MICROBIAL BIOSIGNATURES AND PRIMARY MINERAL PHASES IN HIGH IRON THERMAL SPRINGS

The emerging anoxic source waters at Chocolate Pots hot springs in Yellowstone National Park contain 2.6 to 11.2 mg/L Fe(II) and are 51-54°C and pH 5.5-6.0.  These waters flow down the accumulating iron deposits and over three major phototrophic communities:  Synechococcus/Chloroflexus at 50-54°C, Pseudanabaena at 50-54°C, and a narrow Oscillatoria at 36-45°C.  We are assessing the contribution of the phototrophs to biosignature formation in this high iron system.  These biosignatures can be used to assess the biological contribution to ancient iron deposits on Earth (e.g., Banded Iron Formations) and, potentially, to those found on Mars.  Most studies to date have focused on chemotrophic iron-oxidizing communities; however, recent research has demonstrated that phototrophs have a significant physiological impact on iron hot springs systems.

We recently completed a mineralogical survey of the ecosystem to determine whether any of the different communities are associated with different primary mineral assemblages at the surfaces of the microbial mats.  Primary minerals were characterized across a range of spatial scales using powder X-ray diffraction (XRD), optical and scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS).  Mineral samples associated with microbial mats from the entire ecosystem indicate that 2-line ferrihydrite is the dominant phase.  Other apparently diagenetic mineral phases (quartz, goethite, siderite, and hematite) were detected in dry mineral samples exposed on the surface face of the main iron deposit.  How primary iron precipitates undergo early diagenesis in different parts of the ecosystem, and whether the physiological activity of the different communities affects the diagenetic pathway that accommodates the transformation of two-line ferrihydrite to more ordered crystalline phases, is presently being investigated.

The goal of this research is to provide an initial dataset that will illustrate the maximum amount of paleobiological and paleoenvironmental information expected to form in these types of iron deposits.  Insights from our research may help elucidate 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.