Slime to the rescue

In waters that are anaerobic, metabolism of dead organic matter requires a means of accepting electrons transferred away from the necessary oxidation, other than that which involves oxygen as an electron acceptor.  Some heterotrophic bacteria achieve this by the simple chemical trick of reducing sulphate ions (SO42-) to sulphide ions (S2-).  This form of heterotrophy does not oxidise carbohydrate back to carbon dioxide plus water, but produces methane.  In the context of economic geology, it is the generation of sulphide ions that is more interesting, for any dissolved metal ions will swiftly combine with sulphide to form highly insoluble sulphides – the general form taken by many ore minerals.  This is the process observed to occur around deep-ocean hydrothermal vents, where biogenic sulphide ions cause metals dissolved in the hot water to precipitate and form the dark clouds from which such vents get their name – “black smokers”.  Many metal deposits are now known to have formed in such an environment, notably the volcanogenic massive sulphide or VMS ores.

However, there are many sulphide ores that have no obvious relationship to hydrothermal vents, such as sediment hosted deposits like the massive lead and zinc sulphide deposits of the Mississippi type.  Moreover, most sulphate-reducing bacteria are intolerant of oxygen whereas sediment-hosted deposits often bear isotopic witness to the presence of oxygen.  But, deposits of that kind often show intricate fine banding, suggesting slow deposition of fine-grained sulphides.  Some light is thrown on the problem by a daring piece of research involving sampling from flooded caves in a flooded Pb-Zn mine in Wisconsin (Labrenz, M.  et al. 2000.  Formation of sphalerite (ZnS) deposits in natural biofilms of sulfate-reducing bacteria.  Science, v. 290, p. 1744-1747).  SCUBA divers recovered scum formed by bacterial filaments or biofilm, and analyses showed the clear association of the bacterial cells with nanometre-scale spheres of zinc sulphide.  The species of sulphate-reducing bacteria involved is not exactly oxygen-loving, but will tolerate moderate levels dissolved in water.  Here clearly is a means for the formation of low-temperature massive Pb-Zn sulphide deposits.

The astonishing feature of the results of Lanbrenz and co-workers is that the zinc sulphide forms from water with very low levels of the metal (less than one part per million).  The bacteria, or at least their metabolic products, scavenge the metal, and quite probably dangerous cadmium, extremely efficiently.  Chances are that similar bacteria could also pick out lead and arsenic.  That opens up a new means of  bio-remediation – clean-up of both mine waste and contaminated drinking water.

The activity of sulphate reducers leaves its signature on the sulphur isotopes of ancient sediments, revealing periods when the burgeoned, as in Phanerozoic black-shale strata.  They were most active in this respect before about 2 billion years ago, when atmospheric oxygen levels were so low as to diminish oxidation by that highly active gas.  It seems that sulphate reducers also promote the precipitation of dolomite – (Ca,Mg)CO3 – over that of calcite in sea water.  This tallies with the common association of dolomitization of calcite in many sedimentary sulphide deposits, and also with the predominance of dolomites over limestones in the early Precambrian. [see also:  Vasconcelos, C. and McKenzie, J.A. 2000.  Sulphate reducers – dominant players in a low-oxygen world.  Science, v.  290, p. 1711-1712].

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