Stromatolites are layered rock-like structures built by communities of microorganisms, primarily cyanobacteria, that trap sediment and precipitate minerals over thousands of years. The most famous living examples sit along the shores of Hamelin Pool in Shark Bay, Western Australia, where they form one of Earth’s most remarkable biological landscapes. These structures matter because their ancient relatives were among the earliest visible signs of life on the planet, dating back more than three billion years, and the living versions in Shark Bay offer scientists a rare chance to study processes that shaped the atmosphere, the oceans, and the trajectory of complex life.
How Stromatolites Build Themselves
A stromatolite is not a single organism. It is the cumulative product of microbial mats, thin living films of bacteria and other microbes that grow on sediment surfaces in shallow water. Cyanobacteria in these mats slow water currents enough to cause suspended particles to settle out of the water column, even when the current would normally keep those grains aloft. The sticky extracellular substances the microbes produce then glue these grains to the mat surface, locking them in place.
Over time, the mat grows upward through this combination of trapping, binding, and mineral precipitation. A new layer of living microbes colonizes the top, the older layer below becomes entombed in sediment and newly formed carbonate crystals, and the cycle repeats. This iterative stacking can produce structures meters thick and beds that stretch for kilometers.
The mineral precipitation side of the process involves tiny carbonate crystals that nucleate within the sticky extracellular substances surrounding the cells. As these crystals grow, the organic material that originally held them gradually disperses and disappears, leaving behind a mineralized framework.
Why Shark Bay
Stromatolites were once widespread in Earth’s shallow seas, but today living examples are rare. One reason the Hamelin Pool population thrives is geography. A shallow underwater ridge called Faure Sill partially blocks the exchange of water between Hamelin Pool and the open ocean, restricting circulation and allowing evaporation to push salinity to roughly twice that of normal seawater. This hypersaline environment severely limits the diversity of animals that can survive there. Critically, organisms that would otherwise graze on microbial mats are largely absent, giving the mats the breathing room to grow undisturbed into lithified structures.
In most modern marine environments, snails, sea urchins, and other grazers eat microbial mats before they can accumulate. Shark Bay’s extreme salinity removes that pressure. The result is a coastline where stromatolites and flat algal-mat sheets flourish across intertidal and subtidal platforms, forming a landscape that has no real equivalent anywhere else on Earth at this scale.
The other major site where significant modern microbialites exist is Exuma Sound in the Bahamas, where both stromatolites and thrombolites (a related structure with a clotted rather than laminated internal texture) grow in close proximity. Comparing these two sites has helped researchers understand how different environmental conditions steer microbial communities toward laminated versus clotted architectures.
A Living Laboratory for Early Earth
The oldest known stromatolite fossils are roughly 3.5 billion years old, found in the Pilbara region of Western Australia not far from Shark Bay. For context, Earth itself is about 4.5 billion years old, so stromatolites appear in the rock record astonishingly early. These ancient structures are central to understanding when and how life began reshaping the planet’s chemistry.
Cyanobacteria in stromatolite-like communities were likely producing oxygen through photosynthesis long before the atmosphere contained meaningful amounts of it. Evidence from stromatolites and biomarkers in ancient lake sediments suggests that oxygen-producing cyanobacteria had evolved by around 2.7 billion years ago, hundreds of millions of years before the atmosphere became permanently oxygenated. Earth’s surface did not become durably oxygenated until the Great Oxidation Event, a drawn-out geochemical shift between roughly 2.43 and 2.22 billion years ago. Before that transition, cyanobacterial communities in shallow water likely created local oxygen-rich “oases” in an otherwise oxygen-free world, and iron isotope data from stromatolites of that era show that iron delivered by deep ocean currents was being rapidly oxidized near these communities.
Shark Bay’s living stromatolites let researchers observe analogous processes in real time. Of particular significance, some subtidal structures in the Spaven Province area of Hamelin Pool bear a striking resemblance to 1.9-billion-year-old longitudinal stromatolites found at Great Slave Lake in Canada’s Northwest Territories. And a coccoid cyanobacterium called Entophysalis, common in the lithifying mats of Hamelin Pool, has an ancient lineage: its precursor, Eoentophysalis, dominated stromatolite assemblages during the early and middle Proterozoic. Finding a living descendant of something that built reefs over a billion years ago is about as close as biology gets to a time machine.
What Lives Inside the Mats
A stromatolite’s surface may look like a simple knobby rock, but the microbial mat coating it is a finely stratified ecosystem. Studies using high-throughput genetic sequencing have shown that three bacterial groups dominate all marine mat types in Shark Bay: Proteobacteria, Cyanobacteria, and Bacteroidetes. These same three phyla show up consistently regardless of mat morphology, forming what researchers describe as the core community.
Even at the millimeter scale, the community changes dramatically with depth. In both smooth and pustular mat types, cyanobacteria dominate the uppermost layer (the top two millimeters or so), where light penetrates and photosynthesis occurs. Below that, the distribution of bacterial phyla becomes more even, shifting toward groups associated with sulfur cycling and fermentation. Microelectrode measurements taken through the mat cross-section reveal steep gradients of oxygen and sulfide that track these shifts in community composition: oxygen peaks near the sunlit surface, then drops to zero within a few millimeters, while sulfide increases with depth.
Beyond bacteria, active eukaryotic organisms also inhabit these mats. Research comparing Shark Bay and Bahamas microbialites has documented eukaryotic communities living within both stromatolitic and thrombolitic structures, complicating the older view that these systems are purely prokaryotic. More recently, a novel member of the Asgard archaea, a group of microbes considered among the closest living relatives of the ancestor that gave rise to all complex (eukaryotic) life, was isolated from Shark Bay microbial mats. The organism was named Nerearchaeum marumarumayae, with the species name drawn from the Malgana language spoken by the Indigenous people of the Shark Bay region, meaning “ancient home.”
Growth Rates and the Age of the Structures
Stromatolites grow extraordinarily slowly. Radiocarbon dating of Shark Bay structures indicates growth rates ranging from less than 0.1 millimeters per year to just over 0.5 millimeters per year. At the faster end, a stromatolite would take two thousand years to grow a single meter. At the slower end, it would take ten thousand.
The growth history of Hamelin Pool’s stromatolites appears to unfold in two broad phases. The first phase ran from roughly 2,000 to 1,100 years ago. The second phase began around 900 years ago and continues to the present day, coinciding with a gradual lowering of sea level on the order of about 1.5 meters to its current position. Between these two phases, there is evidence of a period of exposure around 1,000 years ago, when sea level dropped below the surfaces where microbial mats were growing. The structures you can see today are not ancient in geological terms, but their slowness makes even young ones impressive: a knee-high stromatolite in Hamelin Pool may represent centuries of uninterrupted microbial work.
Separate studies using sediment cores have correlated these microbial deposits with Holocene sea-level variations, confirming that the microbial layer constitutes a relatively young and shallow veneer atop older sediments.
Shape, Size, and What Controls Them
Not all stromatolites in Hamelin Pool look the same. Their shapes range from smooth domes and columnar pillars to pustular, irregular knobs. Some are small enough to fit in your hand; others form large platforms. This variety is not random. Environmental gradients across Hamelin Pool, including increasing salinity, greater temperature swings, and decreasing wave energy as you move southward from Faure Sill toward the Nilemah Embayment, appear to influence morphology, internal fabric, and which microbial communities dominate the building process.
The distinction between lithified and non-lithified mats also matters. In Hamelin Pool, well-laminated, hard structures tend to be built by coccoid cyanobacterial mats, while filamentous cyanobacterial mats produce poorly lithified sheets that contribute sediment to the pool but do not form durable buildups. This finding has implications for reading the fossil record, because it means the relationship between mat type and preservation potential is not straightforward. Some mats build lasting monuments; others dissolve into sand.
A Microbial Carbonate Factory
Hamelin Pool is not just home to a few picturesque stromatolites. It functions as what geologists call a microbial carbonate factory. The lithifying mats construct the stromatolites themselves, but the weakly lithified mats that erode and degrade produce enormous volumes of sand-sized micritic grains. Irregular micritic grains from pustular sheet mats and gelatinous pavement mats together make up close to 26% of the total sediment in the pool, with estimates suggesting the system produces roughly 24,000 metric tons of microbial sediment per year. That is a staggering amount of biologically manufactured rock material for a single embayment, and it means microbial processes are not just decorating the landscape but actively building and reshaping the sedimentary environment.
This realization has shifted how researchers think about ancient carbonate platforms. For decades, Hamelin Pool stromatolites were considered poor analogs for Precambrian stromatolites because of their coarse-grained character, while ancient examples typically have a much finer, micritic texture. But detailed microscopy has revealed that precipitation of fine-grained microbial micrite is actually an important mechanism of stromatolite accretion in Hamelin Pool, sometimes forming laminated micritic framework. Internal fabrics of Neoproterozoic stromatolites from elsewhere in Western Australia show a strong resemblance to the clotted peloidal textures found in Hamelin Pool’s grey micrite. The analog is better than it appeared.
What Stromatolites Mean for Mars
If microbial life ever existed on Mars, stromatolite-like structures are among the most plausible biosignatures scientists could hope to find. The oldest known stromatolites on Earth, the roughly 3.5-billion-year-old Dresser Formation structures in Western Australia, have been studied in two and three dimensions using advanced imaging. Their deep, iron-rich weathering profile makes them particularly relevant as analogs for what altered carbonate structures on Mars might look like. Researchers have argued that if similar macro-, meso-, and micromorphologies were identified in habitable Martian settings by rover imaging systems, those materials would be compelling targets for sample return.
This is not idle speculation. Mars rovers are already exploring terrain where liquid water once pooled, and the criteria for recognizing potential biosignatures lean heavily on what we know from Earth’s oldest and most primitive biological structures. Shark Bay’s living stromatolites serve as a reference library: they show the range of shapes, textures, and chemical signatures that microbial construction produces under known conditions, giving astrobiologists a baseline for distinguishing biological from purely geological structures on another planet.
Cyclones, Climate, and Fragility
Despite their geological pedigree, Shark Bay’s stromatolites are not invulnerable. Severe Tropical Cyclone Olwyn, which struck the area in 2015, provided a natural experiment in storm impacts. The cyclone caused erosional sculpting of sediments, mats, and structures; deposition and winnowing of sediments across the platform; and accumulation of mucilaginous products into flocs, slurries, and sludges. The storm’s effects were not uniformly destructive: some of the biological debris reorganized into new mat-forming material (a “floc-to-mat” biogeomorphic response), while in other areas, enhanced bioturbation by animals moving through disturbed sediment worked against mat recovery.
Climate change adds longer-term uncertainty. Sea-level rise could alter the water depth and circulation patterns that maintain Hamelin Pool’s extreme salinity. If salinity dropped closer to normal marine levels, grazing organisms that currently cannot tolerate the conditions might colonize the area, and the competitive advantage that lets microbial mats dominate would erode. Temperature and salinity gradients across the pool already influence which microbial communities thrive and what structures they build; shifting those gradients could change the character of the system even if it does not destroy it outright.
The Malgana Connection
Shark Bay is Malgana country. The Malgana people have lived in the region for tens of thousands of years, and the microbial mats and stromatolites sit within their traditional lands. This connection is not just cultural backdrop. When researchers recently described a new species of Asgard archaeon isolated from Shark Bay’s microbial mats, they named it Nerearchaeum marumarumayae, with the species name derived from the Malgana language: “marumaru” meaning many nights, old, or ancient, and “maya” meaning camp or home. The full name translates roughly to “of the ancient home,” a reference to the deep antiquity of stromatolites in Earth’s biological history.
The choice of name reflects a growing practice in microbiology of honoring Indigenous knowledge systems and land connections when describing organisms from specific places. For the Malgana, the stromatolites are part of a landscape with its own long story, one that predates and runs parallel to the scientific narrative about cyanobacteria and atmospheric oxygen. Gathaagudu, the Malgana name for Shark Bay, is a UNESCO World Heritage Site, recognized for both its natural and cultural significance. The stromatolites sit at the intersection of those two values in a way that few other geological features anywhere in the world can match.