Sean A. Guidry

Bacterial shrubs, crystal shrubs, and ray-crystal shrubs: bacterial vs. abiotic precipitation

Hot-water travertine deposits commonly contain shrub-like morphologies, on the centimeter to meter scale, that range from highly irregular forms (bacterial shrubs) to features that display regular geometric patterns (crystal shrubs and ray-crystal crusts). The bacterial shrubs have been previously recognized as the product of bacterially induced precipitation whereas the ray-crystal crusts have been described as due to dominantly abiotic precipitation. The bacterial shrubs have a highly irregular morphology, similar to woody plants, in contrast, end member crystal shrubs display distinct crystal habits and regular repeating morphologies, i.e., they commonly have attributes of noncrystallographic as well as crystallographic dendrites. There is a complete gradational relationship between the bacterial shrubs and crystal shrubs. Ray-crystal crusts are large fan-shaped features composed of subparallel, extremely coarse bladed crystals of calcite. All three types (bacterial shrubs, crystal shrubs, and ray-crystal crusts) contain either a dense tangle of bacterial body fossils and/or micron-sized pores. The micropores are bacterial molds. The immediately enveloping spar crystals around all three morphologic types are devoid of bacterial body fossils and micropores. The bacterial shrubs, crystal shrubs, and ray-crystal crusts are, to differing degrees, the product of bacterially induced precipitation as well as abiotic mineral precipitation. The differences in morphology are due to the relative contribution of bacterially induced precipitation as compared with that of abiotic mineral precipitation. Bacterially induced precipitates can form in environments too chemically harsh or otherwise inhospitable for other taxa to thrive. Epiphyton and Renalcis commonly formed in occult environments under conditions inhospitable to other taxa. Consideration of the published descriptions and interpretations concerning Epiphyton and Renalcis, and by analogy the origin of bacterial shrubs, have led to the conclusion that Epiphyton and Renalcis also are bacterially induced precipitates.

Polymeric substances and biofilms as biomarkers in terrestrial materials: Implications for extraterrestrial samples

Organic polymeric substances are a fundamental component of microbial biofilms. Microorganisms, especially bacteria, secrete extracellular polymeric substances (EPS) to form slime layers in which they reproduce. In the sedimentary environment, biofilms commonly contain the products of degraded bacteria as well as allochthonous and autochthonous mineral components. They are complex structures which serve as protection for the colonies of microorganisms living in them and also act as nutrient traps. Biofilms are almost ubiquitous wherever there is an interface and moisture (liquid/liquid, liquid/solid, liquid/gas, solid/gas). In sedimentary rocks they are commonly recognized as stromatolites. We also discuss the distinction between bacterial biofilms and prebiotic films. The EPS and cell components of the microbial biofilms contain many cation chelation sites which are implicated in the mineralization of the films. EPS, biofilms, and their related components thus have strong preservation potential in the rock record. Fossilized microbial polymeric substances (FPS) and biofilms appear to retain the same morphological characteristics as the unfossilized material and have been recognized in rock formations dating back to the Early Archaean (3.5 b.y.). We describe FPS and biofilms from hot spring, deep-sea, volcanic lake, and shallow marine/littoral environments ranging up to 3.5 b.y. in age. FPS and biofilms are more commonly observed than fossil bacteria themselves, especially in the older part of the terrestrial record. The widespread distribution of microbial biofilms and their great survival potential makes their fossilized remains a useful biomarker as a proxy for life with obvious application to the search for life in extraterrestrial materials.

Anatomy of siliceous hot springs: examples from Yellowstone National Park, Wyoming, USA

Numerous siliceous hot spring systems in the Norris and Lower Geyser Basins of Yellowstone National Park, Wyoming, provide insights into spring geometries, depositional facies, and lithofacies associated with modern hot springs. Analyses of active (Cistern Spring, Octopus Spring, Deerbone Spring, and Spindle Geyser) and inactive (Pork Chop Geyser) siliceous hot springs have facilitated the construction of a facies model for siliceous hot spring deposits at Yellowstone. Yellowstones siliceous springs tend to group into four broad morphological categories: siliceous spires and cones, domal mounds, terraced mounds, and ponds. Siliceous spires/cones are subconical accumulations up to 5–7 m high and about 2 m in diameter, and are common deposits in Yellowstone Lake. Domal mounds are characterized by siliceous precipitates with a broad lens or shield geometry (2–3 m in vertical relief), discharge channels, and an areal accumulation of approximately 150 m2. In contrast, terraced mounds have a stair-step morphology, a substantial pool (∼8–10 m in diameter), “shrubby” precipitates, and occupy areas of ∼2000 m2. Siliceous ponds are variable in size, have little outflow, and exhibit low amounts of silica precipitation. Of these morphological varieties, domal mounds and terraced mounds are thought to have the best long-term preservation potential. The four spring morphotypes are composed of up to eight cumulative hot spring depositional facies: (1) vent (>95 °C), (2) proximal vent (<95 °C), (3) pool (∼80–90 °C), (4) pool margin (∼80 °C), (5) pool eddy (<80 °C), (6) discharge channel/flowpath (<80 °C to ambient), (7) debris apron (variable temperatures), and (8) geyser (variable temperatures). This facies model based on numerous springs facilitates our ability to interpret ancient hot spring deposits and to infer depositional conditions. Precipitation of siliceous sinter is the result of abiotic and biotic processes. Abiotic precipitational processes are dominant in the vent area, whereas biotic influences on the precipitate fabric become progressively more important downflow.

Depositional Facies and Diagenetic Alteration in a Relict Siliceous Hot-Spring Accumulation: Examples from Yellowstone National Park, U.S.A.

ABSTRACT Siliceous precipitates from portions of an 11-m-long core drilled through a relict Yellowstone hot-spring deposit provide valuable insights into the diagenetic facies in this hydrothermal environment. Most of the core consists of relatively low-temperature siliceous sinter containing abundant plant, diatom, and microbial fossilized remains derived from the distal debris apron and discharge channel/flowpath of the hot spring. A few intervals contain stromatolitic horizons analogous to modern, siliceous pool-margin and discharge channel/flowpath facies. Despite the abundance of silicified microbial remains (e.g., silicified stromatolitic horizons as well as individual filamentous microbes, diatoms), distinctive organic biomarkers are not preserved in the core. The apparent lack of distinctive organic biomarkers is attributable to pervasive diagenetic alteration in the presence of high-temperature, chemically reactive fluids. Numerous diagenetic features have been superimposed on the siliceous sinter, especially considering the relatively shallow depths of burial. The uppermost interval (0 to 1.2 m) consists of highly porous siliceous sinter (opal-A) and interbedded travertine. Amorphous manganese oxides and sparry calcite cements are present and are irregularly distributed throughout this interval. Below is a diagenetic facies (4.8 to 5.3 m) that consists of opal-CT and minor chalcedony. The most pervasive diagenetic alteration in the core is the 6.8 to 11.2 m depth interval, where abundant zeolites (mordenite, heulandite), bladed cristobalite lepispheres, chalcedony cements, and calcite veins are present. Judging by the assemblage of plant and diatom remains, most of this pervasive alteration is superimposed on relatively low-temperature siliceous sinters and is likely the result of migration of hot waters through this interval. The paragenesis of all these intervals is very complex, involving numerous episodes of mineral precipitation and dissolution. This diagenetic alteration is facilitated by the porous nature of the siliceous sinter, thermal convection of superheated fluids or steam, strongly oxidizing spring waters, and fluctuating hydrochemical regimes. Rapid dissolution and/or replacement of organic matter indicates that standard morphological means (e.g., petrographic identification of body fossils) may be the best method by which to identify that organisms were previously present in ancient hot-spring accumulations.

Time of flight secondary ion mass spectrometry (ToFSIMS) of a number of hopanoids

Abstract Time of flight secondary ion mass spectrometry (ToFSIMS) has been applied to a number of bacterial hopanoids in an attempt to characterise these geologically important molecules in situ by a surface sensitive technique. Our results show that these molecules can be detected using this instrumentation to a high degree of mass accuracy. We believe that ToFSIMS can, therefore, be used to identify these molecules in environmental samples where sample size may be an issue and contraindicate the use of more traditional techniques such as GC–MS.