Stromatolites, fossilized cyanobacteria

Stromatolites, formed by the "algal" blooms of cyanobacteria, are one of the oldest biological expressions of planetary evolution.
Stromatolites are rocks that originate from the activity of microorganisms, usually cyanobacteria (also known as cyanophyte or blue-green algae).
At present they are mainly responsible for the so-called "algal blooms" in ponds and water courses during warm days. Their growth is greater when the waters contain abundant nutrients (phosphorus, nitrogen, etc). However, their ubiquity makes them thrive in almost all aquatic environments when certain conditions of temperature and calm waters occur.
These microorganisms induce the crystallization of calcium carbonate during photosynthesis and at the same time trap particles transported by water, producing the formation of thin layers of carbonate that acts as cement. Currently, most stromatolites form in lakes and seabed. The oldest known fossil stromatolites are about 3.5 billion years old, and represent the first evidence of life on Earth.
Some stromatolites may be formed by other biochemical processes,

There are stromatolites formed by oxides of manganese (black), and not by calcium carbonate as usual. These stromatolites can be formed in dark environments.
In such cases the manganese dissolved in the water of underground rivers is used by microorganisms specialized in oxidizing manganese. These bacteria, from the group of "extremophiles", are adapted to live in extremely hostile environments, in this case by the total absence of light and the lack of nutrients. They are "chemosynthetic" organisms because to synthesize their organic matter they use a chemical reaction (the oxidation of manganese), instead of using light as do photosynthetic organisms. It is probable that in some planets of the Solar System also exist microorganisms adapted to this type of underground environment.