FAST-GROWING SLIME CURTAINS REVEAL A DYNAMIC NITROGEN (AND IRON?) WORLD IN THE SHALLOW SUBSURFACE

Manantial del Toro is a flooded cave in the shallow karst plain developed in the La Altagracia province, Dominican Republic. The cave contains more than 3 kilometers of explored passages and has a strongly stratified water column, with oxygenated, nitrate-rich (80 uM) freshwater overlying anoxic sea water. Fast-growing, rust-colored slime curtains are found below the halocline. Water chemistry varies both vertically and horizontally within cave passages colonized by the biofilms. A strict inverse correlation between dissolved sulfate and nitrate concentrations is consistent with complex water flow paths belowground and with diver observations of small seeps and highly permeable wall rock. Multiple biofilm samples analyzed using 16S rRNA gene cloning revealed a community specialized for nitrogen cycling, including ammmonia oxidation, nitrite oxidation, and anaerobic ammonia oxidation (ANNAMOX). Archaeal clone libraries had low diversity and were dominated by close relatives of known ammonia oxidizing archaea and members of the uncultivated Marine Benthic Group E clade. Bacterial clone libraries were more diverse but contained 20-40% of clones affiliated with known nitrite oxidizing or ANNAMOX genera. Relatives of known sulfate reducing prokaryotes were uncommon, and total dissolved sulfide was below detection limits (2 uM) in the water column. Known iron oxidizers were uncommon and affiliated with the putative iron and nitrogen oxidizing clade Deferribacterales. However, based on the rusty appearance of the biofilms we expect that these or other unknown iron oxidizers are active in the biofilms. Future work will be aimed at understanding the niche dimensions of nitrogen and iron cycling taxa within the biofilm, and what role it may play in controlling the release of iron and phosphorous to fragile marine ecosystems at the margin of the karst plain.