Experimental investigation of grain trapping capabilities in cyanobacterial vs. algal mats: Implications for grain size in stromatolites through time

In general, Archean and Proterozoic stromatolites are fine-grained (predominantly composed of micrite) whereas most modern marine examples are comparatively coarse-grained. Given that the modern marine forms are commonly studied as analogues to ancient forms, it is important to understand the processes responsible for these textural differences. Cyanobacteria are typically considered the dominant stromatolite builders through time, but it is well known that many modern marine stromatolites also contain, or are even dominated by, eukaryotic phototrophs. Thus, we conducted experiments to test the grain trapping and binding capabilities of cyanobacterial-dominated mats versus eukaryotic algal mats in order to better understand the grain size trends in stromatolites through time. Cyanobacterial mats were collected from Catalina Harbor by the 2012 International Geobiology Course, and eukaryotic mats consisting of filamentous algae were collected from the outflow of seawater tanks at the USC Wrigley Marine Institute on Catalina Island. Mats were cut into rectangles of known area, placed in Petri dishes of known weight, and inclined at six angles (15° increments from 0-75°) in circulating seawater tanks. Known masses of fine (0.125-0.250 mm), medium (0.50-1.0 mm), and coarse (1.0-2.0 mm) sediments were carefully delivered to the inclined mats. Mats were then exposed to one 24 hour light/dark cycle while the grains settled. The mats were removed from the tanks, dried, and imaged under SEM to assess the degree of binding of the sediment. The sediment left over in the Petri dishes was weighed, and the amount trapped by the mats was calculated by difference. The cyanobacterial mats were able to trap fine grains consistently at all angles. At angles higher than 45°, medium and coarse grains were not trapped as efficiently, essentially obeying the predicted angle of repose response. The algal mats trapped medium and coarse grains at all angles. Interestingly, the algal mats were poor at trapping fine grains, which slipped through the mesh of filaments in our experiment. SEM imaging of the cyanobacterial mats showed that after one day of growth, fine grains were thoroughly covered by cyanobacterial filaments and bound. The rare medium and coarse grains that were trapped were also bound. SEM imaging of the eukaryotic mats revealed that the grains, while trapped by the filaments, were not bound into the structure in the way that the grains were intertwined in the cyanobacterial mats. Although we cannot conclude that all fine-grained stromatolites were formed by cyanobacteria, our results suggest that coarse-grained stromatolites (e.g., most modern marine stromatolites) may require a eukaryotic component in order to trap coarse grains beyond the angle of repose. Similar sedimentation experiments using other microbial mats, such as those dominated by diatoms or that produce copious amounts of EPS, may yield additional information on the origins of coarse and fine-grained stromatolites.