Correspondence Comment on “Abiotic Controls on H2 Production from Basalt-Water Reactions and Implications for Aquifer Biogeochemistry” SIR: Stevens and McKinley (1) suggest that hydrogen, produced by basalt-water interaction, is the primary source of energy supporting the microbial community in the Columbia River Basalt (CRB) aquifer. We have suggested that organic matter, not abiotically produced hydrogen, is the main energy source supporting the microbial community in the CRB (2). Stevens and McKinley (1) state that our study “contained no evidence to substantiate those claims”. We feel compelled to respond to their inaccurate characterization of our previous study and to summarize the four types of data that we previously provided to substantiate our conclusions. (i) A molecular analysis of the CRB microbial community (3) has demonstrated that the composition of the microbial community resembles that of an anaerobic microbial community living on organic matter rather than hydrogen as the primary energy source. As previously discussed in detail (2), in the methanogenic portion of the CRB aquifer, less than 3% of the microbial community was comprised of methanogenic microorganisms. This is consistent with the composition of the complex microbial consortia needed to convert organic matter to methane. However, in a system such as Stevens and McKinley propose (1) in which abiotic hydrogen is the primary electron donor and hydrogen-utilizing methanogens are the primary consumers, hydrogen-utilizing methanogens would be expected to be major components of the microbial community. A similar low percentage of hydrogen-consuming sulfate-reducing microorganisms was found in the portion of the CRB aquifer in which sulfate reduction appeared to be the terminal electron-accepting process (3). Stevens and McKinley should explain why the microbial community in the CRB does not have the composition expected for a community living on abiotically produced hydrogen as the main electron donor. (ii) The CRB aquifer contains sufficient dissolved organic matter to support a heterotrophic microbial community. We substantiated this claim previously (2) with data from a previous study of the CRB aquifer (3) that reported concentrations of dissolved organic matter of ca. 2-5 mg/L. As we previously noted (2), these concentrations of dissolved organic matter are comparable to those found in many deep pristine aquifers in which organic matter oxidation is considered to be the primary energy source supporting the microbial community (4-8). Stevens and McKinley should explain why levels of dissolved organic matter that readily support heterotrophic microbial communities in other aquifers are not utilized by the microbial community in the CRB. (iii) There is not enough reductant in the basalt of the CRB to have continually produced abiotic hydrogen at the rates that Stevens and McKinley suggest. We previously (2) presented the results of calculations demonstrating that at the rates of hydrogen production that Stevens and McKinley previously (9) suggested to take place in the CRB all of the iron reductant would be oxidized within several hundred years. In their latest paper (1), Stevens and McKinley continue to ignore this simple consideration and make calculations of methane production in the CRB from hydrogen over a 1556
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FIGURE 1. Measured pH values of 202 groundwater samples collected from the confined aquifers of the CRB (13). 30 000-yr period. Their calculations are based on estimated rates of hydrogen production of 1-6 nmol m-2 day-1, which they extrapolate from their hydrogen production rates of 1-6 nmol g-1 day-1 and their estimate of 1 m2 of surface area per gram of basalt. Using this information and the fact that the basalt is expected to contain 10-13% (11.5% average) iron by weight (10), the time necessary to oxidize all of the iron in the basalt, even at the slowest rates proposed by Stevens and McKinley, can be calculated as
(
)( )(
)
1 g of basalt × day 0.115 g of Fe × 1 nmol of H2 produced g of basalt 1 nmol of H2 produced nmol of Fe × -9 55.85 × 10 g of Fe 2 nmol of Fe oxidized yr ) 2821 yr 365 day
(
(
)
)
This demonstrates that even if all of the iron in the basalt were used for hydrogen production there would not be enough iron reductant to produce the abiotic hydrogen that Stevens and McKinley suggest has supported methane production over the last 30 000 yr. The possibility of significant sustained production of hydrogen from basalt-water interactions becomes even more remote when it is considered that the aquifer is millions of years old. Stevens and McKinley should explain the source of the reductant that would support the sustained rates of hydrogen production they propose over geologically relevant time scales. (iv) There is no significant, sustained production of hydrogen from basalt-water interaction when the basalt is incubated under conditions that approximate those in situ. We provided evidence to substantiate this claim (2), and data in Stevens and McKinley’s paper (1) support this. Groundwaters in basalt aquifers are buffered at alkaline pH, typically ca. pH 8 or above (11, 12). This includes the CRB aquifer (Figure 1). Yet, Stevens and McKinley continue to report data from studies at artificially low pH values of 5 and 6, which are never found within the confined aquifers of the CRB and which Stevens and McKinley know artificially stimulate hydrogen production from the basalt. They used such low pH values in the studies presented in each of the three figures of their paper (1). When the low pH data are 10.1021/es0015996 CCC: $20.00
2001 American Chemical Society Published on Web 02/22/2001
removed, reducing the clutter in their figures, it is apparent that there is no consistent, sustained hydrogen production from basalt at the pH values typically found in the CRB. Presenting the low pH data and using this to speculate on the rates and mechanisms of hydrogen production in the CRB is misleading. Stevens and McKinley should explain how data collected at artificially low pH is relevant to processes in the CRB. Stevens and McKinley (1) suggest that our results demonstrating a lack of hydrogen production from basalt at environmentally relevant pH cannot be used to interpret processes in the CRB because we did not conduct experiments with CRB materials, but then they conclude in their paper that “While our experiments have focused on the CRB system, we know of no reason these phenomena should be unique to this formation”. We specifically chose the Snake River Plain Aquifer basalt because the earlier studies of Stevens and McKinley (9) had indicated that this basalt had the highest rates of hydrogen production of all the basalts they investigated. Furthermore, we conducted studies with actual aquifer basalt and groundwater whereas Stevens and McKinley used outcrop material and artificial buffers. It seems likely that studies with actual aquifer material and groundwater, incubated under conditions that approximate those in situ, will provide the best insights into the potential for abiotic hydrogen production in basalt aquifers. In summary, neither the microbiological nor the geochemical data support Stevens and McKinley’s claim of a hydrogenbased microbial community in the CRB aquifer. All of the available data are consistent with a microbial community that is supported from the degradation of organic matter. Although it would certainly be exciting to find a hydrogenbased, deep subsurface, microbial community, Stevens and McKinley have not provided any evidence for such a community nor have they demonstrated a mechanism for supporting such a community via basalt-water interactions.
Literature Cited (1) Stevens, T. O.; McKinley, J. P. Environ. Sci. Technol. 2000, 34, 826-831. (2) Anderson, R. T.; Chapelle, F. H.; Lovley, D. R. Science 1998, 281, 976-977. (3) Fry, N. K.; Frederickson, J. K.; Fishbain, S.; Wagner, M.; Stahl, D. A. Appl. Environ. Microbiol. 1997, 63, 1498-1504.
(4) Chapelle, F. H.; Lovley, D. R. Appl. Environ. Microbiol. 1990, 56, 1865-1874. (5) Chapelle, F. H.; Zelibor, J. J. L.; Grimes, D. J.; Knobel, L. L. Water Resour. Res. 1987, 23, 1625-1632. (6) Murphy, E. M.; Schramke, J. A.; Fredrickson, J. K.; Bledose, H. W.; Francis, A. J.; Sklarew, D. S.; Linehan, J. C. Water Resour. Res. 1992, 28, 723-740. (7) Lee, R. W. Open-File Rep.-U.S. Geol. Surv. 1984, No. 84-237, 12-14. (8) Burt, R. A. U.S. Geol. Surv. Water-Supply Pap. 1993, No. 2392, 59-60. (9) Stevens, T. O.; McKinley, J. P. Science 1995, 270, 450-454. (10) Reidel: S. P.; Tolan, T. L.; Hooper, P. R.; Beeson, M. H.; Fecht, K. R.; Bentley, R. D.; Anderson, J. L. In Volcanism and Tectonism in the Columbia River Flood- Basalt Province; Reidel, S. P., Hooper, P. R., Eds.; The Geological Society of America, Inc.: Boulder, CO, 1989; pp 21-53. (11) Hearn, P. P.; Steinkampf, W. C.; White, L. D.; Evans, J. R. Proceedings of the U.S. Geological Survey Workshop on Environmental Geochemistry; USGS Circular 1033; USGS: Denver, 1990; pp 63-68. (12) Wood, W. W.; Low, W. H. Geol. Soc. Am. Bull. 1986, 97, 14561466. (13) Early, T. O.; Spice, G. D.; Mitchell, M. D. A Hydrochemical Data Base for the Hanford Site, Washington; Rockwell Hanford Operations, RHO-SD-BWI-DP-061; U.S. Department of Energy: Richland, WA, 1986.
Robert T. Anderson Department of Microbiology University of Massachusetts Amherst, Massachusetts 01003
Francis H. Chapelle U.S. Geological Survey Stephenson Center, Suite 129 720 Gracern Road Columbia, South Carolina 29210-7651
Derek R. Lovley* Department of Microbiology University of Massachusetts Amherst, Massachusetts 01003 ES0015996
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