Exploring the relative importance of surficial cyanobacteria communities versus physical, chemical and bioloigical processes at depth on sulfur cycling in two microbial mat environments

Abstract

Microbial mats are multi-layered structures built by morphol. and metabolically diverse consortiums of microorganisms. In many mats, the predominant fabric and biovolume of the structures are primarily generated by Cyanobacteria, which carry out photosynthesis at or near the mat surface. Both the mat-building and mat-dwelling communities shape the morphol. and geochem. gradients with depth in the mat. After dioxygen, redox transformations of sulfur fuel mat biogeochem. and provide a useful window into understanding community structure and biogeochem. cycling in the mat. In particular, it has been shown that the top of the zone of sulfide accumulation migrates through the day in response diel light cycles: it lies at a greater depth in the day when dioxygen is produced by photosynthetic Cyanobacteria, and moves towards the surface at night when photosynthesis is no longer active and oxygen accumulation is limited by rates of diffusion through the mat surface. We analyzed sulfur isotope patterns in pore water sulfide in order to understand how different types of microbial communities and environmental conditions influence microbial-driven geochem. reactions in two Cyanobacteria-rich systems:. (1) saline microbial mats lining a shallow lagoon at Little Ambergris Cay, Turks and Caicos Islands, and. (2) benthic microbial mats underlying brackish, low-O_2 waters in Middle Island Sinkhole in Lake Huron, near Alpena, Michigan. We find that diel variations in pore water sulfide sulfur isotope gradients penetrate to depths of up to 8cm below the zone of active photosynthetic activity. This implies that, although photosynthetic activity may play a role in influencing vertical migrations of the top of the zone of sulfide accumulation, other phys., chem. and biol. processes influence sulfur cycling at greater depths. By comparing these two sites, we provide insight into how both environmental conditions and microbial communities shape biogeochem. patterns in microbial mats.