EVIDENCE OF MICROBIAL FRAMBOIDAL PYRITE FORMATION IN A TERRESTRIAL HOT SPRING

Iron-bearing structures resembling microbial cells were found within microbial mats attached to siliceous sinters from a terrestrial hot spring within the Uzon Caldera, Kamchatka, Russia by transmission electron microscopy (TEM). The structures are approximately 0.5 µm, and composed of nanocrystalline ferrihydrite balls (2-4 nm) attached to the outside of a remnant cell wall. Inside the cells, ferrihydrite (4-8 nm) and pyrite were identified by selected area electron diffraction, high resolution TEM, and energy-filtered TEM (EFTEM). Phosphorous mapping by EFTEM did not reveal any incorporation of organic material within the structures. Mineralogical and morphological relationships suggest that ferrihydrite bound extracellularily (as a byproduct of cellular respiration) when the cells were alive. The ferrihydrite spheres are likely produced by iron oxidizing bacteria that derived ferrous iron locally from volcanic glass, which is commonly seen hydrolyzed and converted to kaolin group minerals. The cell structures have since become preserved through pyritization. A gap that exists between the ferrihydrite spheres on the outside of the structure and the interior phases represent a vestigial cell wall or an exopolymeric substance that once surrounded the cell that has since degraded away. This gap suggests that ferrihydrite (and possibly pyrite) began to replace the organic material within the cell presumably as the cell was dying or quickly after cell death. Pyrite is believed to have formed as hydrogen sulfide diffused through the system, sourced either by sulfur reducing bacteria or from volcanic gasses emitted from the spring. In many of the structures, pyrite crystals appear to initially form along the inside edges of the structures and grow inward towards the center of the structures taking on a framboidal appearance. Framboidal pyrite (2 µm in diameter) has also been observed by scanning electron microscopy, commonly residing on the surface of microbial mats and within the rocks on which the sinters are forming. The juxtaposition of phases with disparate iron oxidation states in the presence of thermophilic microbial forms is an observation consistent with hypotheses that sulfide mineral surfaces can act as catalysts in organosynthesis (Cody, 2005).

Cody, G.D.(2005) Geochemical connections to primitive metabolism. Elements, V.1 p. 129-143.