Coupling of Biogeochemical Cycles

• Part I • Stoichiometry • Redox Chemistry and Metabolism

• Part II • The Coevoution of Life and the Earth

System !

• Part III (This afternoon) Introduction to Stable isotopes and a few problems

Abundance of elements depends on (1) Nucleosynthesis (2) Solar system formation (3) Earth Differentiation

http://www.daviddarling.info/images/cosmic_abundance.jpg

H,  He  are  most  abundant  (from  big  bang,  fusion  of  H  to  form  He  in  stars)  !

Boron,  Li,  Be  not  abundant  (also  made  in  Big  bang)  !

Other  elements  that  are  lighter  than  iron  are  made  in  stellar  interiors,  heavier  ones  are  made  during  supernova  explosions.  !

Model  has  changed  in  the  past  decade–  Where  did  Earth’s  water  come  from?

-­‐  gravitational  interaction  of  planetoids

Where  did  the  atmosphere  and  oceans  come  from?  Hydrogen,  Helium  and  other  gases  swept  away  by  solar  wind

Addition  of  water  from  accreting  planetoids/asteroidsDifferentiation  (melting  and  separation  into  layers)

Composition of Earth as a whole and just in crust

•Evidence  for  internal    Composition  and  structure    of  Earth:  

• seismic  waves  • meteorites  

Behavior  of  elements  depend  on  the  number  and  configuration  of  electrons  

Elements take different forms in different parts of the Earth System so transfers from one sphere to another involve change of chemical form or change of phase

Atmosphere Hydrosphere Biosphere Lithosphere

Carbon (C)

COvolatile organics

HCO

Organic C (~CH

CaCOOrganic C graphite

Nitrogen (N)

NNH

HNONHDON

Organic N (amino acids)

N-salts

Phosph-orous (P)

Small amounts aerosols

!PO

Organic P (DNA)

Apatite (CaPO

Gas/liquid Liquid/  dissolved  ion

Solid/liquid/  dissolved  ion

Solid

H2O                                  water  vapor                              liquid                                              liquid                                        ice  (cryosphere)

LAND  Surface

Atmosphere

OCEAN  (hydrosphere)

LITHOSPHEREWeathering  Volcanism  metamorphism

Weathering  Volcanism  metamorphism

Photosynthesis/Respiration  Nitrogen  Fixation/Denitrification  Precipitation/evaporation  Momentum/Energy  

transport  (rivers,  aersols)

Exchanges  of  major  elements:    C,  O,  N,  P,  S,  Si,  Fe,  Mg  Most  exchanges  are  mediated  by  Biological  processes  (hence  Biogeochemistry)

Biogeochemistry  involves  the  biological  processes  that  transfer  elements  between  ‘spheres’  as  well  as  the  forms  they  take  in  each  ‘sphere’

Gas  exchange  Precipitation/evaporation  Momentum/Energy  exchange

What determines which chemical form of an element will be found?

• Energetics (thermodynamics)

• How much chemical energy is released/required when bonds are broken/formed? (Lowest chemical potential energy is more favored; look up in tables)

• Takes energy to go from more to less random states (e.g. from gas to dissolved to solid).

CH4 + 2O2 ⬄ CO2 + 2H2OEnergy required to break 4 C-H bonds 4*99kcal/mol = 396 kcal/mol 2 O=O bonds 2*119kcal/mol = 238 kcal/mol Total = 634 kcal/mol

Energy released by forming 2 C=O bonds 2*177kcal/mol = 396 kcal/mol 4 O-H bonds 4*111kcal/mol = 238 kcal/mol Total = 798 kcal/mol

164 kcal/mol released Reaction is exothermic (we knew this….)

CH4 + 2O2 ⬄ CO2 + 2H2OCurrent concentrations of gases in the atmosphere: CH4: 2 ppm (2 x 10-6 mol CH4/mol air)

O2 : 21% (0.21 mol O2/mol air)

CO2 380 ppm (380 x 10-6 mol CO2/mol air

H2O: ~1% water vapor in atmsophere (.01 mol H2O/mol air)

224

22

2

))(()()(

aOaCHaCOOaHKeq =

Where a is activity = concentration or ideal pressure times a correction factor to represent the role of molecular interactions that might interfere with the ability to participate in reactions –’nonideal’ conditions. !Keq is determined (from thermodynamic constants) to be ~10140 !

Is methane in the atmosphere at equilibrium??? i.e. Q equals Keq?

Gaia hypothesis proposes that organisms  interact  with  their  inorganic  surroundings  on  Earth  to  form  a  self-­‐regulating,  complex  system  that  contributes  to  maintaining  the  conditions  for  life  on  the  planet.    (Wikipedia  definition)

• The atmosphere is not at chemical equilibrium with ocean/land surfaces

• Gases are present in nonequilibrium concentrations in the atmosphere because they are continuously produced and consumed by biological processes

• Feedbacks between those gases and the life that produces them maintain the atmosphere in a state conducive to the continuation of life

James  Lovelock

Lynn    Margulis

Stoichiometry and Metabolism

• Metabolic needs of organisms couple biogeochemical cycles (e.g. enzymes required for various metabolic processes require amino acids, trace metals, etc. in specific quantities)

• Productivity as a measure of biology and ecosystem element flow: GPP, NPP, NEP

• Concept of a ‘limiting’ nutrient

More than C … biogeochemical cycles involve any element

needed for life

Where%do%components%of%plants%come%from?%

Global ‘stoichiometry’ Ocean Redfield Ratio

for photosynthesis (Redfield, Ketchum, and Richards,1963) Coupling of the major element cycles

106CO2+ 16 NO3

- + HPO42- + 122 H20 + 18 H+ <=>

(CH2O)106(NH3)16(H3PO4) + 138 O2

Ocean C:N:P is ~ 106:16:1 This is not a constant ratio though it is often used as if it were

On land, ratios are different – C:N and C:P can be much higher because plant structural material is built out of C-rich materials

like cellulose (C6H12O2)

Internal recycling in the ocean will

dominate the ratio of dissolved N:P to be about the same as

that of N:P falling as particles into the

deep ocean

Question - nutrient distributions in the ocean

http://publishing.cdlib.org/ucpressebooks/data/13030/6r/kt167nb66r/figures/kt167nb66r_fig050.gif

Evidence of relative constancy of the Redfield ratio from ocean observations

(Tyrell, Nature 1999)

• GEOSECS: water samples from all over the world

Strong correlation of N to P as predicted by Redfield

ratio

Cease,  A.  J.  &  Elser,  J.  J.  (2013)  Biological  Stoichiometry.  Nature  Education  Knowledge  4(5):3Nature  

But .. it is more complicated

Marine Redfield ratio varies:

Martiny et al. 2013 Nature

Geoscience

http://www.nature.com/scitable/knowledge/library/biological-­‐stoichiometry-­‐102248897

Figure  3:  Consumer  community  composition  can  determine  the  nutrient  by  which  autotrophs  are  limited.  ©  2013  Nature  Education  Elser  et  al.  1988.  All  rights  reserved.  

Limiting nutrient is not always the same

24

Liebig‘s  Law  of  the  minimumN  is  a  limiting  nutrient  in  many  ecosystems

...“Das Wachstum von Pflanzen wird durch die knappste Ressource eingeschränkt“

Dealing with nutrient deficiency

• Conserve  what  you  have  (Water  use  efficiency,  nitrogen  use  efficiency)  

• Spend  what  is  abundant  to  get  what  is  required  (spend  C  to  get  P  through  mycorrhizae;  spend  water  to  get  C  through  stomates)  

• (New  way)  Find  a  human  to  give  you  what  you  need    (ferelizer)

!Organic matter today is nearly all made by one process – photosynthesis !Energy is required to make C-C bonds !Breaking of C-C bonds and reformation of CO2 releases energy, and fuels a wide range of microbial and heterotrophic activity.

Redox chemistry and metabolic pathways

Metabolic  Pathways  

Oxidation  and  Reduction  

   The  main  energy  capture  and  release  mechanisms  of  life  consists  of  Redox  systems  -­‐  coupled  oxidation  and  reduction  reactions  

         Oxidation  is  removal  of  electrons  -­‐  releases  energy  (e.g.  rusting)    Fe2+  ➔  Fe3+  +  e-­‐      or  2H2O  ➔ O2    +  2H+  +    2e-­‐            Reduction  is  the  addition  of  electrons  -­‐  takes  energy  

   Fe3+  +  e-­‐    ➔  Fe2+    or  2CO2  +  4e-­‐  +  4H+  ➔ 2CH2O  +  O2  

When  chemical  reactions  occur  bonds  are  formed  and  broken;  electron  transfer  in  these  reactions  can  be  deduced  from  an  elements'  electronegativity    the  degree  of    'attraction'  an  element  has  for  electrons).    Metabolic  pathways  either  use  external  energy  (chemical  or  photon)  to  create  new  bonds,  or    release  chemical  bond  energy  by  breaking  them  apart.

For  every  oxidation  in  a  metabolic  pathway,  there  is  a  corresponding  reduction.    

!

         The  reductant  (electron  donor)  supplies  the  electrons  and  is  oxidized.  

         The  oxidant  (electron  acceptor)  receives  the  electrons  and  is  reduced.  

For  example,  in  the  reaction  

                               CH2O  +  O2(g)  ==>  CO2  +  H2O(liq)  +  energy        (remineralization/metabolism)  

the  C  is  oxidized  (from  0  to  +4)  and  the  O  is  reduced  (from  0  to  -­‐2)  

In  this  case,  CH2O  is  the  reductant  (it  is  oxidized),  and  O2  is  the  oxidant  (it  is  reduced)

Oxidation  States  !Element     Oxidation  State   Species  Nitrogen     N  (+V)       NO3

-­‐  

                   N  (+III)       NO2-­‐  

                N  (O)       N2  

                  N  (-­‐III)       NH3,  NH4+  

Sulfur                  S  (+VI)       SO42-­‐  

                   S  (+II)       S2O32-­‐  

                     S  (O)       S°                      S(-­‐II)       H2S,  HS-­‐,  S2-­‐  

Iron       Fe  (+III)     Fe3+                      Fe  (+II)         Fe2+  Manganese   Mn  (+VI)     MnO4

2-­‐  

        Mn  (+IV)     MnO2  (s)           Mn  (+III)     MnOOH  (s)           Mn  (+II)     Mn2+

Many  elements  in  the  periodic  table  can  exist  in  more  than  one  oxidation  state.  Oxidation  States  are  indicated  by  Roman  numerals  (e.g.  (+I),  (-­‐II),  etc).    The  oxidation  state  represents  the  "electron  content"  of  an  element  which  can  be  expressed  as  the  excess  or  deficiency  of  electrons  relative  to  the  elemental  state.

How to determine: Assign O = -2 Assign H = +I Charge on species

V IV III II I O -I -II -III

oxid

ized re

duce

d

Heterotrophs  (use  reduced  carbon  as  an  energy  source)

Autotrophs  (reduce  C  to  organic  forms)  require  energy

Organisms  can  work  in  consortia  to  help  overcome  some  energetic  limitations

CO2 + H2O ⬄ CH2O + O2C is reduced (+4 to 0) O Is oxidized (-2 to 0)

i.e. electrons moved from O to C

transport  of  electrons  coupled  to  pumping  protons  !Just  to  remind  you  that  there  is  actually  very  complex  biochemistry  behind  everyone  one  of  these  equations….    

glucosee-­‐

CH2O          à CO2  +  4  e-­‐  +  H+  0.5  O2  +  4e-­‐  +  4H+  à H2O

Oxidizing agent Is reduced to Reduced form Δ

Per mole glucoseO H dissolved -3190 kJ

MnO Mn mineral -2920 kJ

NO N dissolved -3030 kJ

NO NH dissolved -2750 kJ

Fe Fe mineral -1410 kJ

FeOOH Fe mineral -1330 kJ

SO S dissolved -380 kJ

CH CH dissolved -350 kJ

• Organic matter is oxidized by these electron acceptors • These are the metabolic pathways of decomposition • ΔG0 is free energy of the reaction – negative means energy is released • The more negative, the greater the energy to fuel the organism

anoxic

Redox in soils – !White area around roots !Soil was likely saturated with water !O2 consumption by roots exceeded supply; Anoxic conditions Used up the next oxidant (NO3

-, likely not in abundant supply) !Next in line is Iron (as Fe2O3, or Fe (III)) !Oxidized iron (Fe2O3) is red colored (rust) and Fe(III) will react quickly with free O2 to form insoluble Fe oxides !Reduced iron (FeO) is uncolored or grey FeS (pyrite) is black (Fe(II) is stable in soluble form, so can migrate with water) ! “Gleyed” soil – indicates flooding

Mottling

Water saturated

O2 enters slowly due to slow diffusion in water (10000 times slower than in air)

Organic matter

O2

NO3 ➔ N2 (bubbles)

SO4 ➔ S-- (precipitates with Fe+2 or lost as H2S)

2CH2O ➔ CO2 + CH4

Wetlands/saturated soils

Fe+3 ➔ Fe+2 (precipitates with S or O)

Methane emissions from !Freshwater wetlands are generally less than those of Saltwater weltands ---- why?

Why should we care?• The world was not always as it is today — life and

the atmosphere have co-evolved (e.g. no atmospheric O2 for the first 2 billion years)

• Coupling of biogeochemical cycles through metabolism of organisms breaking down organic matter

• Interntal transformations (e.g. of S or N in different oxidation states) define the forms and cycling of these elements