Bacterial sulphides from the Archaean

Most of the sulphide mineralization involved in base-metal ore bodies formed by reaction between metal ions and those of sulphur released by bacteria that reduce sulphate ions in water.  They do that while oxidizing organic matter or hydrogen in their metabolism, under completely anaerobic conditions.  Like other biological processes, sulphide production at the cell level fractionates the isotopes of sulphur so that it becomes possible to chart sulphate-reducing bacteria through time.  Depletion of 34S in sedimentary sulphides relative to that in co-existing sulphates (such as baryte) was previously known with certainty back to 2.7 Ga.  Danish and Australian bio-geochemists have now pushed this particular bacterial metabolism back by 750 Ma (Shen, Y.  et al. 2001.  Isotopic evidence for microbial sulphate reduction in the early Archaean era.  Nature, v. 410, p. 77-81).

The data from the Pilbara Craton of Western Australia helps calibrate the evolutionary bush of the prokaryotes, which is based on comparisons between RNA in different living organisms.  The trouble is, sulphate-reducing species with very primitive genetics and similar lifestyles (hyperthermophilic) occur among both the Bacteria and Archaea.  Shen et al. go for the Bacteria Thermodesulfobacterium as the most likely organism responsible.  Their argument is that the mineralization replaces originally sedimentary gypsum, formed at low temperatures, and probably represents hydrothermal processes in which thermophilic organisms could have thrived.  Bacteria that reduce sulphate ions at low temperatures – gram-positive and purple bacteria – are genetically more advanced than their candidate.

See also:  Slime to the rescue Earth Pages December 2000

“Piltdown” bird

Fragmentary remains of vertebrates in particular are notoriously prone to misguided reconstruction – Gideon Mantell placed the Iguanodon’s thumb on its nose, thereby obscuring evidence for the first hitchhiking dinosaur for many decades.  The forger of fossils has two possible motives – spite in the case of Piltdown Man, or profit.  The skilled forgeries of Silurian trilobites by quarrymen from Dudley in Britain’s West Midlands are now more valuable because they were made for rapacious Victorian antiquaries, than bona fide Calymene specimens.  Missing links sought by professional palaeontologists and archaeo-biologists are in a field of their own.  It has long been suspected that birds evolved from small carnivorous dinosaurs, and the early Cretaceous of China has provided spectacular transitional fossils.  Archaeoraptor was announced as the final missing link in 1999.  Within a year it was denounced as a forgery that combines very skilfully the bones of a primitive bird with those of a non-flying dromeosaurid dinosaur.  How it was assembled has finally been revealed using X-ray tomography, which shows that as many as 5 different specimens were “cut and pasted” together (Rowe, T. et al. 2001.  The Archaeoraptor forgery.  Nature, v. 410, p. 539-40).

Cretaceous water lilies

Readers of Earth Pages will be delighted to learn that fossil flowers of Nymphaeales (water lilies) have been found in the Lower Cretaceous of Portugal.  (Friis, E.M. et al. 2001.  Fossil evidence of water lilies (Nympaeales) in the Early Cretaceous.  Nature, v. 410, p. 357-360).

When modern corals emerged

Fossil corals  fall into three taxonomic groups or Orders: tabulate, rugose and scleractinian.  Only the last group is alive today.  Scleractinian corals have been central to the “carbonate factories” that have drawn down CO2 from the atmosphere throughout the Mesozoic and Cainozoic Eras to form reef limestones.  They are major regulators of long-term climate fluctuation.  However, there is something very odd about their appearance in the fossil record, as discussed recently by George Stanley and Daphne Fautin (Stanley, G.D. and Fautin, D.G. 2001.  The origins of modern corals.  Science, v. 291, p. 1913-1914).

The rugose and tabulate corals were exclusively Palaeozoic colonial, carbonate-secreting organism.  Their record ends abruptly with the end-Permian mass extinction.  No examples of scleractinian corals have been found in rocks older than Triassic.  The oddity is a 14 Ma gap in known coral fossils in the earliest Triassic.  Scleractinians secrete calcium carbonate as aragonite, whereas rugose corals formed from calcite; an important difference in processes at the cellular level.  It is hard to avoid the conclusion that the ancestors of scleractinians did not secrete carbonate and were entirely soft-bodied taxa during the Palaeozoic Era.  If Permian Rugosa and Tabulata happily secreted carbonate, while proto-Scleractinia did not, there ought to be a biochemical or geochemical explanation for the last taking on a reef building role in Mesozoic times.

Molecular evidence suggests that scleractinian ancestry goes back to the Late Carboniferous, and that there is a complex “lawn” (as opposed to tree or bush) of genetic relationships between modern hard corals and soft-bodied organisms that are closely related.  The puzzle can potentially be resolved if modern corals and their ancestral lines lost and regained skeleton building several times in the Mesozoic and Cainozoic.  Exploring that requires more understanding of how carbonate is secreted at the cell level, and the geochemical conditions in seawater that underpin the need for secretion.

Following the greatest ever mass extinction at the end of the Permian, early Triassic oceans were almost sterile and anoxic.  Global CO2 levels were high, yet little carbonate was deposited in the marine environment.  That would have increased the amount of calcium and bicarbonate ions in sea water.  Many corals harbour algal symbionts that are involved in calcification.  As calcium carbonate saturation drops so too does carbonate secretion, and vice versa.  Calcium is a two-edged sword in cell metabolism.  On the one hand it is vital in “information” transfer, yet above a threshold it combines with CO2 to form crystalline carbonate within the cell wall, that spells cell death.  In Palaeozoic oceans rugose and tabulate corals, as well as a host of other carbonate secreting animals, would have buffered calcium concentrations below levels tolerable by other, soft-bodied animals.  Their sudden demise 251 Ma ago, along with most everything else, would have left calcium to build up in the early Triassic “Strangelove” ocean.  Survivors of the holocaust would have had a fierce task coping with potential calcium toxicity, and the scleractinians may well have adopted calcification as a survival mechanism.  Thereafter, oceans restocked with reef building organisms would have had tolerable calcium concentrations for most organisms, those now able to secrete carbonate having the benefit of armour against predation and a solid substrate for colony building.


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