Through a Glass Darkly
Microbes may have lived in volcanic glasses that date back to the Archaean era (3.8 to 2.5 billion years ago). These organisms colonize glass in order to extract energy, “eating” metals such as iron or manganese contained within. As they do so, the chemolithoautrophs in volcanic glass may create etchings that remain long after their bodies decay and disappear.
Volcanic glass forms when hot lava is quickly quenched by cooler waters. A centimeters-thin veneer of glass forms on the outside of pillow lavas, and when they cool further and are buried the glass on the outside fractures.
The connection between microbes and volcanic glass was first made by Ingunn Thorseth and Harald Furnes of the University of Bergen, who noticed dimples and depressions on the surface of volcanic breccias from Iceland. The sizes and shapes of the pits were similar to the microbes that they found on the same pieces of glass, so they figured the microbes were somehow dissolving the glass.
But some scientists question whether these marks left in glass are in fact “trace fossils” created by microbes. Water moving through fractures in the glass can ferry microbes inside, but scientists still debate exactly how the organisms survive within the glass, or how they go about dissolving the glass. Critics suggest that the pits, grooves and tubular channels could be created by processes such as glass fracturing, fluid moving through the glass, or maybe even crystals growing within the glass. In other words, microbes live in glass houses, but they may not build them.
For McLoughlin, there is little doubt that the structures in volcanic glass are created by microbes. “The morphologies are so intricate -- the really well-preserved modern examples show spiral and branched shapes -- that I think we can discount bubbles or crystals moving through glass to make channels,” she says. “I’ve convinced myself these are biological.”
In glasses formed recently on the seafloor, the organic remains of microbes can be found plastered on the walls of the tiny glass structures, and there are still traces of DNA. Sequence analysis of this DNA has found bacteria and archea which are 1,000 to 10,000 times more abundant than microbial cells found in the overlying deep sea water. The glass microorganisms include those which use iron and manganese oxidation pathways to produce energy (basaltic glass is rich in this food source).
By studying the microbes in modern glasses, scientists can compare the structures they may create to those found in more ancient rocks where all physical traces of the organisms are long gone. Similar-shaped granular and tubular textures radiating in from the edges of rims and fractures are preserved in volcanic glass that is millions of years in age.
McLoughlin says that titanium-bearing minerals in the glass help to preserve the structures, and also can indicate when the microbes were at work. As the glass was altered on the ancient seafloor a mineral called titanite was precipitated out of the glass and filled the tubular channels left by the microbes, preserving the structures.
“It makes a cast, like pouring Plaster of Paris into a footprint,” says McLoughlin.
The ancient glass subsequently went through crystallization changes as it was buried under the seafloor, metamorphosed, and eventually turned into chlorite. (Chlorite is green, which is why layers of these metamorphosed glasses are known as Greenstone belts). If the titanite had not filled in the structures before the glass was altered, they would have transformed into chlorite as well. “So it preserves the textures, and not only that, because it contains minor amounts of radioactive uranium, it can be dated,” says McLoughlin.
Radiometric dating of the titanite found in Archean pillow lavas by Neil Banerjee and Antonio Simonetti confirmed that the structures also formed in the Archean era. If microbes made these structures, they had to be nearly as ancient as the once glassy lavas they lived in.
“The idea of hydrothermal vents as the cradle of life has been around for some time, but when considering volcanic lavas most people thought these were too hot and uninhabitable,” says McLoughlin. “Traditionally trace fossils have been found in muds and sediments, and so we’re now opening up a new field of looking for trace fossils in volcanic environments.”
McLoughlin and her colleagues are continuing work on structures found in volcanic glasses, trying to understand where and how they are distributed in the earliest seafloor. They are also trying to refine the dates on some of the earliest examples, and to better understand precisely how they formed. "We are hoping to see more clearly through this glassy window into the early Earth," says McLoughlin.