cycling of key elements and nutrients • Reference • Butcher S.S. et al Global Biogeochemical Cycles

  2. Carbon Cycling • The Major Carbon Reservoirs • CO2 in Air 750 Pg • CO2 in water 38,000 Pg • Soil Humics 1500 Pg • Coal/Oil 10,000 Pg • Carbonate mineral 107 Pg • Marine Biota 3 Pg • Terrestrial Biosphere 560 Pg P = Peta = (1015) • The Major Carbon Fluxes • Biological Assimilation 170 Pg • Biological Dissimilation 170 Pg • Fossil Fuel burning 5 Pg

  3. Sulphur Cycle • Differences from N cycle • More valences • spontaneous reactions Important Oxidation States Form Example Oxidation State S2- sulphur proteins -2 S0 sulphur 0 SO32- sulphite +4 SO42- sulphate +6

  4. -2 H2S Desulphurylation -2 Assimilative sulphate reduction Waste Animal protein -2 -2 SO4 plant protein +6 -2

  5. Assimilative Sulphate reduction • Sulphate v. stable • Incorporated into protein • Many organisms in assimilative reduction • SO4 R-SH • Decay H2S • Desulphurylation

  6. -2 H2S intermediate SRBs Dissimilatory Sulphur Reduction +4 SO32- SRBs SO42- +6

  7. Sulphate stable in high pE environments • If • no oxygen • energy source (eg, H2 or acetate) • Sulphate reducing bacteria (SRB) • then SO42-H2S • SO4 electron acceptor • Competes with methanogens Dissimilatory Sulphate Reduction

  8. Sulphate reduction and sulphide very important • H2S is: • toxic • to bacteria, algae, workers (ppm) • smelly • reactive with metals • gives FeS • precursor for acid formation (when oxidised by Thiobacillus) • cause of steel and concrete corrosion

  9. H2S Equilibria and pH • H2S(aq) HS- + H+ S2- + 2 H+ • pKa = 7.1 pKa = 14 • H2S also a gas • Only H2S toxic/smelly, others dissolved • pH >8 little H2S • pH <7 mostly H2S

  10. -2 H2S Sulphur Bacteria intermediate Spontaneous with O2 Sulphur Bacteria +4 -2 SO 3 0 S Sulphur Bacteria -2 SO 4 +6 Sulphur Oxidation

  11. H2S S , spontaneous in O2 • Not fast (only HS- reacts) • H2S and S electron donors • Variety of organisms • Sulphur Bacteria (Beggiatoa; Thiobaccillus, Thiothrix) • require oxygen • Photosynthetic (Green and Purple sulphur bacteria) • anaerobic • Waste Stabilization Ponds (WSP) Sulphur oxidation

  12. Bacteria in Water Droplets (H2S + O2 H2SO4 ) Sewer Corrosion H2S AIR H2S Sewage SO42- S2- S2- + H+ H2S

  13. Nitrogen cycle • Less spontaneous reactions than S Form Species Oxidation State NH3 ammonia proteins -3 N2 0 NO Nitric oxide +2 NO2- Nitrite +3 NO3- Nitrate +5

  14. Nitrogen fixation 0 N2 N Fixers Cyanobacteria Plant Protein -3

  15. Nitrogen fixation • Energy intensive • Reduction of nitrogen • Major path biological • bacteria • free-living (Azospirillum) • and symbiont (Rhizobium) • Blue green algae • Also lightening and feritilizers

  16. Nitrate Reduction and Ammonification -3 NH3 Ammonification Bacteria Dead Organic Matter -3 Excretion Death Death Animal Proteins -3 NO3- Grazing Nitrate Reduction Plant & Bacterial Proteins +5 -3

  17. Ammonification • Organic nitrogen into ammonium • ammonium taken up by plants • Assimilative Nitrate reduction • N incorporated into organic nitrogen. • minor pathway • Bacteria, fungi, algae Organic Nitrogen Transformations • Organic nitrogen measured as Kjeldahl-N (TKN) • Organic N is digested by H2SO4 to release N as ammonia. • Ammonia then measured (distillation)

  18. Nitrification -3 NH3 Ammonia Oxidation Ammonia Oxidising Bacteria +3 NO2- Nitrite Oxidation +5 NO3-

  19. Two Stage Process • 1 Ammonia oxidation (Nitrosomonas and Nitrosospira) • 2NH3 + 3O2 2NO2- + 2H+ + 2H2O • 2 Nitrite oxidation (Nitrobacter) • 2NO2- + O2 2NO3- • Measurement of ammonia • Nesslers method • Use Nesslers reagent to get a brown colour • Borate method • NH3 + H3BO3 NH4+ + H2BO3- • H2BO3- + H+ H3BO3 Nitrification

  20. Denitrification +3 0 Low O2 NO2- N2 Nitrite Reduction Low O2 Nitrate Reduction +5 NO3-

  21. Dissimilatory Reduction of NO3- and NO2- to N2 • Variety of bacteria • Occurs in absence of oxygen • Small amounts of NO and N2O also formed • Nitrate and Nitrite measured • ion chromatography (Dionex) - best • Nitrite • Indirect Colorimetric method - tedious • Nitrate • UV absorbtion - interfering species • Additional reduction methods • ion specific electrodes Denitrification

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