INVESTIGATING THE EFFECTS OF ENVIRONMENT ON SHORT-TERM MICROBIAL MAT SEDIMENT BINDING

Polygonal microbial mats lives in the intertidal zone of Little Ambergris Cay (LAC) in the Turks and Caicos Islands, a <2000-yr-old island composed mainly of ooid sand. They occur in locations both with and without mangroves. Mangroves play an important role in stabilizing and accumulating fine sediments, which can contribute to coastlines keeping up with or outpacing sea-level rise. Microbial mats can also trap and bind sediment, but we do not know whether this process can contribute to large-scale sediment accumulation in the same way as mangrove-driven sediment stabilization. We hypothesized that trapping and binding of sediment by microbial mats on LAC has contributed to the formation of this island during a period of net sea-level rise. We designed a series of lab experiments to determine how quickly polygonal mats can trap and bind sediment in different environmental conditions: supratidal (fully subaerial), subtidal (fully subaqueous) without current agitation, and subtidal with wave agitation. Environmental conditions were simulated in the lab with different aquaria; mats were exposed to a 12-hr light cycle to simulate natural diurnal cycles and subtidal mats were submerged in artificial seawater. Each experiment began with ooid sand being added to the surface of replicate mat samples; the mats were then incubated undisturbed for 8 hours. Ooid trapping and binding was characterized by imaging each mat hourly with a stereoscope for the next twelve hours. Each mat image was point counted to quantify how the proportion of ooids bound by cyanobacterial filaments increased over time. We found that in the subaerial environment, no filaments grew across the ooids, whereas in both subaqueous environments, filament growth occurred throughout the experiment. There was more filament growth in the wave-agitated subaqueous environment than the non-agitated subaqueous environment. The results of these experiments demonstrate that cyanobacterial filament growth over sediment occurs most efficiently when the intertidal polygonal mats are underwater, and that some current agitation can increase filament growth. As the consequences of climate change continue to drive sea-level rise, microbial mat communities like those on LAC may be able to help protect islands by accumulating and stabilizing sediment rapidly.