Shark Bay, at the westernmost point of Australia is known for white sandy beaches, abundant marine life and exceptional fishing. But for me the greatest attraction is the stromatolites. Stromatolites don’t look like much—clumps of bumpy dark brown or black rocks poking up from the salty waters of a shallow bay, but they have a remarkable story to tell. These rocks are actually alive with communities of billions of photosynthetic microbes. They consist of layers of single-cell organisms called cyanobacteria that produce sticky films that glue together grains of sand and fine sediment. Ever so slowly, layers of calcium carbonate like tree rings build up rock-hard lumps stone. Typically they grow less than one millimeter a year, but if conditions are favorable for thousands of years, they can grow the size of boulders. We owe our lives to the cyanobacteria in stromatolites, because if it weren’t for the ability of this microscopic bacterium to harness sunlight, taking in carbon dioxide from the air and releasing oxygen, the earth’s atmosphere would not be suitable for humans or other animals to breathe.

The cyanobacteria of stromatolites evolved early in the earth’s history, about 3.5 billion years ago. At that time, the atmosphere consisted primarily of carbon dioxide, carbon monoxide, methane and hydrogen sulphide. Oxygen represented less than 1% of the air. Yet once this humble bacterium invented photosynthesis, it was poised to transform the atmosphere. Vast tracts of seashore became inhabited by bacterial ecosystems which were able to extract carbon from carbon dioxide in the air (and in the process release oxygen) and generate a mucous-like sticky substance that bound together particles of sand and clay. Other species fed on the mucous and produced microbial mats—thin films forming layers and binding to substrate beneath them. Another kind of bacteria then flourished on the mats using calcium in the environment to crystalize the mineral aragonite. Aragonite, a form of calcium carbonate, turned the mats into rock-like structures. A fourth kind of bacterial community was available to perforate the rock with tiny holes and cause additional crystallization. This process then repeated itself countless times, generating massive stromatolite colonies wherever conditions allowed.

By 2.7 billion years ago, cyanobacteria were thriving all over the planet, having produced vast tracts of stromatolites. By 2.3 billion years ago, stromatolites had raised the level of atmospheric oxygen to several percent, a concentration high enough to oxidize dissolved iron in the oceans. Huge deposits of iron oxide (rust) were laid down. Today most sources of iron ore date back to this time of iron oxidation. Around 2 billion years ago the process of iron oxidation came to a close. At 1.5 billion years ago, photosynthetic plants came onto the scene, producing more oxygen. Plants and cyanobacteria continued to raise the level of oxygen in the atmosphere to over 20%. By about 1 billion years ago, animals evolved that could breathe oxygen, and during the Cambrian period 600 million years ago an explosion of marine life took place. Within the next several hundred million years or so, land animals including dinosaurs, birds and mammals evolved on the planet. Then in the final few million years, a blink of an eye in geological terms, humans appeared on the scene. So in a real sense, we all owe our lives to stromatolites.

Cynthia and I saw our first stromatolites at Hamelin Pool at the southern end of Shark Bay. Hamelin Pool is partially cut off from the rest of the Bay by a sandy sill of sea grass. There is very little fresh water running into Hamelin Pool, so as seawater flows in, it evaporates and concentrates the level of salt. These are conditions unsuitable for many marine organisms, but are ideal for cyanobacteria.

From the boardwalk, we couldn’t get close enough to see stromatolites expelling oxygen. Fortunately however, at a museum in the nearby telegraph station, someone has been dutifully caring for an aquarium that contains possibly the world’s only captive stromatolite. By looking closely in the aquarium, we were indeed able to see bubbles (presumably oxygen) stuck to the crevices on the stromatolite.

Where stromatolites have been cut to reveal their inner structure, it is easy to see the layers of microbial mats. This particular specimen, also at the telegraph museum was extracted from an iron ore deposit in Northwest Australia.

The information I have used in this blog post have been gleaned from the information signs at the Hamelin Pool boardwalk, the Shark Bay World Heritage Discovery and Visitor Centre, and the book Stromatolites by Ken McNamara (2009, Western Australian Museum). I have not examined original research papers about stromatolites.

One of the most interesting factoids presented in Ken McNamara’s book concerns the use of fossilized stromatolites to infer the number of days in the year hundreds of millions of years ago.

“Because many of the microbes involved in stromatolite growth are photosynthetic, they are only active during the day. It has been argued that this is why it is possible, in some exceptionally well-preserved stromatolites, to identify daily growth layers.”

McNamara references a research paper (Awramik, SM and Vanyo, JP 1986. ‘Heliotropism in modern stromatolites.’ Science 231: 1279-1281.):

“Such calculations were carried out by Awramik and Vanyo, who showed that the stromatolite Anabaria juvensis from the late Proterozoic in the Northern Territory that lived about 850 million years ago recorded about 435 days in the year. This means that the day length would have been about 20.1 hours.”

However, I am skeptical of this claim. If the growth rate of a stromatolite is less than 1mm per year, then evidence of daily growth layers of ~400 per year would I think be exceedingly difficult to see. It would require microscopic resolution of a fossilized specimen. So I would like to track down the article in Science and see what evidence they present.

David