Chapter 11—Part 2 Cyanobacteria and the Rise of Oxygen and Ozone

Different forms of cyanobacteria a) Chroococcus b) Oscillatoria c) Nostoc (coccoid) (filamentous) (heterocystic) Nitrogen-fixing

Trichodesmium bloom Berman-Frank et al., Science (2001) Fix N in the morning Produce O 2 in the afternoon

Cyanobacteria and the rise of O 2 Prokaryotic (no cell nucleus) Facultative aerobes (able to photosynthesize oxygenically or anoxygenically) Lipid biomarker evidence (2-  methyl hopanes) at 2.7 b.y. ago –This leads to a big question: Why did atmospheric O 2 not rise until ~2.4 b.y. ago? Phylogenetically related to chloroplasts in higher plants

cyanobacteria Chloroplasts in algae and higher plants contain their own DNA Cyanobacteria form part of the same branch on the rRNA tree Interpretation (due to Lyn Margulis): Chloroplasts resulted from endosymbiosis

Implications Oxygenic photosynthesis was only invented once! Cyanobacteria invented it, and then some eukaryote imported a cyanobacterium (endosymbiosis) and made a living from it. All higher plants and algae descended from this primitive eukaryote.

Early evidence for photosynthesis J. Brocks et al., Science 285, 1033 (1999) Presence of 2  -methyl- hopanes in the 2.7 b.y-old Jeerinah formation in northwestern Australia  cyanobacteria were present at this time Similarly, steranes (from sterols) indicate the presence of eukaryotes Both lines of evidence have now been questioned -- Methyl hopanes have been found in other bacteria -- The biomarkers may not be indigenous

Let’s now look at the geologic evidence for the rise of O 2 …

Geologic O 2 Indicators H. D. Holland (1994) (Detrital)

Permian and Triassic Redbeds A redbed from the Palo Duro Canyon in West Texas links/permo_triassiac.htm

Caprock Canyon (Permian and Triassic) redbeds Redbeds contain oxidized, or ferric iron (Fe +3 ) –Fe 2 O 3 (Hematite) Their formation requires the presence of atmospheric O 2 Reduced, or ferrous iron, (Fe +2 ) is found in sandstones older than ~2.2 b.y. of age

Banded iron- formation or BIF (>1.8 b.y ago) Fe +2 is soluble, while Fe +3 is not BIFs require long- range transport of iron  The deep ocean was anoxic when BIFs formed

What BIFs tell us about O 2 Need to have an anoxic deep ocean filled with ferrous iron, Fe +2, in order to supply the iron (Holland, 1973) –Rare Earth element patterns  Much of the iron comes from the midocean ridges Banding is probably caused by seasonal upwelling

Witwatersrand gold ore with detrital pyrite (~3.0 Ga) Pyrite = FeS 2 oxidized during weathering today  Atmospheric O 2 was low when this deposit formed P. Cloud, Oasis in Space (1988)

What detrital pyrite and uraninite tell us about O 2 Uraninite: UO 2 (U +4 ) –U +4 insoluble, U +6 soluble Oxidative weathering of the land surface was not occurring prior to ~2.3 Ga Atmospheric O 2 was therefore fairly low (< about PAL (times the Present Atmospheric Level)

The best evidence for the rise of O 2 now comes from sulfur isotopes…

S isotopes and the rise of O 2 Sulfur has 4 stable isotopes: 32 S, 33 S, 34 S, and 36 S Normally, these separate along a standard mass fractionation line In very old (Archean) sediments, the isotopes fall off this line Requires photochemical reactions (e.g., SO 2 photolysis) in a low-O 2 atmosphere SO 2 + h  SO + O –This produces “MIF” (mass-independent fractionation)

Production of sulfur “MIF” by SO 2 photolysis J. Lyons, UCLA, in press UV absorption coefficientsBlowup of different forms of SO 2 The different forms of SO 2 (e.g. 32 SO 2 and 33 SO 2 ) absorb UV radiation at slightly different wavelengths

S isotopes in Archean sediments Farquhar et al. (2001)

 33 S versus time Farquhar et al., Science, Phanerozoic samples High O 2 Low O 2

Updated sulfur MIF data (courtesy of James Farquhar) Includes new data at 2.8 Ga and 3.0 Ga from Ohmoto et al., Nature (2006) New low- MIF data glaciations

“Yo-yo atmosphere” model (Watanabe et al., 2006) glaciations pO 2

Finally, let’s think about what this implies for stratospheric ozone…

The rise of ozone Ozone (O 3 ) is important as a shield against solar UV radiation Very little ozone would have been present prior to the rise of atmospheric O 2 We can calculate how the ozone layer develops as atmospheric O 2 levels increase 

Ozone and temperature at different O 2 Levels 1-D climate modelPhotochemical model A. Segura et al. Astrobiology (2003) The ozone layer The ozone layer does not really disappear until O 2 levels fall below ~1% of the Present Atmospheric Level (PAL)

Ozone column depth vs. pO 2 Kasting et al. (1985) Why the nonlinearity? O 2 + h  O + O O + O 2 + M  O 3 + M As O 2 decreases, O 2 photolysis occurs lower down in the atmosphere where number density (M) is higher

Conclusions Atmospheric O 2 levels were low prior to ~2.4 b.y. ago Cyanobacteria were responsible for producing this O 2 An effective ozone screen against solar UV radiation was established by the time pO 2 reached ~0.01 PAL, probably around 2.4 b.y. ago