Deep Time History
The rock-like structures dotting the shallow waters of Hamelin Pool are modern examples of one of the earliest forms of life on Earth. These microbial communities, known as microbialites, are built by complex communities of microorganisms, primarily cyanobacteria. They look like rocks, only the thin outer layer contains living microbes, which trap and bind sediment with sticky secretions and precipitate minerals like calcium carbonate from the water. This process builds the structure very slowly. The mounds at Hamelin Pool grow at a rate of less than 0.5 millimeters per year.
The structures here are often called stromatolites, which means "layered rock," but many display a clotted internal structure, earning them the name "thrombolites" from the Greek thrombos, meaning clot. This clotted form results from a different construction process than their layered cousins. The community of organisms is surprisingly diverse. Genetic analysis shows that cyanobacteria, long thought to be the main builders, make up less than 5% of the microbial population. The majority consists of other bacteria like proteobacteria and planctomycetes, as well as archaea. This complex ecosystem is shows how life may have functioned 3.5 billion years ago, before the evolution of plants or animals.
The Hypersaline Haven
Hamelin Pool provides a perfect, protected environment for these microbial structures to exist, a condition now rare on Earth. The primary reason is its hypersaline water. A massive seagrass bank, the Faure Sill, restricts the flow of water between the pool and the rest of the Indian Ocean. This, combined with a hot, dry climate and high evaporation, makes the water's salinity range from 55 to 70 parts per thousand—about twice as salty as normal seawater.
This extreme saltiness is a barrier to most marine animals. Grazing creatures like snails, which would otherwise feed on the microbial mats, cannot survive in these conditions. This lack of predation allows the slow-growing thrombolites to flourish undisturbed across 100 kilometers of shoreline, from the intertidal zone to depths of about 4 meters. This unique environment, recording Earth's ancient past, is why Shark Bay's designation as a UNESCO World Heritage site in 1991.