Electrons, Life and the Electrons, Life and the Evolution of the Oxygen Evolution of the Oxygen

Cycle on EarthCycle on Earth

Paul G. Falkowski

Rutgers University

Presented at MIT / October 9, 2007

http://video.mit.edu/watch/electrons-life-and-the-evolution-of-the-oxygen-cycle-on-earth-9298/ Start video at 4:30

Sustaining life on a planet

All organisms derive energy for growth and maintenance by moving electrons from a substrate to a product

All substrates and products must ultimately be cycled

Biological processes are pairede.g., photosynthesis and respiration

All metabolic processes on Earth are prokaryotic and were derived in the Archean and/or Proterozoic Eons

Chemistry of life centers on the disequlibriumredox chemistry of “the big six”

……and at least 54 other and at least 54 other ““trace elementstrace elements”” (Fe)(Fe)

H, C, N, O, P, and SH, C, N, O, P, and S

Primary role of P is in forming phosphates, Primary role of P is in forming phosphates, binding to C, forming organics, whereas binding to C, forming organics, whereas other 5 facilitate electron transferother 5 facilitate electron transfer

H

Li

C N O

FeS

P

O is at least as abundant as C; H< C< N< O…S & P, the most “scarce” or limiting on Earth.

Oxygen chemistry

mainly found as oxidation state (-II)water, oxide & hydroxide metalsalso as (-I); H2O2also as superoxide anionalso as hydroxyl radicals

moderately soluble: ~284μM @ 20oCslow kinetics for abiotic oxidation of organic matterfaster abiotic oxidation of reduced iron and sulfur speciesslower abiotic oxidation of reduced manganese, ammonium, methane

To oxidize Earth, reduced (organic) carbon must be removed before it is reoxidized.

A very small fraction (~0.01%) of the organic matter produced by photosynthesis in the ocean escapes respiration & is buried.

What is the sink for the organic matter on geologic time scales?

Global distribution of chlorophyll

Global sediment type distribution; where is the C deposition?

Evolution of oxygenic photosynthesis• When did it first

occur?– ~3 Ga or

earlier…probably

Cyanobacteria are only bacteria that evolve oxygen

The oxygen problem

Assume O2 evolution by ~3000 Ma-Cyanobacterial microfossils (Knoll 1996)-Biomarkers (Summons 1999)

Large increase in atmospheric O2 2400-2200 Ma-Sulfur isotopes (Farquhar 2000)-Carbon isotopes (Des Marais 1992)

Delay between the emergence of oxygenic photosynthesis and the rise of free atmospheric O2 by ~600 Myr

Abiotic, photocatalyzed reduction of N2 to NH3

6Fe(II)Sn + 6H2O + N22NH3 + 6Fe(III)OH + 6Sn

3.8 bya, with no O2, how to make stable Fe & supply N to the planet?

Alternate way to form banded iron formations?

This reaction would be totally inhibited by even low O2

Master Equations in the N/C/O CyclePhotosynthesis/Remineralization

106CO2 + 16NO3 + H2PO4 + 122H2O =

C106H263O110N16P + 138O2

Nitrogen fixation (function of O2)2N2 + 4H+ + 3CH2O 4NH4

+ + 3CO2

Nitrification (function of O2)NH4 + 2O2 NO3 + 2H + H2O

Denitrification (function of O2)C106H263O110N16P + 84.8HNO3

106CO2 + 55.2N2 + 16NH3 + H3PO4 + 1177.2H2O

Oxic-Anoxic Transition of the Black Sea

Normalized nitrification/denitrification rates

Shelf width drastically affects amount of fixed N in model

(Fennel et al. 2005)

How can a balance of N & O sources & sinks be achieved despite continued organic matter production?

Nitrification > 20uMDenitrification <5uM

Falkowski 2004

Falkowski 2005

Falkowski 2005

N cycle constrained timing of oxygen on Earth and provides a major feedback that constrains atmospheric [O2]

Global N, C, O cycles are constantly fluctuating on time scales of hundreds of millions of years to thousands of years

Anthropogenic alterations of these cycles is much faster than we observe throughout most of the geological record

Questions for chemistry students

What are 5 (bio)chemical reactions, that if industrially scaled,would fundamentally change the world in which we live?

Hydrogen generation from photochemical splitting of water? oxidation

Nitrogen fixation? reduction

Formation of alkanes from cellulose? (hydrolysis & reduction)