Lab - Cyanobacteria and the formation of stromatolites

Official Extract:
Senior Year, Specialty: Atmosphere, hydrosphere, climates: from the past to the future.
Climate and atmospheric variations on a geological time scale.

Scientific Problem:
The primitive Earth's atmosphere was rich in carbon dioxide and devoid of oxygen.
Conversely, the current Earth's atmosphere is rich in oxygen and relatively poor in carbon dioxide. The evolution of the primitive atmosphere is to be linked to the appearance of life, particularly cyanobacteria, photosynlanthropic prokaryotes, probably 3.5 billion years ago.

Procedure:
Dating the appearance of life on Earth with certainty is currently difficult. However, sedimentary evidence dating back 3.5 billion years and containing numerous fossils seem to attest to biological activity in the oceans during that period. This is the case with the oldest stromatolites (Australia, South Africa, Quebec, etc.), consisting of successive concentric layers alternating limestone beds and fossilized bacterial mats.
The most common hypothesis to explain the construction of stromatolites is the action of cyanobacteria that would facilitate the precipitation of calcium carbonate, the origin of the limestone beds.
To test this hypothesis, a suspension of current cyanobacteria (strain Anabaena PCC6309) is used along with a calcium hydrogenocarbonate solution and a polarizing microscope. The objective is to place the cyanobacteria in the presence or absence of calcium hydrogenocarbonate in order to understand how stromatolites form (observation of the formation of calcium carbonate crystals upon contact with cyanobacteria).