Assessment of empty microbial sheaths in sediment stabilization, Catalina Harbor, California

Tidal mudflats in Catalina Harbor, California, represent a unique opportunity to examine how microbial mats alter and stabilize their environment under dynamic conditions. Previous work has demonstrated that complex microbial communities are responsible for trapping and binding new sediment grains onto the surface of actively growing microbial mats, however little work has been done on the internal structure of the mat communities within the sediment and their resilience to disruption. We examined the microbial communities within the mats, the associated sediment system and the mechanisms used to maintain stability despite diurnal tides and physical disruption. Filaments and sheaths ( 10µm wide by >100µm long) were present on the surface of the mat and up to two centimeters below the mat surface. In contrast to the sheaths observed on the mat surface, the majority of the sheaths within the sediment were empty. Despite the fact that living cells no longer populated the sheaths, they maintained a bundled structure and appeared to stabilize sediment. To test the stability of the sediments without actively growing organisms, one set of mat and associated sediment samples were killed by autoclaving and another was autoclaved and solvent washed to remove organic material. Results demonstrated that the autoclaved mat and associated sediment maintained coherency despite the absence of living organisms, as the empty sheaths remained and supported sediment stability. In contrast, the solvent-washed samples lost coherency. This work demonstrates that microorganisms in mat communities can fundamentally alter the behaviour of sediments, and provides insight into how these communities stabilize marine, tidal mudflats even after the death of filament-producing organisms.