Microbial Life on Early Earth - Lecture Notes

Microbial World: Life on the Early Earth

Major Events in Earth's History

  • PreCambrian: 4565 – 541 Ma
  • Archaean: 4000 - 2500 Ma (4.0 – 2.5 Ga)

Early Earth Challenges and Origins of Life

  • Sparse and patchy Precambrian fossil record due to:
    • Rock cycle
    • Chemical alteration
    • Simple morphologies
  • Darwin’s Dilemma: Lack of rich fossiliferous deposits in early periods.
  • Origin of replication and translation: No compelling scenarios exist.
  • Origins of organic molecules:
    • Terrestrial: UV, impact, electric shock
    • Extraterrestrial: pseudopanspermia / panspermia
  • Self-assembly: Amino acids in vitro; deep-sea hydrothermal vents

First Life on Earth

  • Oldest rocks: 4.375 Ga
  • First oceans: ca. 4.2 Ga
  • Hydrosphere: low pH, high [Fe^{+2}]_{aq}
  • Atmosphere: low pO2, high pCO2, high UV flux, ‘faint young sun’
  • Lithosphere: heat flow ca. 15 TW (today only 4.4 TW)
  • Late Heavy Bombardment: peak 3.85 Ga
  • Survival of early microbes within cryptic crustal nurseries?

Evidence of Early Life

  • Prokaryote body fossils, stromatolites, chemical fossils.
  • Earliest putative evidence: 3.77 Ga
  • Oldest well-accepted evidence: 1.9 Ga

Carbon Isotope Ratios

  • Important isotopes: C12/C13, S34/S36, N14/N16
  • Biological fractionation can be obscured by metamorphism
  • Phanerozoic OM: \delta^{13}C = -27‰
  • Can abiotic CH_4 production produce similar light isotopic values?

Haematite Tubes and Oldest Life

  • 4.28 Ga haematite tubes from NSB vent deposits, Canada.
  • Carbon isotopes: Isotopically light carbon: - 5–15‰ consistent with biological activity.
  • Carbon: Graphite; crystallization temperature ~500° C
  • Carbonate rosettes: microbial activity
  • The oldest life on Earth: 4.28 Ga (Nuvuaggituk Supracrustal Belt Canada)

Carbon Isotope Ratios and Photosynthesis

  • Microbial methanogenesis: \delta^{13}C = -27‰
  • Oxygenic photosynthesis: \delta^{13}C = -22‰
  • Oldest sedimentary rocks:
    • Isua, Greenland
    • Barberton, South Africa
    • Pilbara Craton, NW Australia

Isua Supracrustal Belt, Greenland

  • 3.77 Ga
  • Carbonaceous microspheres – not necessarily fossil microbes
  • Carbon inclusions within fluid inclusions - origins uncertain
  • Bulk rock analyses: isotopically light: -25‰ Indicates anoxygenic photosynthesis
  • High-resolution ion microprobe analysis of apatite – strongly negative C isotope values: -30‰
  • Oldest evidence for photosynthesis: Isua

Akilia Island, Greenland

  • 3.85 Ga
  • \delta^{13}C = -37‰
  • Early Archaean already had complex communities of photoautotrophs?
  • BUT: The lithological bands are not beds: gneissic banding. Syngenetic / diagenetic?

Iron Oxidation and Photosynthesis

  • \delta^{56}Fe values from magnetite grains
  • Oxidation of FexOy: preferential use of ^{56}Fe
  • Each band has a distinct signature; primary NOT diagenetic
  • Oldest evidence for photosynthesis: Isua 3.77 Ga
  • Deposition of Fe(III) oxides
  • Partial oxidation of Fe(II) rich water
  • How could these volumes of Fe-oxide form without photosynthesis?

Importance of Photoautotrophy

  • Dominant mechanism for energy production for most of Earth's history.
  • Produces 99% of O_2 in the atmosphere.
  • Altered global chemical and ecosystems, facilitating respiration and underpinning eukaryote evolution.
  • Main problem: anoxygenic photosynthesis doesn’t produce oxygen as waste.
  • Big question: When did cyanobacteria evolve?
    * Green sulphur bacterium

Microfossils of the Apex Chert, Australia

  • 3.465 Ga
  • Filaments with ‘compartments’ + Isolated circular structures
  • Rich in C and Fe
  • Encased in rounded pebbles of chert
  • Cyanobacteria?
  • Not sedimentary chert – a hydrothermal vein
  • Structures had been selectively photographed
  • Internal divisions are artefacts
  • Graphite flow stringers

Raman Spectroscopy of Microfossils

  • Spectra similar to abiological amorphous graphite.
  • ‘Fossils’ and irregular globules of carbon have the same signal – identical to amorphous graphite.

SIMS Analyses of Microfossils

  • SIMS analyses document taxon-correlated carbon isotope compositions.

Stromatolites

  • Shark’s Bay, Australia: mm-scale laminations of sediment and microbes
  • Shallow marine
  • Domes or columns
  • Microbially mediated precipitation of CaCO_3
  • Tens of cm to several metres diameter

Stromatolites of Strelly Pool, Pilbara Craton, Australia

  • 3.43 Ga
  • Continuous laminae on steep slopes
  • Different S isotopes signatures in different laminae
  • S-based anoxygenic photoautotrophs

Stromatolites of Dressner Fmn, Pilbara Craton, Australia

  • 3.48 Ga
  • Botryoidal textures
  • Slumped laminae
  • Outward-facing Botryoids
  • Possible biosignatures: gas bubbles? 20x larger than most known fluid inclusions
  • Spherical outline: growth within an elastic medium that deformed around them and prevented passage to the surface
  • Would allow internal radiating crystals to form during H-T alteration
  • Bubbles often preserved today in pools at 45-55 deg C

Late Archaean Rocks and Oxygen

  • Quartz pebble conglomerates (3.0 – 2.5 Ga)
  • Shallow-water delta systems
  • Detrital pyrite (FeS2) and uraninite (UO2): direct precipitates
  • In presence of O_2:
    • FeS_2 -> FeO
    • UO2 -> UO4(aq)
  • NO FREE O_2
  • When did oxygenic photosynthesis evolve?

Stromatolites in the Rock Record

  • 48 Archaean localities

Stromatolites and Cyanobacteria

  • Oscillatoracean cyanobacteria
  • Motile
  • Rejuvenation of mat
  • Most stromatolites do not preserve the framework microorganisms

Bitter Springs Chert

  • Oscillatoracean cyanobacteria
  • 850 Ma
  • Gleobotrydion
  • Globular microstructures
  • Dark central 'nucleus'
  • Oldest body fossils of eukaryotes?

Biomarkers

  • Organic molecules preserved in rocks or fossils that are chemical indicators of life
  • Most are lipids
  • Highly polymerised, progressively aromatised, but can retain core skeleton with diagnostic # C atoms
  • Or robust functional groups e.g. ring structures
  • Cholesterol -> Cholestane -> Sterane
  • CAUTION! contamination

Biomarkers in Kerogenous Shales

  • 2-methylhopanes (derived from cyanobacterial cell membranes)
  • C28-C30 steranes (derived from eukaryotic sterols)
  • Biomarkers of Wittenoom, Pilbara Craton, Australia 2.7 Ga
  • Requires molecular O_2
  • First eukaryote body fossils = 1.8 Ga

Banded Iron Formations

  • Hancock Gorge, Western Australia (2.4-1.9 Ga)
  • Global distribution
  • Vertically persistent
  • Laterally extensive
  • Alternating layers of magnetite/haematite and Fe-stained shale / chert
  • Banded iron formations 2.4 – 1.9 Ga

The Great Oxygenation Event

  • Hadean: no O_2
  • End Archaean: O_2 = < 10^{-9} of current values
  • Cyanobacteria began to produce O_2 between ca. 3.7 and 2.4 Ga (Wittenoom: 2.7 Ga)
  • In 100 Ma O_2 levels rose dramatically from < 10^{-9} to 10^{-1} / 10^{-2} of current values
  • The Great Oxygenation Event 2.4 – 1.9 Ga
  • Decrease in H (sink for O_2)
  • Linked to rise of anoxygenic photosynthetic bacteria (consume H)
  • Increase in Ni availability