Global Biogeochemical Cycles and. II. Biological Metabolism

Transcription

1 Global Biogeochemical Cycles and Biological Metabolism I. Biogeochemistry & Biogeochemical Cycles A. Global cycles: nitrogen, water, carbon B. Carbon cycle through time II. Biological Metabolism A. Redox reactions: basis of metabolism B. The metabolic pathways C. Microbial Habits I. What is Biogeochemistry? Biogeochemistry = the study of how the cycling of elements through the earth system (water, air, living organisms, soil and rock) is governed by physical, chemical, geological and biological processes hydrosphere atmosphere biosphere pedosphere lithosphere Key Names Vladimir Vernadsky ( ): 1945) Russian scientist known as the father of biogeochemistry, invented the terms geosphere, biosphere, and noosphere G. Evelyn Hutchinson ( ): famous limnologist (considered to be founder of limnology) (also studied the question of how biological species coexist) 1

2 Global Change and the N-cycle The nitrogen dilemma is not just thinking that carbon is all that matters. But also thinking that global warming is the only environmental issue. The weakening of biodiversity, the pollution of rivers... Smog. Acid rain. Coasts. Forests. It s all nitrogen. - Peter Vitousek Global Change and the N-cycle Human alteration of N-cycle is LARGE (> 100% in fixation relative to the background rate of Tg N/yr): No place on earth unaffected by this Vitousek et al., 1997 (terrestrial) 80 to 90 Tg 20 Tg 40 Tg 2

3 Atmospheric N 2 : 3.9 x 10 9 Reactive N (NOx, NH 3 ) Vegetation: 4,000 Soils: 100,000 Units: Stocks (Tg N) or Flows (Tg N/yr) Atmospheric N 2 : 3.9 x 10 9 Huge but inaccessible Reactive N (NOx, NH 3 ) Vegetation: 4,000 Soils: 100,000 Units: Stocks (Tg N) or Flows (Tg N/yr) 3

4 Atmospheric N 2 : 3.9 x 10 9 Reactive N (NOx, NH 3 ) Vegetation: 4,000 Note big difference in N turnover times in terrestrial vs. Marine biota Terrestrial: 4000/1200 = 3-4 yrs Marine: 300/8000= Soils: 2 weeks 100,000 Atmosphere: 3.9x10 9 /~ years! Units: Stocks (Tg N) or Flows (Tg N/yr) Atmospheric N 2 : 3.9 x 10 9 Reactive N (NOx, NH 3 ) natural Nfixation Vegetation: 4,000 Soils: 100,000 Units: Stocks (Tg N) or Flows (Tg N/yr) 4

5 Atmospheric N 2 : 3.9 x 10 9 Reactive N (NOx, NH 3 ) Human N fixation Vegetation: 4,000 Soils: 100,000 Units: Stocks (Tg N) or Flows (Tg N/yr) I. The Global water cycle Atmosphere: 13,000 Ice: 33 x 10 6 Freshwater available for interaction with terrestrial biosphere is small fraction Lakes: 230,000 Vegetation: 10,000 Oceans: x 10 9 groundwater: 15.3 x 10 6 Soil water: 120,000 In km 3 (pools) and km 3 /yr (fluxes) 5

6 I. The Global water cycle Atmosphere: 13,000 Precip: 111,000 Ice: 33 x ,000 Atmospheric supply to Land (precipitation) is greater than land return to atmosphere (evapotranspiration) Transpiration Lakes: 230,000 Vegetation: 10,000 Oceans: x 10 9 groundwater: 15.3 x 10 6 Soil water: 120,000 In km 3 (pools) and km 3 /yr (fluxes) I. The Global water cycle Atmosphere: 13,000 Evaporation: 425 5,000 Precip: 385 5,000 40,000 Precip: 111,000 Opposite is true over the ocean (precip < evapotranspiration) Ice: 33 x ,000 Transpiration Atmospheric supply to Land (precipitation) is greater than land return to atmosphere (evapotranspiration) Lakes: 230,000 Vegetation: 10,000 Oceans: x 10 9 groundwater: 15.3 x 10 6 Soil water: 120,000 In km 3 (pools) and km 3 /yr (fluxes) 6

7 I. The Global water cycle Atmosphere: 13,000 Evaporation: 425 5,000 Precip: 385 5,000 40,000 Ocean water powers land precip Precip: 111,000 Ice: 33 x 10 6 Oceans: x 10 9 groundwater: 15.3 x ,000 Transpiration Vegetation: 10,000 Plants power water return to atmosphere on land Lakes: 230,000 Soil water: 120,000 Atmospheric turnover time: 13,000/ 496,000 = yrs 1 week In km 3 (pools) and km 3 /yr (fluxes) Facts about the global water cycle 496 x 10 3 km 3 Global precip: 385,000 (ocean) + 111,000 (land) 496 x 10 3 km x 10 6 km 2 =097x km Earth surface area 1 meter of precip (global average) Land area is 148 x 10 6 km = 0.75 m average land precip Comparison: Tucson gets 0.30 m, tropical forests 2 or 3 m 7

8 Overview: The Modern Global carbon cycle (1990s) Total FF. emissions Total landuse change emissions Values in black are natural, values in red are anthropogenic Note: 1990s values (IPCC 2007, Ch. 7) 8

9 What is residence time of C in atmosphere? One estimate is from land+ocean flux: =190; 597/190 3yrs BUT: this is not an estimate of removal rate to ocean of excess CO 2 (2.2). Better: 597 / 2.2 = 270 years. But in truth, for CO 2, no single residence time. Total FF. emissions Total landuse change emissions Values in black are natural, values in red are anthropogenic Note: 1990s values (IPCC 2007, Ch. 7) Fun problem in global biogeochemistry: How likely is it that in your next breath, you will inhale at least one molecule of nitrogen (N 2 ) that Julius Caesar exhaled in his last breath? (Julius Caesar, the famous Roman emperor, was assassinated on March 15, 44 BC by a group of Roman senators). This requires some basic chemistry: 1 mole of air is 6.02 x molecules At STP, 1 mole occupies ~22 liters. Also helpful: whole atmosphere: 1.8 x moles of air What is atmospheric mixing ratio of N 2? What is N 2 mixing time in the atmosphere? What is N 2 residence time in the atmosphere? 9

10 First, the concentration, in the whole atmosphere, of Caesar s last-breath N 2 molecules # N2 moleculesin Caesar ' s last breath Concentration # molecules in whole atmosphere 3 N 2 1mol air 23 1L of air inbreath 6 10 molecules / mol 4air 22L air mols air / atmosphere 6 10 molecules / mol Next, the average number you inhale per breath = Concentration ( molecules Caesar ' s N 2/ molecules air) # molecules inhaled 22 1mol air L 6 10 molecules / mol 22 Lair So on average you inhale 5 molecules of Caesar s-breath N 2 in every breath! 22 II. What is Biological Metabolism? Metabolism, simplified oxygenic photosynthesis H 2 O + CO 2 CH 2 O + O 2 Carbon in atmospheric CO2 Aerobic Respiration More generally: autotrophy Reduced Carbon in organic matter (plant biomass & energy supply) e - donor + CO 2 CH 2 O + e - acceptor heterotrophy 10

11 Life is catalyzed Redox Reactions Master variables in soil & water chemistry: H + and e - Control the direction, rate and endproducts of many reactions. Electron transfer reactions near earth s surface are driven largely by C cycling: Photosynthesis: CO 2 (g) + H 2 O(l) CH 2 O(s) + O 2 (g) Respiration: CH 2 O(s) + O 2 (g) CO 2 (g) + H 2 O(l) Both are full, balanced redox reactions. Oxidation-Reduction Reactions A redox reaction involves the transfer of one or more electrons (e - ) from one species to another. Reduced Species A + Oxidized Species B Oxidized Species A + Reduced Species B e - e - The reaction proceeds in either direction, depending on magnitude and sign of G r (change in Gibbs free energy = energy stored or released) 11

12 Some Redox Definitions Oxidation: The loss of e - Reduction: The gain of e - Oxidized Species: Oxidizing agent or oxidant in the reaction. Atom or compound that oxidizes the other atom or compound. The electron acceptor. Is itself reduced (gains an e - ) in the process. Reduced Species: Reducing agent or reductant t Atom or compound that reduces the other atom or compound. The electron donor. Is itself oxidized (loses an e - ) in the process. The full redox reaction: Reduced Species A + Oxidized Oxidized Reduced + Species B Species A Species B e - e - contains two half reactions, where each pertains to a given element that undergoes oxidation or reduction: Red A Ox A + m H + (aq) + e - (aq) Ox B + m H+ (aq) + e-(aq) Red B 12

13 Thermodynamics of Redox The e - 's represent a common currency for examining i conditions of thermodynamic spontaneity and equilibrium when comparing reactions. The thermodynamic spontaneity of the reaction is determined by the potential for e - to be transferred in one direction or another. Half-Cell Conventions Half-Reaction: The elemental reaction used to describe a redox reaction. Normally written as a reduction: 0.25 O 2 (g) + e - (aq) + H + (aq) 0.5 H 2 O (l) log K = 20.8 E h 0 = standard half-cell potential (electromotive force) [V] = 0.059*log K Describes the spontaneity of the reaction as written (tendency of reaction to occur, driving force) under standard state conditions of reactants and products. Measure of affinity of an electron acceptor for electrons under standard conditions. 13

14 Standard Hydrogen Electrode H = 1 atm 2 Platinum electrode a H + = 1 ½H 2 (g) H + + e - E h0 = V Log K = 0.0 Thus given free energy of formation for compounds E 0 h ΔG 0 r < 0 (E 0 h > 0): reduction has greater tendency to occur than does the reduction of H + (aq) to H 2 (g) (under standard conditions). 14

15 Overall redox reaction in soil is sum of two half-reactions MnO 2 (s) + 4H + (aq) + 2e - Mn 2+ (aq) +2H 2 O (l); log K = 41.6 is a reduction half-reaction. This reaction must be combined (coupled) with another half-reaction that is inverted to represent an oxidation half-reaction. No free electrons appear in balanced full redox reactions. For example, the reduction of Mn(IV) to Mn (II) could be coupled with the oxidation of nitrite to nitrate (see next slide) Summing two half reactions MnO 2 (s) + 4 H e - Mn H 2 O log K = x (0.5 NO H 2 O 0.5 NO 3- + H + + e - ) log K = MnO 2 (s) + 2 H + + NO 2 - Mn 2+ + NO 3- + H 2 O log K = 13.2 Will this reaction proceed under standard conditions? 15

16 Terminal Electron Accepting Processes TEAPs refers to the fact that in natural aqueous systems, the availability of O 2 to act as a terminal electron acceptor in respiration is limited by its availability. Other oxidizing agents are used as O 2 becomes depleted. The order of use of these alternative ti TEAPs is the same as that of their energetic yield. Sequence of TEAPs is consistent with the decrease in energy yield per mole of organic matter oxidized. 16

17 Falkowski et al. (2008) 17

18 The change in Gibbs free energy G (and Eh) can be calculated for all pairs of redox reactions, giving theoretical thermodynamic energy yields. Reactions that run downhill (release energy) offer opportunities to power metabolism, and hence, a possible habit for life. Falkowski et al. (2008) 18