Growth of Microbial Mats on Hard Nuclei in Shallow Sandy Environments

The growth of most stromatolites is a result of interactions among the growth of microbial mats, mineral precipitation, water flow, and sediment movement. Here, we ask how oxygenic photosynthetic microbes colonize surfaces and interact with sediments in high-energy environments that contain constantly moving sand. For this, we investigate cyanobacterial growth on centimeter-scale concrete spheres in a continuously agitated wave tank. Cyanobacteria are unable to colonize moving sand, but establish biofilms on spheres within 5-6 weeks. These biofilms trap up to 0.5 g/cm² of sand on the top and 0.3 g/cm² on the sides within 25 weeks. The colonization does not depend on the size of the spheres, but instead depends on their surface roughness. Cyanobacteria easily colonize spheres with a surface roughness that matches the bed grain size (0.3 mm), but cannot colonize the initial topographic highs with a roughness of ~0.001 mm. In both cases, recesses on the surfaces of the spheres protect cyanobacteria from sandblasting. Thus, microbial biofilms can become established even in high-energy environments, if topographic highs are large enough not to be rolled around by the flow and rough enough to provide attachment loci. If cementation occurs within biofilms, the interplay among biofilm growth, sediment trapping, and cementation can lead to the upward as well as lateral growth of stromatolites. These experimental observations can explain the preferential upward growth of stromatolites on topographic highs in areas with frequently mobilized sediment grains, including modern stromatolites in the intertidal zone in Shark Bay and the subtidal zone in The Bahamas.