Living Fossils: Microbialites

Some of the oldest known fossils in the world are mounds of mud—now turned to rock—formed by communities of bacteria. These mounds, called microbialites, are known from as many as 3.5 billion years ago.

Fossil specimen of the stromatolite Collenia versiformis from the Proterozoic of Montana. Specimen is from the Cornell University Paleobotanical Collection (CUPC), Ithaca, New York.

Fossil specimen of a stromatolite from the Silurian of Herkimer County, New York. Specimen is on display at the Museum of the Earth, Ithaca, New York.

Microbialites form when bacteria cause chemicals—especially calcium carbonate (CaCO3), which forms limestone—to precipitate out of water. The bacteria also form sticky mats that can trap sediment from the surrounding environment. The resulting structures are layered (stromatolites) or lumpy (thrombolites). Although rare, structures like this are still forming around the world today, including in New York State! Shark Bay, Australia is another world-famous site, containing the most diverse and abundant living stromatolites in the world.

Microbialites of Green Lake State Park, Fayetteville, NY

Green Lake (and its close neighbor Round Lake) are small, glacially-formed lakes east of Syracuse, NY.

Green Lake (and its close neighbor Round Lake) are small, glacially-formed lakes east of Syracuse, NY. These lakes are meromictic, meaning that they do not mix or overturn when the weather cools in the autumn. This is because salty ground water seeps into the lakes through the surrounding rocks. The seeping groundwater also contains abundant carbonate, dissolved from nearby ancient limestones. This mineral-rich water is heavier than fresh water, so it stays at the bottom of the lakes. The life that inhabits this unique environment is distinctive, especially the bacteria. At least 123 kinds of bacteria have been identified in Green Lake, as well as sponges, animals which are very rare in fresh water.

Many bacteria live in the upper layer of the lake, where sunlight penetrates the water. The bacteria use the sunlight to make food through photosynthesis. This process causes calcium carbonate to precipitate out of the water, creating thrombolites, which are masses of cream-colored lime mud. The mud forms ledges up to 30 feet thick along the best-lit parts of the lake shore.

"Microbialites: The Freshwater reefs of Green Lake" by ESFResearch (Youtube).