Increased interest in microbial interactions with marine basalts began more than 10 yr ago when research revealed that microorganisms can have an effect on the weathering of Fe silicate rocks and minerals such as basalt glass (Thorseth et al., 1992, 1995a, 1995b; Giovannoni et al., 1996; Furnes et al., 1996; Fisk et al., 1998; Bennett et al., 2001). It was suggested that the chemical energy required for chemolithoautotrophs could be provided by both direct interactions of microbes with olivine and microbial utilization of methane and hydrogen, produced when olivine reacts with water (Neal and Stanger, 1983; Stevens et al., 1993; Stevens and McKinley, 1995; Stevens, 1997; Fisk and Giovannoni, 1999; McKinley et al., 2000; Freund et al., 2002). These authors further suggested that the subseafloor biosphere on Earth might be an ideal analog to subsurface ecosystems that could exist on other solar bodies.
Over the past 5 yr, the study of olivine dissolution via microbial activity has focused on two different metabolic pathways. Santelli et al. (2001) and Welch and Banfield (2002) used iron-oxidizing bacteria (Acidithiobacillus ferrooxidans) in low-pH cultures to examine changes in both the olivine surface morphology and the chemistry in the culture media. Longazo et al. (2001, 2002) placed unidentified bacillus bacteria from the Columbia River aquifer into cultures with olivine but with no added Fe or Mg to show that environmental microbes can create weathering features on olivine. Garcia et al. (2004) used Escherichia coli at 37°C in culture with olivine grains to trace changes in the concentration of Fe in the systems.
Research into olivine end-member (fayalite and forsterite) dissolution both with and without microorganisms have created multiple dissolution mechanisms typically based on the pH of the system. Santelli et al. (2001) conducted both biotic and abiotic fayalite (Fe-rich end-member of olivine) dissolution reactions at pH = 2–4 and found that at these pHs Fe3+ inhibited fayalite dissolution. Welch and Banfield (2002) also conducted biotic and abiotic fayalite dissolution reactions at pH = 2 and also found that the dissolution of fayalite is significantly inhibited by ferric iron. Welch and Banfield hypothesized that the removal of Fe from M1 sites and oxidation of Fe3+ or adsorption of Fe2+ into M2 sites create a laihunite-like surface layer that produces the pits and rough texture seen in weathered fayalite and inhibits further dissolution.
Around pH = 9, the mechanism of forsterite (Mg-rich end-member of olivine) dissolution changes from a system controlled by the production of a Si-rich surface layer through Mg-H exchange (at pH < 9) to one with a Mg-rich surface layer because of the preferential release of Si (at pH > 9) (Pokrovsky and Schott, 2000a, 2000b). At pH < 9, the dissolution rate of forsterite decreased with increasing pH, but at pH > 9, the dissolution rate remained constant with increasing pH. In alkaline conditions, the dissolution processes may lead to the formation of a brucitelike or Mg-bearing sheet silicate layer. However, once both systems reached equilibrium, the molar ratio of Mg:Si released into solution was the same (~2); at pH = 3, equilibrium was reached within 100 min, and at pH = 11, equilibrium was attained after 150 hr (Pokrovsky and Schott, 2000b). Oelkers (2001) found that at pH = 2 forsterite dissolution rates are independent of aqueous Mg and Si concentrations and indicate that dissolution occurs in a two-step process. First, the surface of the forsterite is protonated, forming rate-controlling precursor complexes. Second, the Mg–O octahedral chain linking bonds break, which liberates both Mg and Si from the forsterite structure. Pokrovsky and Schott (2000b) showed that at pH > 9 the dissolution rates of forsterite decrease as a function of increasing dissolved Si or CO2 concentrations. However, Mg concentrations had no effect on the dissolution rate. For olivine, the initial step in dissolution is the formation of a leached layer (either M1 sites in fayalite, or the removal of Mg or Si in forsterite), which can either inhibit or enable further dissolution.
The focus of this research is to quantify the rates of dissolution that can be achieved by a consortium of environmental bacteria. This work builds on the culturing work started by M.M. Moeseneder et al. (unpubl. data) in 2002 with basalt glass and olivine cultures, which resulted in enhanced dissolution of both basalt glass and olivine in the presence of a basalt glass inoculum. M.M. Moeseneder et al. (unpubl. data) collected samples of pillow lavas from seamounts in the Cobb-Eickelburg chain to inoculate cultures with either sterile basalt glass or sterile olivine. The cultures and controls were incubated at 4 and 15C for 3 yr. Periodically, the media within the cultures were analyzed for changes in dissolved Si and Li. The inoculated cultures showed enhanced concentration of dissolved Li and Si through time over the uninoculated controls (M.M. Moeseneder et al., unpubl. data). The idea behind this research was to estimate microbially driven dissolution rates of peridotite in the seafloor using a subsurface peridotite as our source of inoculating microbes. We wanted to have an environmental source of microbes for our cultures so that our estimates of microbially driven dissolution would reflect in situ rates. Our experimental design did not test for the presence of microorganisms in the enrichments. Instead, we assayed the enrichment cultures and controls for soluble products of olivine dissolution.