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.
Architects of the atmosphere
The ancient relatives of the Phi Phi cyanobacteria performed a planetary-scale transformation. Through oxygenic photosynthesis, they gradually released immense quantities of oxygen, a waste product for them, into the atmosphere. This led to the Great Oxygenation Event around 2.4 billion years ago, which fundamentally altered the chemistry of the planet and allowed for the evolution of complex, oxygen-breathing life. Today, stromatolites are rare, found only in specific, often harsh environments where conditions prevent grazing animals from consuming the mats and the water chemistry is just right.
The caves of the Hat Noppharat Thara–Mu Ko Phi Phi National Park provide such a refuge. The combination of a limestone (karst) area providing calcium-saturated water, intense sunlight for photosynthesis, and protection from grazers allows these communities to thrive. Genera of cyanobacteria found in these types of mats include Scytonema and others from the order Oscillatoriales. Studying these living systems shows geobiologists how the earliest lifeforms interacted with their environment and built the world we know today.