NATURAL 2-LINE AND 6-LINE FERRIHYDRITE CHARACTERIZATION AND MICROBIAL BIOMINERALIZATION IN A HIGH IRON THERMAL SPRING

Chocolate Pots hot springs in Yellowstone National Park is a group of actively accumulating iron deposits. The anoxic vents waters are 50-53ºC with a pH of 5.6-5.8 and [Fe²⁺]=4.7-5.9 mg/L. The Fe²⁺ is rapidly oxidized down outflow channels by ambient air as well as by oxygen produced by cyanobacterial mats (Pierson et al., 1999; Trouwborst et al., 2004). The pH increases down outflow channels due to photosynthetic CO₂ fixation as well as CO₂ outgassing. Chocolate Pots hot springs provides a means to examine the role of cyanobacteria and green anoxygenic phototrophs in the oxidation of Fe²⁺, formation and diagenesis of primary precipitates, iron biomineralization of the bacteria, and the formation of microbial biosignatures.

Primary precipitates associated with the microbes located at the vents and along the outflow channels were characterized using SEM, TEM, SAED, XRD, and EMPA.

2-Line ferrihydrite (2-L Fh) was the primary phase identified by XRD at all locations. However, in some XRD scans small peaks of the more ordered 6-line ferrihydrite (6-L Fh) appeared. The highest concentration of 6-L Fh was found in optically opaque thin bands (100-800 µm) that alternated with optically red silica-rich material (20-50 µm). The red material appeared as 2-L Fh by XRD, although ED indicated the presence of crystalline phases yet to be identified. EMPA of the red layer revealed Fe/O and Fe/Si ratios of 1.0 and 5.4, respectively. The Fe/O and Fe/Si ratios of the 6-L Fh layer were 1.3 and 5.5, respectively. We are presently using HRTEM to determine whether 6-L Fh is a primary or secondary phase in these shallow, fresh deposits. No other diagenetic phases were identified.

Bulk XRD of the mineralized microbial mats revealed the presence of 2-L Fh. Heavy encrustation of some cyanobacterial cells was studied by TEM. Photosynthetic membranes remained intact and allowed positive identification of the cyanobacteria, even when cells were extensively degraded. In contrast, other cyanobacterial cells contained iron minerals that precipitated in the periplasmic space, indicating two different modes of mineralization.

Our research has implications for the role of phototrophs in the deposition of BIFs on Earth, and may assist in the search for evidence of fossilized microbial life in iron deposits on Mars.