Biofilms and BIFs

Biomineralization is a growing topic that ranges from life’s influence on the production of economic deposits of metal ores to even the suspicion that it might play a role in Alzheimer’s syndrome.  The most common, and enduring evidence of the influence of micro-organisms in making rocks are stromatolites made of carbonates that blue-green bacteria have secreted, perhaps from as early as 3500 Ma ago.  Something similar, though it involves eukaryotic algae, is the formation of tufa or travertine where springs emerge from limestones.  Many a child, including my young self, consigned a cuddly toy to “petrifying” springs, such as Mother Shipton’s Well in Knaresborough, Yorkshire.  Few retrieved them, which is why there aren’t many rock-like Teddies around..  Another childhood memory, that bears on biomineralization, is a spring surrounded by orange and brown slime that we supposed was so deadly that only bathing in helicopter fuel would ward off a dreadful end brought on by the faintest splash of the loathsome gunk.  It is a great surprise to learn that such ochreous springs, common where coal mines drain to the surface, might hold a key to the formation of Precambrian banded iron formations (BIFs) (Brake, S.S. et al. 2002.  Eukaryotic stromatolite builders in acid mine drainage: implications for Precambrian iron formations and oxygenation of the atmosphere.  Geology, v. 30, p. 599-602).

Groundwater that has passed through iron-sulphide bearing rocks, becomes both acid and charged with iron-2 after oxidation of pyrite.  It is high acidity and low Eh that dissolves toxic heavy metals and arsenic, rather than their iron content, that make springs of such waters so hazardous to small boys bent on careers as hydraulic engineers (check their shins and fingers for the lingering water blisters that are a sure sign of the onset of arsenic poisoning).  It seems that Euglena, a common “animalcule” in such springs that is easily seen with a cheap microscope, is an ochre (iron-3 hydroxides and sulphates) forming agent.  It is an acid-tolerant, oxygenic photosynthesizer that builds slimy mats.  Given time and substantial supplies of dissolved iron, Euglena actually builds hard structures reminiscent of stromatolites.  Brake and colleagues from Indiana State and Kansas universities, and the Colorado School of Mines, studied Euglena from coal-mine drainages under lab conditions, and provide details of their metabolism.  The modern iron-stromatolites are so like some variants of BIFs from the Archaean and Palaeoproterozoic, when they were at their acme, that the authors suspect their origins in biofilms formed by prokaryotic organisms with similar metabolism to the more complex Euglena.  Until their work, most geologists regarded BIFs as products of inorganic precipitation of iron-3 compounds and silica when iron-2 rich seawater met oxygen produced by photosynthesizing cyanobacteria.  Indeed they speculate that the biofilm makers could have been early eukaryotes, despite the first unambiguous evidence for nucleus-bearing organisms being no older than 2100 Ma.  If they are correct, then such communities would have needed free oxygen, and would themselves have contributed to oxygen build-up in the early atmosphere.

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