White Paper Title: 
The effect of microorganisms on chemical speciation within the hydrothermal plume

TWG - Biogeochemical processes – Interaction between life and minerals

Sarah A Bennett, University of Southern California

Over the history of hydrothermal research, geochemists have reported the chemistry of the deep-sea plume environment to be inorganically dominated, following abiotic oxidation kinetics and thermodynamically favorable products (Field and Sherrell, 2000; German and Von Damm, 2004).  Meanwhile, biologists have detected elevated cell concentrations within the plume and have more recently reported the presence of both auto- and heterotrophic microorganisms (Sunamura et al., 2004; Lam et al., 2004; Dick and Bradley, 2010; German et al., 2010; Sylvan et al., In prep).  The biological utilization of reduced chemical species adds an additional component to both the kinetic and thermodynamic controls on the chemistry within the plume.  

For example, oxidation could either become slower or faster under microbiological controls than predicted from kinetic calculations.  Statham et al. (2005) demonstrated that the Fe oxidation rates in samples collected from a hydrothermal plume in the Indian Ocean were slower than predicted, whereas in the Guaymas basin oxidation rates of Mn were elevated within the plume (Dick et al., 2009).  More recently, Fe(II) was found co-located with organic carbon within sediment trap samples collected at 9°50’N East Pacific Rise, even though Fe(II) is kinetically unstable in oxic conditions (Toner et al., 2009).  Toner et al. (2009) determined that the organic carbon was composed of exopolymeric material, i.e. organic substances excreted from microbial species.  Organic complexation of inorganic minerals within the plume can lead to stabilization of intermediate phases, which would otherwise crystallize and form more complex mineral structures (Bennett et al., 2008).  This has the potential to impact the chemical budgets of the ocean on a global scale (Tagliabue et al., 2010).  

These recent studies highlight the importance of collaborative research between microbiologists and geochemists within the plume environment and more directed research to address the impact of microbiology on chemical speciation.  Even though the elevated concentration of constituents within the plume compared to the open ocean should enable us to exploit with ease current techniques, we must address issues associated with the deep-sea environment.  The time delay between sampling and sample processing has produced artifacts in our results because of changes in the chemical speciation and biological communities over time (Bennett et al., 2009).  On the flip side, the elevated concentrations within the plume make in-situ sensing devices a potential solution.