Unexplained mountains are always disconcerting for geologists. But for certain sludge-dwelling bacteria, making dolomite is no problem at all.
There was a time when Earth made dolomite in great piles--piles like the Dolomite Mountains, in the Italian Alps, where French mineralogist Déodat de Dolomieu discovered the mineral in 1791. Today, though, dolomite forms in only a few select salt flats and lagoons. The mineral’s ingredients--magnesium, calcium, and carbonate ions--are common enough in seawater, but the conditions necessary for arranging the ingredients in neatly ordered, alternating layers have apparently become rare. Geologists for the last two centuries have been puzzled by the Dolomite Problem, and Judith McKenzie of the Swiss Federal Institute of Technology in Zurich has been preoccupied with it for almost two decades. But now she thinks she may have cracked it. Dolomite, she says, is made by a family of sulfate- consuming bacteria that may once have been far more prevalent.
McKenzie and a Brazilian graduate student, Crisogono Vasconcelos, discovered that dolomite crystals were still forming at the bottom of a lagoon near Rio de Janeiro, in an oxygen-free, sulfate-rich sludge. In general, geologists had thought that sulfate inhibits dolomite formation because negatively charged sulfate ions tend to tie up positively charged magnesium ions in water, leaving them less available for making dolomite. Magnesium is the key ingredient that distinguishes dolomite from other calcium carbonate minerals, such as limestone.
But McKenzie noticed two things about the Red Lagoon (or Lagoa Vermelha in Portuguese): the water was murky and reddish because it had lots of tiny particles in it, and it smelled of rotten eggs. She concluded that the particles were sulfate-reducing bacteria, which get their oxygen from sulfate; in the process they produce hydrogen sulfide, which smells like rotten eggs. Perhaps, McKenzie decided, those bacteria might have something to do with the formation of dolomite in the lagoon.
The general idea was not new: geologist Robert Folk of the University of Texas has for years claimed to see bacteria on electron micrographs of dolomite and other carbonate crystals. McKenzie decided to test the idea experimentally. She extracted bacteria from the Red Lagoon and mixed them into a simulated sludge consisting of sand, nutrients, sulfate, and all the ingredients for making dolomite. Then she put the mixture into a refrigerator. When she examined it a year later, she found a white precipitate clinging to the sand grains, holding them loosely together. Closer inspection under a scanning electron microscope revealed that the bacteria themselves were encrusted with the white crystals--which, from the way they bent X-rays, turned out to be dolomite. Bacteria-free samples of the sludge showed no crystals.
The bacteria, McKenzie thinks, convert the sulfate in the Red Lagoon from a liability into an asset, as far as dolomite formation goes. When they take in sulfate, they’re absorbing magnesium ions too. They use some of them as nutrients and excrete the rest. They do the same with calcium, and they also excrete bicarbonate ions as a by-product of respiration. In other words, the bacteria excrete all the ingredients of dolomite--thus giving those ingredients a chance to come together on their cell walls.
Sulfate-reducing bacteria probably couldn’t have made mountains of dolomite all by themselves. But once they had provided the first crystals, inorganic processes could have taken over, building on the bacterial template. Billions of years ago, McKenzie points out, before higher plants evolved, Earth’s atmosphere contained much less free oxygen, a gas sulfate reducers can’t tolerate. The bacteria may have thrived in many more places then, and that might explain why dolomite formed abundantly.
The Dolomite Mountains, Italy’s Alpi Dolomitiche, however, are a special case: the rock in them was originally simple calcium carbonate made from dead corals and seashells on the seafloor. Only later was it transformed into dolomite through the orderly addition of magnesium--and through the action, McKenzie suspects, of sulfate-reducing bacteria. Many people have thought that the problem with forming the mineral under natural conditions is to get such a high degree of ordering, explains McKenzie. What we have seen in our experiment is that a biological factor can overcome this barrier.