White Paper Title: 
Techniques for evaluating the variety and rates of biogeochemical processes in seafloor hydrothermal systems

Within the shallow subsurface of hydrothermal systems, microorganisms simultaneously depend on and alter their mineral and fluid substrates. The products of these biogeochemical interactions alter the nutrient flux available for water column processes, ultimately becoming incorporated into the world’s oceans. Key to these subsurface processes are biological and chemical diversity and reaction rates, which have been studied in the field, with models, and by recreating systems in the laboratory.

Field studies still provide the clearest snapshot of biogeochemical processes in seafloor hydrothermal environments. New in situ technology has allowed the collection of high resolution datasets monitoring chemical and heat fluxes (Ding et al., 2001; Tivey et al., 2002; Ding et al., 2005). Detailed studies of microbial communities on fine spatial scales linked to mineralogical zonation within chimney deposits (Kormas et al., 2006; Schrenk et al., 2003) inspired new studies to characterize colonization succession (Page et al., 2008). Recent findings from field studies, however, indicate additional levels of complexity, including the realization that current methods may vastly underestimate true diversity (Huber et al., 2007). Likewise, the chemical complexity in time and space is correspondingly underestimated (Luther et al., 2008) and yet is critical to accurate descriptions of biogeochemical processes due to rapid rates of microbial metabolism over small spatial scales. In addition, meta-analysis of these field data is hindered by asynchronized biogeochemical datasets and the lack of a centralized or cross-referenced data repository. These issues also impede the calibration of biogeochemical models incorporating microbial activity.

To constrain habitat regimes for microorganisms in the primarily inaccessible regions of hydrothermal systems, reactive transport models linked to calculations of energetics have been successful (Tivey, 2004; Shock and Holland, 2004). Models have been constructed to describe generic systems and predict microbial metabolic niches (McCollom, 2000) and recently to describe specific systems for which microbial surveys were previously conducted (Houghton  and Seyfried, in press). However, geochemical models are limited by their thermodynamic assumptions, including both the equilibrium constants at in situ conditions (Foustoukos and Seyfried, 2007; Luther et al., 2001) and the relative reaction rates of key redox reactions (Foustoukos et al., in review). Careful evaluation of chemical speciation at both equilibrium and disequilibrium conditions on time scales relevant to microbial metabolic rates will continue to be critical in transforming modeling techniques from generalized to specific predictive tools for biogeochemical processes

Experimentally simulating biogeochemical processes is a blend of traditional techniques used to determine kinetic and equilibrium behavior (Seewald, 2001) and traditional enrichment culture techniques used to evaluate microbially-mediated reactions and rates in monoculture (Vetriani et al., 2005; Holden and Adams, 2003). Recent work in this lab is focused on determining chemical and physical modifications caused by mineral-precipitating microbial communities using both open-system experimental observations and corresponding reactive transport modeling to quantify changes in permeability and chemical fluxes along sharp thermal gradients. The rates of microbially-mediated reactions under non-ideal growth conditions simulating subsurface hydrothermal environments are critical data necessary to begin evaluating interspecies reactions within subsurface communities and biofilms.