HR: 17:00h
AN: B42E-05 [PDF]
TI: Nitrogen isotope evidence of ammonia vapor assimilation by cave wall microbial biofilms in a sulfidic
cave, a novel mechanism of nutrient acquisition
AU: * Stern, L A
EM: lstern@mail.utexas.edu
AF: University of Texas, Department of Geological Sciences, Austin, TX 78712-0254 United States
AU: Engel, A S
EM: aengel@mail.utexas.edu
AF: University of Texas, Department of Geological Sciences, Austin, TX 78712-0254 United States
AU: Bennett, P C
EM: pbennett@mail.utexas.edu
AF: University of Texas, Department of Geological Sciences, Austin, TX 78712-0254 United States
AB:
Ammonia volatilization provides a source of fixed nitrogen to the microorganisms living at low pH on the walls of Lower Kane
Cave. Sulfuric acid speleogenesis is actively enlarging this cave; hydrogen sulfide dissolved in ground water is oxidized to
sulfuric acid both in the cave streams and after volatilization to the cave walls. The result of this process is
replacement the host limestone with a rind of gypsum on the cave walls upon which droplets with pH values of 1-2 sulfuric
acid accumulate. The radically different pH values of the cave stream and cave wall habitats results in an unusual mechanism
of fixed nitrogen acquisition by the cave wall microbial community, accumulation of volatilized ammonia. The spring water
entering the cave has 25 to 40 $\mu$M NH$_{4}^{+}$ and circum-neutral pH allowing a small but significant amount this
ammonium to volatilize as NH$_{3}$. This gaseous ammonia may then partition into the acidic cave wall droplets accumulating
to concentrations of up to 800$\mu$M where it serves as a nitrogen source to the cave wall microorganisms. The effects of
this ammonia volatilization may be seen in the extremely low $\delta^{15}$N value of the cave wall biofilms, as low as
-16$\permil$. These $\delta^{15}$N values are among the lowest observed in organic materials. Therefore nitrogen isotope
ratios represent a marker of ammonia volatilization in subaerial sulfuric acid speleogenesis that may be applied to cave
systems that were once sulfidic to delineate regions of subaerial versus subaqueous cave formation.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1806 Chemistry of fresh water
DE: 1824 Geomorphology (1625)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting