Geoscientist Online

Tucked away at the southern end of Shark Bay in Western Australia is a quiet spot called Hamelin Pool. It's a rather unusual place. The seawater here is twice normal marine salinity, and the beach is made of billions of fingernail-sized shells of the bivalve Fragum erugatum. These bivalves are one of the few forms of life able to live in this hypersaline water, along with sea-snakes – and stromatolites; for it was here, in the 1950s, that scientists first realised that these structures - hitherto known only from the fossil record - were still forming today.

Rising from the sea like serried ranks of concrete cauliflowers, these domes of aragonite are essentially living rocks, each having been constructed by benthic microbial communities, dominated by cyanobacteria, along with sulphate-reducing bacteria, microalgae and true bacteria. Formerly known as blue-green algae, the filamentous and spherical cyanobacteria are, like the microalgae, photosynthetic. Only in the last few years have we begun to understand the complex sequence of events by which different microbial communities construct these living rocks. Moreover, the discovery of a number of other different types in a variety of lakes in south-western Western Australia is showing that the archetypal Shark Bay stromatolites may not be as typical as once thought. These other stromatolites are also, intriguingly, providing insights into the reason for the great decline in stromatolite diversity that took place during the late Proterozoic. Because not only does Western Australia boast arguably the world’s greatest diversity of modern stromatolites, it also possesses their richest fossil history - including the earliest evidence of life on Earth (early Archaean, Pilbara region).

Modern stromatolites

Stromatolites are constructed by the activity of microbial communities that trap and bind sediment, but are also able to precipitate carbonate minerals. Researchers from the University of Miami, led by Pam Reid, have recently discovered the intricate way in which stromatolites are constructed by studying ones that flourish in shallow, warm seas around the Bahamas. This research has revealed how a complex sequence of changing benthic microbial communities colonise and then build the stromatolites. Of vital importance to how they grow is the dynamic balance between the rate of sedimentation and periodic biomineralisation by the colonies of microbes living on the stromatolite’s surface.

Bahaman stromatolites are built in four phases. Firstly, a pioneer microbial community is established which is dominated by filamentous cyanobacteria (mainly Schizothrix) that arrange themselves vertically, then wrap around grains of sand, trapping them in a mucus-like film. These are then replaced by another microbial community containing quite different microbes, known as 'heterotrophic' bacteria. These new bacteria are basically organic sludge-degraders, similar to the type that decompose compost heaps. These bacteria form a continuous mucilaginous sheet on top of the first layer of sediment. Next, a third group of bacteria comes to the party. These are sulphate-reducing bacteria that, by feeding on the mucus-like film left by the first community, promote the growth of aragonite crystals and the formation of a thin crust.

The fourth, and final, bacterial community to colonise the surface is dominated by spherical coccoid cyanobacteria (mainly Solentia). These are active microbes that bore into the previously crystallised aragonite crust, leaving behind tiny tunnels that become filled by new crystal growth - a sort of bacterial reinforced concrete. Rather than destroy the mats’ fabric, these cyanobacteria contribute to the construction of the stromatolite. This sequence of colonisation by different types of bacterial communities is repeated thousands and thousands of times, resulting in the slow growth (0.1 to 0.5mm per year) over hundreds, or even thousands, of years.

Although the perception has been that when fossil stromatolites are found this indicates hypersaline conditions, because of the environment in which they form in Hamelin Pool, this is not the case. Salinity probably has little influence on their development. Moreover, arguments that the lack of grazers on the Hamelin Bay stromatolites is due to hypersalinity - implying that the Proterozoic decline in stromatolite diversity was caused by there being more grazers - are not supported by observations on other Western Australian stromatolites.

In Western Australia stromatolites are also known to occur in saline lakes (Lake Thetis), brackish lakes (Lake Clifton and Lake Walyungup) and freshwater lakes (Lake Richmond). What these sites, along with Hamelin Pool, have in common is that they are found close to limestone (the coastal Pleistocene Tamala Limestone, an aeolian calcarenite). This means that the groundwater feeding the lakes is rich in calcium bicarbonate. The other significant factor that determines whether stromatolites form is that the groundwater providing the carbonate for construction should be low in nutrients. This means that the microbial communities have very few competitors. In other parts of the world stromatolites have been found growing in what are, to most organisms, highly stressful environments. Hot springs in Yellowstone National Park, USA, freezing lakes in Antarctica, and highly alkaline lakes in East Africa all contain stromatolites - but few nutrients to encourage competing organisms.

Microbial communities construct stromatolites in two ways. One is by trapping fine sediment with a sticky film of mucus that each cell secretes, then binding the sediment grains together with aragonite precipitated from the water. Because the cyanobacteria are both photosynthetic and able to move toward the light, they can keep pace with the accumulating sediment. They therefore always remain on the outer surface of the stromatolite. Some of the Shark Bay stromatolites are formed by this method, but others form in the second way - mainly from the precipitation of aragonite, forming a framework that incorporates little sediment.

This variability in internal structure depends on their position relative to the shoreline, as hydrodynamic activity affects the amount of sediment in the water, while water depth affects the components of the microbial communities. Understanding the subtle difference in their morphology at different water depths is potentially very important in interpreting the palaeoecology of Precambrian stromatolites. In the intertidal zone at Hamelin Pool so-called 'pustular-mat' stromatolites occur. These are formed principally by the coccoid cyanobacterium Entophysalis. These stromatolites are not laminated and have a poorly-defined internal structure. They are predominantly formed by the trapping and binding of coarse, sandy carbonate sediment.

From the lower intertidal region down to the upper subtidal zone, 'smooth-mat' stromatolites occur. These are constructed by a community dominated by the filamentous cyanobacterium Schizothrix, which are laminated, have a well defined internal texture and a smooth outer surface. They are also formed by the trapping and binding of coarse, sandy carbonate sediment. The deepest stromatolites found live at a depth of about 3.5m, where they occur as weakly laminated, coarse-structured 'colloform-mat' stromatolites up to a metre tall. These are constructed by a complex microbial community, including cyanobacteria Microcoleus and Phormidium, as well as Entophysalis. Some algae, mainly diatoms, are also found in this zone. Like cyanobacteria, some diatoms can secrete copious quantities of sediment-trapping mucus. Studies by Pam Reid and colleagues have shown that these subtidal stromatolites are formed from micritic carbonate mud precipitated by the microbes.

Whereas many of the Hamelin Pool stromatolites are typically well layered, the stromatolites in lakes are generally either poorly layered, or not layered at all, developing a thrombolitic texture (box). Situated about 100km south of Perth, brackish Lake Clifton is a 21.5km long, narrow lake, no more than one kilometre wide, and reaching a maximum depth of only about 3.5m. Like other coastal lakes in Western Australia, the water level rises during the winter and drops during the summer as the discharge of groundwater into the lake oscillates seasonally. Consequently, many of the stromatolites become emergent during the summer. At the northern-eastern end of the lake the thrombolitic stromatolites are so numerous that they have coalesced to form a reef about 30m wide and extending for at least 5km.

The Lake Clifton stromatolites are formed largely by the product of the precipitation of aragonite by the filamentous cyanobacterium Scytonema. Linda Moore, while working at the University of Western Australia, discovered that variations in groundwater discharge to Lake Clifton affect not only the distribution of stromatolites but also their shape. The brackish water in which they grow is populated by a diverse invertebrate fauna containing many grazers, mainly crustaceans (isopods, amphipods and ostracods) and worms (polychaetes and nematodes). The stromatolites provide both a source of food and a refuge for these animals, showing that the salinity of the water has little to do either with stromatolite formation or whether or not they are able to persist under grazing pressure.

To test the observation that stromatolites can grow effectively under high grazing pressure Linda Moore and her colleagues at the University of Western Australia established a tank in the laboratory in which stromatolites were kept with grazing invertebrates. After 14 weeks they found that not only had the stromatolites not deteriorated, but they had actually grown, severely weakening the old yarn about the late Proterozoic decline of stromatolites being due to increased grazing pressure from newly evolved invertebrates.

Stromatolites in the other lakes have their own unique features. For instance, those in the saline Lake Thetis, about 240km north of Perth, are broad, low domes up to 3m across, within which coarsely laminated stromatolites show evidence of columnar growth, like many Precambrian forms. Also, many of the stromatolites, when eroded during seasonal exposure, form doughnut-like structures, the central part having eroded away, possibly because it was less well lithified. Interestingly these mirror very closely the structures seen in the Purbeck Limestone in Dorset, most famously at the “Fossil Forest” at Lulworth Cove. Here, what are clearly thrombolitic stromatolites are said to have grown around tree trunks. Apart from the fact that modern stromatolites are not known to grow in this manner, these structures have a close modern analogue in the Lake Thetis stromatolites that have never been near a tree in their lives.

Stromatolites, as I have indicated, are formed of aragonite. However in Lake Walyungup, close to the southern outskirts of Perth, stromatolites are preserved most unusually as hydromagnesite. Studies at Curtin University in Perth suggested that this mineral is an early diagenetic replacement for aragonite. Possibly so much calcium carbonate is removed from the water in the lake that it becomes enriched in magnesium. This high level of magnesium is shown also by the growth in the lake of primary dolomite.

Ancient stromatolites

About 800km northeast of Hamelin Pool, deep in the heart of the Pilbara region of Western Australia, lies the North Pole Dome (Australian humour – it’s actually one of the hottest places on Earth). Remarkably, the layered red, white and black rocks that poke up through the green spinifex grass are little changed from when they were deposited as sediments about 3.43 billion years ago. In the early 1980s geologists Roger Buick and John Dunlop first suggested that these mysterious, structures were stromatolites. Although some that doubted this, subsequent discoveries have demonstrated their biological origin.

The first stromatolites found by Buick and Dunlop in the Strelley Pool Chert were simple domes; but more recently more complex structures have been found. Some are small, cone-shaped stromatolites, arranged in clusters, not unlike an egg carton. Others are the more typically domed-shaped, while some are wavy or columnar in shape, and in a few instances branched. Until these discoveries, this range of morphologies was not thought to occur until the Proterozoic. Seeing this relative complexity of structures at such an early date suggests a reasonably long evolutionary prehistory, perhaps pushing back the origin of life to closer to 4000 million years.

The environment in which these stromatolites grew is still open to question. The Warrawoona Group, within which the Strelley Pool Chert occurs, consists largely of volcanic rocks, mainly basalt, several kilometres thick along with a variety of sedimentary rocks. Because of the similarity of some of the stromatolites, especially the 'egg carton' forms, to stromatolites growing in some hot spring environments today, geologists at the Geological Survey of Western Australia have argued for a similar hot-spring setting for these Archaean forms. However, a team at Macquarie University consider it more likely that these most ancient of stromatolites grew in an extensive shallow sea into which lava periodically erupted from nearby volcanoes.

Stromatolites are so common in many Proterozoic rocks in Western Australia and show such high diversity that Kath Grey of the Geological Survey of Western Australia has demonstrated that they are valuable tools for correlation between sedimentary basins in the region. Moreover, because different stromatolite forms seem to be restricted to certain sediment types, which reflect deposition under different environmental conditions, they are also providing invaluable information on interpreting ancient environments. For instance, Grey has shown how, in the 2000 million year-old Duck Creek Dolomite in south-western Pilbara, two forms of stromatolites can be recognised. One is a columnar variety called Pilbaria, which can be interpreted as having grown in shallow lagoons and into the lower intertidal zone. The other, a broader-domed branching variety called Asperia, appears to have inhabited pools of water in high intertidal or supratidal regions. These two types of stromatolites can be recognised in many Proterozoic rocks, and similar analogues can still be seen living today.

Conservation

Despite the fact that microbial communities and the stromatolites they construct have lived on this planet for three-quarters of its existence, they are not escaping the devastation that Homo sapiens has inflicted on other ecosystems. Although environments such as tropical rain forests are fast disappearing because of direct exploitation, stromatolites may be suffering more indirectly. This is most clearly shown in the brackish water stromatolites of Lake Clifton. Increases in human population density in the area, and extensive application of superphosphate to agricultural areas over the last few decades, have the potential to affect stromatolite growth adversely. Artificially increased nutrient levels in Western Australian lakes and the consequent potential decline in the stromatolite-producing microbial communities is a good, albeit sad, analogue for what really occurred in the Late Proterozoic when animal and plant communities began to expand, along with rising nutrient levels, resulting in the rapid decline in stromatolite diversity.

Attempts have been made to preserve some of the Western Australian stromatolites by creating reserves. The Hamelin Pool stromatolites are now part of the Shark Bay Marine Park and a World Heritage area. At Hamelin Pool and Lake Clifton boardwalks have been constructed that allow visitors to walk over the stromatolites and the sensitive microbial mats. The Lake Clifton stromatolites, which occur in Yalgorup National Park, are currently under assessment for listing as an endangered ecological community under the Australian Environment Protection and Biodiversity Conservation Act.

The vulnerability of living stromatolites arises from their sensitivity to the quality of the groundwater and their very slow growth. Until community awareness of the fragility of these structures is heightened, this ecosystem that has survived 3500 million years is in danger of being destroyed. As a guide to the health of this planet, there can surely be no better measure than this most ancient of all ecosystems.

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