Sulphur: Harbinger of life?

Picture by JJ Harrison (Wikipedia) - Pyrite (Iron Sulphide)

Almost all life on earth today depends on oxygen for its survival, as it is the agent responsible for respiration and metabolism i.e. the breakdown of food for production of energy. There are some species of anaerobic (without oxygen) bacteria though, living in hot springs and hydrothermal vents on the ocean floor, that survive on sulphur and its compounds.

Scientists at the University of Western Australia and the University of Oxford in UK have now discovered the oldest fossilized cells known to date, which seem to be 3.4 billion-year-old sulphur-metabolizing bacteria. These cells are evidence for life on earth just 1 billion years after our planet’s formation, when the atmosphere was devoid of oxygen. The relatively well-preserved cells were found in areas rich in pyrite (iron-sulphide) in the old sandstone rocks of the Strelley Pool Formation in the Pilbara region of Western Australia.

The carbon-rich spheroidal and ellipsoidal microfossils ranged in size from 5 to 25 microns (a micron is a millionth of a meter), and were only found in pyrite (sulphide) rich regions of the sandstone. Clusters and chains of cells, frequently seen in other bacterial fossil remains, were observed that are suggestive of successful cell division. Cell walls rich in carbon and nitrogen were clearly visible, but they were damged or punctured at many places indicating the release of intracellular material leaving behind ‘hollow’ interiors.

Carbon isotope tests, which measure the relative concentrations of C-13 and C-12, also confirmed the organic nature of the fossils. C-12 is the most stable and abundant form of the carbon nucleus, containing 6 protons and 6 neutrons, while C-13 is a rare isotope containing an extra neutron. Organic matter has a lower C-13 to C-12 ratio than inorganic or non-living matter, and the microfossils indeed showed a low concentration of C-13 corroborating their biological origin. Optical spectroscopy tests further revealed that the carbon was in a disordered form, ruling out abiotic carbonaceous material like graphite.

Sulphur was found to be present locally in the cell walls, and there was a high concentration of micron-scale pyrite grains in the vicinity of the cells, putatively formed from the metabolic by-products of the sulphur-consuming bacteria. Moreover, the isotopic concentrations of sulphur (S-33 and S-34) were found to be consistent with microbial processing of sulphur and its compounds e.g. the reduction of sulphates. This is strong evidence for a primitive ecosystem of unicellular organisms living on sulphur-rich sediments in an oxygen-less earth.

 

Wacey, D., Kilburn, M., Saunders, M., Cliff, J., & Brasier, M. (2011). Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia Nature Geoscience DOI: 10.1038/ngeo1238