Lecture 8: Cyanobacteria and the Rise of Oxygen and Ozone

Abiol 574

Different forms of cyanobacteria

(formerly “blue-green algae”)

a) Chroococcus b) Oscillatoria c) Nostoc (coccoid) (filamentous) (heterocystic)

Nitrogen-fixing

Trichodesmiumbloom

Berman-Frank et al.,Science (2001)

• Fix N in the morning• Produce O2 in the afternoon

• Cyanobacteria are a key part of the modern marine ecosystem because of their ability to fix nitrogen

• We also know that oxygenic photosynthesis was first invented by the cyanobacteria, as clearly demonstated by molecular phylogeny

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 Lynn Margulis*): Chloroplasts resulted from endosymbiosis

*and Konstantin Meresch- kowski long before this

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.

Cyanobacteria and therise of O2

• Prokaryotic (no cell nucleus)– By contrast, eukaryotes have a cell nucleus

• Facultative aerobes (able to photosynthesize oxygenically or anoxygenically)

• Purported evidence for cyanobacteria at 2.7 Ga from 2-alpha methylhopanes preserved in sediments from the Pilbara Craton, Western Australia (R.E. Summons et al., Nature, 1999)

• These sediments were also thought to contain lipid biomarker evidence (steranes) for O2-requiring eukaryotes (J. Brocks et al., Science, 1999)

Early evidence for photosynthesis

• Presence of steranes in the 2.7 b.y-old Jeerinah formation in northwestern Australia (Summons et al., Nature, 1999; Eigenbrode & Freeman, PNAS, 2006) free O2 was present at this time, and thus cyanobacteria must have been present, as well--This is disputed by Jochen Brocks, who now thinks that the biomarkers are from later contamination

• Direct evidence for cyanobacteria (2- methyl-hopanes) is also questioned by some because these compounds are found in other anaerobic bacteria

http://www-eaps.mit.edu/geobiology/biomarkers/steroids.html

• Steranes come from sterols, e.g., cholesterol

New Agouron drill core

• New drill holes (2 of them) done by the Agouron Institute, summer (2012)

• Second hole went through the 2.69-2.63 Ga Jeerinah Formation in western Australia

• Water (with fluorescent green dye) was used as the drilling fluid

• No biomarkers of any sort were found..

Interim Agouron Pilbara Drilling Project ReportRoger Buick, Christian Hallmann, Katherine French and Roger Summons

• That said, there is still pretty good evidence that O2 was being produced before the GOE…

Science (2007)

• Mb is forms an insoluble sulfide in reduced environments• A Mb enhancement in shales requires oxidative weathering of sulfides on land, followed by transport of soluble molybdate ion to sediments• The best way to do this, in my view (and that of Reinhard and Planavsky, Nature, 2013) is for the entire atmosphere to become O2-rich for short time periods

Nature, Sept. 26, 2013

• In a paper that is coming out today, Crowe et al. argue that atmospheric O2 reached levels of ~310-4 PAL at ~3.0 Ga• The analysis is based on 53Cr depletion in the Nsuze paleosol (above) and enrichment of 53Cr and U in the contemporaneous Ijzermyn iron formation (next slide)

Cr isotopes and pO2

• Cr has two accessible oxidation states, Cr+3 and Cr+6. As with U, the oxidized state is soluble, while the reduced state is insoluble

• 53Cr is enriched in the +6 state relative to 52Cr when Cr is oxidized

• If O2 is present during weathering, then 53Cr is preferentially removed from soils and deposited in sediments, such as BIFs

Nature, Sept. 26, 2013

• These data are from the Ijzermyn iron formation• Both 53Cr and U are enriched relative to the mantle and/or crust

Let’s now look at the geologic evidencefor the (main) rise of O2…

Geologic O2 Indicators

H. D. Holland (1994)

(Detrital)

• Blue boxes indicate low O2

• Red boxes indicate high O2

• Dates have been revised; the initial rise of O2 is now placed at 2.45 Ga

Permian and Triassic Redbeds

• A redbed from the Palo Duro Canyon in West Texas

http://www.utpb.edu/ceed/GeologicalResources/West_Texas_Geology/links/permo_triassiac.htm

Caprock Canyon (Permian and Triassic) redbeds

• Redbeds contain oxidized, or ferric iron (Fe+3)– Fe2O3 (Hematite)

• Their formation requires the presence of atmospheric O2

• Reduced, or ferrous iron, (Fe+2) is found in sandstones older than ~2.2 b.y. of age

http://www.utpb.edu/ceed/GeologicalResources/West_Texas_Geology/links/permo_triassiac.htm

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 O2

• 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 orewith detrital pyrite

(~3.0 Ga)

• Pyrite = FeS2

• oxidized during weathering today

Atmospheric O2 was low when this deposit formed

P. Cloud, Oasis in Space (1988)

What detrital pyrite and uraninite tell us about O2

• Uraninite: UO2 (U+4)– U+4 insoluble, U+6 soluble (opposite

behavior from iron)

• Oxidative weathering of the land surface was not occurring prior to ~2.3 Ga

• Atmospheric O2 was therefore fairly low (< about 10-2 PAL (times the Present Atmospheric Level)

• The best evidence for the rise of O2 now comes from sulfur isotopes…

S isotopes and the rise of O2

• Sulfur has 4 stable isotopes: 32S, 33S, 34S, and 36S

• 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., SO2 photolysis) in a low-O2 atmosphere

SO2 + h SO + O

– This produces “MIF” (mass-independent fractionation)

“Normal” isotope mass fractionation

• Vibrational energy levels depend inversely on the reduced mass = (k/mR)1/2

En = (n+½) h• Increasing the mass

of one or both atoms decreases the vibrational frequency and energy, thereby strengthening chemical bonds

A simple harmonicoscillator

S isotopes in Archean sediments

• Sulfides (pyrite) fall above the mass fractionation line• Sulfates (barite) fall below it

Farquhar et al. (2001)

(FeS2)

(BaSO4)

33S

33S versus time

Farquhar et al., Science, 2000

73 Phanerozoic samples

High O2 Low O2

Updated sulfur MIF data (circa 2008)(courtesy of James Farquhar)

glaciations

(Note the increase in vertical scale)

MIF data summer 2013

• The pattern in 33S continues to hold as new data are added

• Note the significant disparity between positive and negative 33S values– Need a larger sulfur

reservoir for the negative values so that the isotopic signal is diluted

MIF data summer 2013

• Note also the apparently smaller values of 33S near 3.0 Ga, the same time that the new Cr isotope data point to finite levels of atmospheric O2

– The MIF signal wouldn’t disappear entirely even if pO2 was high, because of reworking of older MIF-rich sediments

Question: What does the sulfur MIF tell us?

1. Must have had low enough O2 (and O3) to allow SO2 to be photolyzed

2. Must have had low enough O2 to prevent all volcanic SO2 from being oxidized to sulfate, as it is today

Start with point 2

Archean sulfur cycle

Kasting, Science (2001)[Redrawn from Kasting et al., OLEB (1989)]

• In a low-O2 atmosphere, volcanic SO2 can be either oxidized or reduced (or it can exit the atmosphere as SO2)• By contrast, today, virtually all SO2 is oxidized to sulfate; thus, any MIF signal is eliminated by homogenization

“Archean” sulfur removal rates

Pavlov and Kasting (2002)

• Typically, SO2, H2S, and S8 are the main exit routes for sulfur from a low-O2 atmosphere

Production of sulfur “MIF” by SO2 photolysis

J.R. Lyons, GRL (2007)

UV absorption coefficients Blowup of different forms of SO2

• The different isotopologues of SO2 (e.g. 32SO2 and 33SO2) absorb UV radiation at slightly different wavelengths

MIF signal forms here

PNAS, 2009

• Shielding by OCS (or O3) produces the right sign for 33S because of the slope of their absorption x-sections in the 190-220 nm region• Mark Claire and I don’t think this actually works, though. One reason is that OCS is very short-lived in a low-O2 environment. We’re still arguing about how exactly the MIF signal is generated.

190 200 210 220

Wavelength (nm)

• There still are some who don’t believe that sulfur MIF requires low O2

• Hiroshi’s group has suggested that sulfur MIF was caused by reactions between sulfate and organic matter in sediments, not by atmospheric processes• They have produced one datapoint with 33S 2‰ (diagram on right). This was done by reacting solid glycine with sulfate in the lab.• The proposed mechanism for producing MIF is surface reactions --To my knowledge, this has not been supported by theory or, more precisely, the supporting theory has been shown to be incorrect (Balan et al., EPSL, 2009)

Science, 2009

Laboratory data

Sedimentarysulfides

Possible alternative mechanisms for causing

sulfur MIF• Spin-orbit (hyperfine) interactions

between the nucleus and the electron cloud (referred to by geochemists as a magnetic isotope effect, or MIE)– Ruled out for Farquhar’s data because the

effect is seen in both 33S (nuclear spin 1) and 36S (nuclear spin 0)

– Significant 36S measured in 2 of 9 of Yumiko’s experiments inconclusive?

• Nuclear field shift effect– Electrostatic interaction between a non-

spherical nucleus and the electron cloud

http://www.ornl.gov/info/ornlreview/v36_1_03/article_34.shtml

Caption:The likely shape of a deformeduranium-234 nucleus was determinedin 1971 by ORNL physicists.

• This and other unstable nucleides have measureable electric quadru- pole moments

234U nucleusHalflife: ~250,000 yrs

http://ie.lbl.gov/systematics/chart_thb2.pdf

Quadrupole deformation chart

http://ie.lbl.gov/systematics/chart_thb2.pdf

Sulfur

• Bottom line: It’s very unlikely that sulfur MIF was caused by nuclear field shift effects• So, the only fractionation mechanism that appears plausible is still SO2 photolysis Farquhar’s story still stands

Sulfur

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

The rise of ozone

• Ozone (O3) is important as a shield against solar UV radiation

• Very little ozone would have been present prior to the rise of atmospheric O2

• We can calculate how the ozone layer develops as atmospheric O2 levels increase

Ozone and temperature at different O2 Levels

1-D climate modelPhotochemical model

A. Segura et al. Astrobiology (2003)

• The ozone layer• The ozone layer does not really disappear until O2 levels fall below ~1% of the Present Atmospheric Level (PAL)

Ozone column depth vs. pO2

Kasting et al. (1985)

• Why the nonlinearity? O2 + h O + O O + O2 + M O3 + M

• As O2 decreases, O2 photolysis occurs lower down in the atmosphere where number density (M) is higher -- So, O3 column depth is virtually unaffected• Eventually, the photolysis peak moves into the tropo- sphere, where H2O also photolyzes, producing O3- destroying HOx radicals

Conclusions• Atmospheric O2 levels were low prior to ~2.4 b.y.

ago– Cyanobacteria were responsible for producing

this O2

– The best evidence for this comes from sulfur MIF• An effective ozone screen against solar UV

radiation was established by the time pO2 reached ~0.01 PAL, probably around 2.4 Ga

• (Not discussed) O2 probably went up for a second time near the end of the Proterozoic, 600-800 m.y. ago, possibly triggering the Cambrian explosion