MICROBIALLY DRIVEN NITROGEN CYCLING IN YUCATAN CENOTES

Coastal karst terrains contain conduit networks that allow for rapid exchange of water between the aquifer and the coast. Biogeochemical reactions that control the composition of exchanging water, particularly nitrogen dynamics, are poorly understood. In the Yucatan aquifer, coastal cenotes (water-filled sinkholes) contain a pycnocline, offering a window into nitrogen cycling during transition from fresh to salt water. This research focuses on nitrogen cycling within three coastal cenotes located in Quintana Roo, Mexico. Water was sampled for salinity, temperature, dissolved oxygen, nutrients, N₂/Ar and microbial DNA in depth profiles across pycnoclines and associated chemoclines.

These profiles reveal three distinct zones of microbially driven nitrogen cycling: 1) a shallow nitrate bearing oxygenated zone, 2) a transition zone dominated by denitrification and sulfide production, and 3) a deep anoxic zone dominated by ammonium. In the shallow zone, functional gene ratios indicate that nitrate can either by allochthonous or autochthonous, depending on the dissolved oxygen concentrations and if the cenote surface contains aquatic vegetation. At the top of the transition zone nitrate is removed from the system by denitrification and possibly dissimillatory nitrate reduction to ammonium (DNRA) as evidenced by both a buildup of nitrogen gas (N₂) and an increase in ammonium production functional gene ratios. The deep anoxic zone is characterized by ammonium concentrations as high as 1mmol L^-1, at least partially due to high concentrations of nifH functional genes which fix dissolved N₂. Stoichiometric calculations of N₂ depletion cannot account for all the ammonium present, thus the breakdown of dissolved organic matter at the cenote bottom may contribute to ammonium concentrations in this zone. Nitrogen species evolve from nitrate to ammonium with increasing salinity in the cenotes. The greatest buildup of ammonium in bottom waters co-occurs with well-developed haloclines. Cenotes provide a framework for understanding microbially controlled nitrogen transformations along a pycnocline during groundwater-surface water exchange. Vertical gradients of N speciation and microbial communities found in cenotes should occur in horizontal gradients from inland aquifer to the coast in karst conduits.