HR: 0830h
AN: B41D-0927    [PDF]
TI: Limits of Microbial Photosynthesis in Hot Spring Ecosystems
AU: * Cox, A D
EM: alysia.cox@asu.edu
AF: Dept. of Geological Sciences, Arizona State University, Tempe, AZ 85287 United States
AU: Shock, E L
EM: eshock@asu.edu
AF: Dept. of Geological Sciences, Arizona State University, Tempe, AZ 85287 United States
AB: The limits of microbial photosynthesis are determined by many environmental factors including light availability, temperature, pH, flow rate, nutrient abundance, chemical composition and the presence or absence of other microorganisms. In an effort to determine which factors have the greatest influence on the limits of photosynthesis, we conducted a field study in the summer of 2003 at Yellowstone National Park. At more than 75 locations temperature, pH, conductivity, and sulfide measurements were made in the field, and at many of these locations samples were collected for major and trace element measurements and organic analyses. Temperatures ranged from 32 to $93\deg$C, conductivities from 790 to 11500$\mu$S, {\it in situ} pH from 1.87 to 8.97, and total sulfide concentrations from 1850$\mu$g L$^{-1}$ to below detection ($\sim$2$\mu$g L$^{-1}$). These data indicate that the previously established upper temperature limit for photosynthesis of $73\deg$C is reached in many alkaline hot springs, but that upper temperature limits decrease with decreasing pH below $\sim$7. As an example, we found no strong evidence for photosynthesis above $45\deg$C at pH $\sim$2. In several locations, photosynthesis appears to be suppressed despite temperatures and pH values that permit photosynthesis elsewhere. Preliminary results indicate that salinity variations are not responsible for suppression of photosynthesis, but that sulfide concentrations may be. Studies of five hot spring outflow channels, together spanning pH values from 2.5 to 8.6, show that photosynthesis appears once sulfide concentrations drop to $<$ 20% of the source values. In one channel where total sulfide varies by only a factor of two over more than 100 m of flow, we found no evidence of photosynthesis despite temperatures ranging from 60 to $35\deg$C and mild pH values of 5.8 to 6.5. These observations lead us to propose that photosynthesis becomes possible after a large decrease in initial sulfide concentration. Although abiotic processes (degassing, mineral precipitation, surface reactions) may explain decreases in total sulfide concentrations, preliminary results suggest that rates of microbial sulfide oxidation determine whether hot spring habitats become suitable for microbial photosynthesis. Alternatively, once sulfide concentrations decrease below some threshold level sulfide oxidizers may be outcompeted by photosynthesizers.
DE: 0400 Biogeosciences
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 8424 Hydrothermal systems (8135)
DE: 9810 New fields (not classifiable under other headings)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting