TRAPPING AND BINDING OF GRAINS BY MICROBIAL MATS: EXPERIMENTS WITH LIVING MATS AND IMPLICATIONS FOR THE STROMATOLITE FOSSIL RECORD

Fine-grained stromatolites dominate the Proterozoic and coarse-grained forms do not appear in abundance until much later in the Phanerozoic. Sediment trapping and binding in stromatolites is commonly attributed to the activity of filamentous cyanobacteria, but the trapping and binding abilities of cyanobacteria have not been well-characterized. We conducted experiments in a controlled tank environment with filamentous cyanobacterial mats (dominated by Coleofasciculus) from a tidal flat on Catalina Island, California, in order to better understand the ability of filamentous cyanobacteria to trap and bind grains.

The results were striking: cyanobacteria trapped grains <200 µm, even at angles well beyond the angle of repose. However, they were unable to trap coarse grains >2000 µm at high angles. Trapping of coarser grains at intermediate angles depended on filament protrusion from the mat. The cyanobacteria actively bound grains that they had trapped, regardless of angle. However, binding was impeded by the introduction of flow where grains were lost before they could be bound. The experiments demonstrated the ability of filamentous cyanobacteria to trap and bind fine grains; however, they failed to trap coarse grains except at low angles under low-flow conditions.

We repeated the experiments with a filamentous green alga (Chaetomorpha) that forms thick mesh-like nets. Chaetomorpha have been found in stromatolite-forming environments and were chosen as a model organism to understand how the size and orientation of a filament impacts grain-trapping ability. The algal mats trapped very coarse grains regardless of incline angle, but the fine grains simply passed through the mesh created by the large filaments. In contrast to the cyanobacteria, the Chaetomorpha algae used here did not appear to possess grain-binding abilities. Thus, morphology and habit appear to be critical to the ability of photosynthetic filamentous organisms to trap grains, and binding is an active process that cannot be assumed.

The results of this study support the hypothesis expressed by Awramik & Riding (1988) that the stromatolite “grain size conundrum” can be explained by the evolution of microbial communities containing [larger] eukaryotes capable of trapping coarser-grained sediment.