Ancient earth
In the limestone sea caves and intertidal zones of Thailand's Phi Phi Islands, a dark, gelatinous film coats the rock surfaces. These are living microbial mats, and complex communities dominated by cyanobacteria. These organisms are direct descendants of some of the earliest life on Earth. By studying them, scientists can observe processes that are nearly identical to those that built the planet's first ecosystems billions of years ago. The mats are modern analogues of stromatolites, the oldest fossil evidence of life on Earth, with some examples dating back 3.5 billion years.
These bacterial communities are not passive inhabitants of the caves. They are active geological agents, building new rock layer by layer. The process is called biomineralization. During photosynthesis, the cyanobacteria absorb carbon dioxide from the surrounding seawater. This metabolic activity increases the pH in the microenvironment around their thread-like filaments. The change in water chemistry causes dissolved calcium carbonate (CaCO3) to precipitate out of the water and crystallize directly onto the bacterial sheaths. Over time, as the bacteria trap sediment and precipitate more minerals, they construct hard, laminated structures that are identical to their fossilized ancestors.