Characterization of elemental release during microbe–granite interactions at T = 28 °C Lingling Wu a, * , Andrew D. Jacobson a , Martina Hausner b,1 a Department of Earth and Planetary Sciences, Northwestern University, 1850 Campus Drive, Evanston, IL 60208-2150, USA b Department of Civil and Environmental Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA Received 1 June 2007; accepted in revised form 28 November 2007; available online 4 December 2007 Abstract This study used batch reactors to characterize the mechanisms and rates of elemental release (Al, Ca, K, Mg, Na, F, Fe, P, Sr, and Si) during interaction of a single bacterial species (Burkholderia fungorum) with granite at T = 28 °C for 35 days. The objective was to evaluate how actively metabolizing heterotrophic bacteria might influence granite weathering on the conti- nents. We supplied glucose as a C source, either NH 4 or NO 3 as N sources, and either dissolved PO 4 or trace apatite in granite as P sources. Cell growth occurred under all experimental conditions. However, solution pH decreased from 7 to 4 in NH 4 - bearing reactors, whereas pH remained near-neutral in NO 3 -bearing reactors. Measurements of dissolved CO 2 and gluconate together with mass-balances for cell growth suggest that pH lowering in NH 4 -bearing reactors resulted from gluconic acid release and H + extrusion during NH 4 uptake. In NO 3 -bearing reactors, B. fungormum likely produced gluconic acid and con- sumed H + simultaneously during NO 3 utilization. Over the entire 35-day period, NH 4 -bearing biotic reactors yielded the highest release rates for all elements considered. However, chemical analyses of biomass show that bacteria scavenged Na, P, and Sr during growth. Abiotic control reactors followed different reaction paths and experienced much lower elemental release rates compared to biotic reactors. Because release rates inversely correlate with pH, we conclude that proton-promoted dissolution was the dominant reaction mecha- nism. Solute speciation modeling indicates that formation of Al–F and Fe–F complexes in biotic reactors may have enhanced mineral solubilities and release rates by lowering Al and Fe activities. Mass-balances further reveal that Ca-bearing trace phases (calcite, fluorite, and fluorapatite) provided most of the dissolved Ca, whereas more abundant phases (plagioclase) con- tributed negligible amounts. Our findings imply that during the incipient stages of granite weathering, heterotrophic bacteria utilizing glucose and NH 4 only moderately elevate silicate weathering reactions that consume atmospheric CO 2 . However, by enhancing the dissolution of non-silicate, Ca-bearing trace minerals, they could contribute to high Ca/Na ratios commonly observed in granitic watersheds. Ó 2007 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Quantifying sources of Ca to streams draining granitic watersheds is crucial for understanding the role of granite weathering in geochemical processes ranging from the long-term evolution of atmospheric CO 2 to nutrient cy- cling in forested ecosystems (e.g., Berner et al., 1983; Ber- ner, 1993; Likens et al., 1996; Likens et al., 1998; Gaillardet et al., 1999; Krishnaswami et al., 1999; Berner and Kothavala, 2001; Blum et al., 2002; Dalai et al., 2002; Bullen and Bailey, 2005). Beginning with the pio- neering works of Garrels (1967) and Garrels and Macken- zie (1967), numerous studies have recognized that waters interacting with granite bedrock have Ca/Na ratios that greatly exceed the Ca/Na ratio of plagioclase, the 0016-7037/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2007.11.025 * Corresponding author. Fax: +1 847 491 8060. E-mail address: lingling@earth.northwestern.edu (L. Wu). 1 Present address: Department of Chemistry and Biology, Ryer- son University, 350 Victoria Street, Toronto, Ont., Canada M5B 2K3. www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 72 (2008) 1076–1095