The distribution of bacteriohopanepolyols in terrestrial geothermal environments.

Abstract

Organic geochemical investigations of terrestrial geothermal vents have provided a veritable treasure chest of unusual results. In particular, recent investigations of microbial lipids preserved in mineral and organo-sedimentary depositions have been shown to be useful in reconstructing vent populations, recording changes in physico-chemical conditions over time and inferring the presence of uncharacterised microbiota. Studies centred on geothermal environments provide a rich source of information that is directly applicable to a range of scientific disciplines including those concerned with the early evolution of life on Earth and other planetary bodies. The organic geochemistry of a particular group of bacterial lipid biomarkers, bacteriohopanepolyols, and their degraded counterparts, geohopanoids, contained within sinters from silica-depositing geothermal vents from New Zealand and Chile provides the basis for this study. This has been complimented with an investigation of BHP distributions that derive from mat-forming microbial consortia that colonise the outflow channels of geothermal vents from California, Nevada and New Zealand. Examination of BHP and hopanoid distributions preserved in siliceous sinters using Atmospheric Pressure Chemical Ionisation-High Performance Liquid Chromatography-Multistage Mass Spectrometry (APCI-HPLC-MSn) and Gas Chromatography-Mass Spectrometry (GC-MS) shows that polyfunctionalised composite BHPs deposited in sinters are well preserved. BHPs and geohopanoids have been analysed from five geothermal locations within the Taupo Volcanic Zone (TVZ). Composite-BHPs, such as bacteriohopanetetrol cyclitol ether and bacteriohopanepentol cyclitol ether, are commonly observed as the most abundant compounds in sinters from Champagne Pool (CP), Loop Road (LR) and Rotokawa (RK). A diverse array of additional BHPs is detected in sinter samples collected from these sites, including a number of novel BHPs. A suite of BHPs have been tentatively-assigned as ‘oxo-BHPs’, i.e. with a ketone group, most likely at C-32 or C-31 of the side chain. Guanidine- substituted bacteriohopanepentol cyclitol ether was also identified and may indicate the presence of methylotrophic bacteria at the sites. Sinters collected from Opaheke Pool (OP) contain BHPs that indicate the presence of methanotrophic bacteria and cyanobacteria. Likewise, 2-methyl BHPs indicative of cyanobacteria were identified in sinters collected from Orakei Korako (OK), including a vent of 97.8oC, potentially indicating another source of 2-methyl BHPs at this site. Sinters from the El-Tatio Geyser Field (ETGF) show a predominance of cyanobacterial signatures that likely derive from endolithic cyanobacterial colonies. Concentrations and the structural diversity of BHPs are greater in sinters from the TVZ when compared to those from ETGF. A survey of the BHP distributions of mat-forming microbial consortia from geothermal locations in Nevada, California and New Zealand highlighted the presence of six currently uncharacterised composite BHP structures with the same novel terminal group at C-35 that appear to comprise a diagnostic BHP signature for mat-forming microbial consortia. Hopanoic acids are the most abundant geohopanoids identified in older sinters and C32 hopanoic acid is the predominant diagenetic product. Geohopanols are more abundant in sinters deposited under acidic conditions, such as those collected from CP, LR and RK and C31 homohopanol is the most abundant geohopanol identified in sinters from these sites. The observed difference is thought to derive from reduction of hopanoic acids to hopanols. The diagenetic end products from each location appear to be different to any previously studied environmental setting indicating an unusual degradation pathway affecting BHP preservation in silica sinters. This indicates that depositional setting not biological input is the key factor determining geohopanoid distributions in these environmental samples. The results of this study complement previous investigations concerning the influence of environmental conditions upon BHP and hopanoid distributions and further elaborate upon the ecology of known BHP-producing bacteria contributing to the sedimentary record.