CO(2) uptake and fixation by a thermoacidophilic microbial community attached to precipitated sulfur in a geothermal spring

Abstract

Carbon fixation at temperatures above 73 degrees C, the upper limit for photosynthesis, is carried out by chemosynthetic thermophiles. Yellowstone National Park (YNP), Wyoming possesses many thermal features that, while too hot for photosynthesis, presumably support chemosynthetic-based carbon fixation. To our knowledge, in situ rates of chemosynthetic reactions at these high temperatures in YNP or other high-temperature terrestrial geothermal springs have not yet been reported. A microbial community attached to precipitated elemental sulfur (S(o) floc) at the source of Dragon Spring (73 degrees C, pH 3.1) in Norris Geyser Basin, YNP, exhibited a maximum rate of CO(2) uptake of 21.3 +/- 11.9 microg of C 10(7) cells(-1) h(-1). When extrapolated over the estimated total quantity of S(o) floc at the spring's source, the S(o) floc-associated microbial community accounted for the uptake of 121 mg of C h(-1) at this site. On a per-cell basis, the rate was higher than that calculated for a photosynthetic mat microbial community dominated by Synechococcus spp. in alkaline springs at comparable temperatures. A portion of the carbon taken up as CO(2) by the S(o) floc-associated biomass was recovered in the cellular nucleic acid pool, demonstrating that uptake was coupled to fixation. The most abundant sequences in a 16S rRNA clone library of the S(o) floc-associated community were related to chemolithoautotrophic Hydrogenobaculum strains previously isolated from springs in the Norris Geyser Basin. These microorganisms likely contributed to the uptake and fixation of CO(2) in this geothermal habitat.

FIG. 1.

Schematic (A) and image (B) of the CFR system used to measure CO₂ uptake by the S^o floc-associated microbial community under conditions of continuous replenishment of fresh spring water.

FIG. 2.

Uptake of ¹⁴CO₂ by a S^o floc-associated microbial community under static conditions in the presence of 15.3 μM NaH[¹⁴C]O₃ after no incubation (0 h) or after 4 h (4 h) of incubation. All incubations were performed in the light with the exception of those labeled D, denoting incubations performed in the dark. Live (L) and glutaraldehyde-killed control (K) reactions are indicated. All incubations contained 1 mM citrate buffer (pH 3.0) unless otherwise indicated (NC), which contained no citrate. n, Number of biological replicates.

FIG. 3.

CO₂ uptake under static conditions by the microbial community associated with 0.5 mg (⧫) or 1.0 mg (▪) of S^o floc after 2 h of incubation in Dragon Spring source water supplemented with a range of NaH[¹⁴C]O₃ concentrations (A) or associated with 1.0 mg of S^o floc over a 4-h incubation in Dragon Spring source water supplemented with 7.7 μM NaH[¹⁴C]O₃ (B). Means and standard deviations are based on results from experiments performed in triplicate (n = 3).

FIG. 4.

CO₂ uptake by microbial community associated with 1.0 mg of S^o floc after a 5-h incubation under static conditions in Dragon Spring source water supplemented with 7.7 μM NaH[¹⁴C]O₃ after addition in the final hour of either 750 nM hydrogen (hydrogen), 65 μM sulfide (sulfide), 191 nM O₂ (oxygen), 750 nM hydrogen plus 191 nM O₂ (hydrogen/oxygen), 65 μM sulfide plus 191 nM O₂ (sulfide/oxygen), or no electron donor or acceptor (control). Means and standard deviations are based on results from experiments performed in triplicate (n = 3).