S-LAYER REACTIVITY OF A HYPERTHERMOPHLIC DEEP SEA METHANOGEN IN A COMPLEX ELECTROLYTE SOLUTION

Methanogens play a major role in the global carbon cycle accounting for over half of all methane production on Earth. Their importance throughout geologic history is also evident. Reactions occurring at methanogenic cell surfaces are important for cellular functioning and impact the geochemistry of the surrounding environment. Archaeal cell envelopes contain a distinct semi-crystaline proteinaceous surface layer (S-layer), which is a structure fundamentally different from most bacterial cell surfaces. Surface interactions between archaeal cells and the surrounding environment has been relatively understudied compared to bacterial cells. This includes investigating S-layer reactions under conditions similar to in-situ environments. Towards this end, the surface reactivity of a hyperthermophilic deep sea methanogen Methanocaldoccocus sp. FS406-22 was investigated within (1) a 0.4M NaCl solution and (2) a simulated seawater solution. Potentiometric titrations and isothermal calorimetry coupled with surface complexation modeling and FTIR spectroscopy reveal the surface of FS406-22 contains three proton-ionizable functional groups. Thermodynamic parameters are consistent with carboxylic acid, polyphosphate, and amine sites. Amine site concentrations decreased significantly within the simulated seawater solution, which corresponded to a loss in buffering capacity at pH values >8. Calorimetric and FTIR data suggest that differences observed between the two solutions could be attributed to conformational changes associated with hydration of the cell surface introduced by the presence of divalent cations. Results indicate that cell surface reactivity is, in part, controlled by the composition of the chemical system. These data improve our ability to accurately predict surface interactions within natural systems, including those not directly observed.