Bacterially induced lithification of microbial mats

Below the photic zone within live microbial mats from tidal flats, the cyanobacteria are dead and bacteria are the dominant living biota. Field data indicate that precipitation of calcium carbonate occurs predominantly within the mats in the aphotic zone. In order to determine whether bacteria could be responsible for inducing the precipitation of calcium carbonate within the microbial mats, live microbial mats and bacteria were collected from modern tidal flats. Over 50 experiments were set up in the laboratory in which live, naturally dead, and sterilized (autoclaved) dead filamentous cyanobacteria were inoculated with bacterial cultures. The experiments were conducted within liquid and gelatinous (firm) media under normal laboratory lighting conditions as well as in the dark. Sterilized, non-inoculated microbial mats were used as controls. The experimental data demonstrated that precipitation of calcium carbonate only occurred on cyanobacterial filaments in the presence of live bacteria. Furthermore, dead cyanobacteria were coated with calcium carbonate much more quickly and to a greater extent than live cyanobacteria. In addition, some of the naturally occurring carbonate precipitates within modern microbial mats are essentially identical to those produced in the laboratory by bacterial cultures without cyanobacteria as well as by bacterial cultures in association with cyanobacteria. Therefore, we conclude that bacteria can play a significant role in producing stromatolites by inducing the precipitation of calcium carbonate on dead cyanobacterial filaments in the aphotic zone within modern microbial mats. The micritic, clotted fabric, exhibited by many ancient stromatolites as well as the mesoclotted fabric of thrombolites are inferred to be the result of bacterially induced precipitation of calcium carbonate.