Microbial communities of two New Zealand hydrothermal systems

Abstract

A striking feature of most alkali-chloride hydrothermal systems is the array of coloured microbial mats that typically cover all but the most extreme temperature regions of the system. In silica-depositing systems these mats are gradually entombed in silica, and through the process of diagenesis, become part of the fossil record. The fossilised deposits from ancient hydrothermal systems often contain structures and cell morphologies resembling microbial mats that are seen in present day systems raising the question of whether modern-day systems may serve as analogs for studying conditions that prevailed on early earth (Schopf, 1993; de Ronde & Ebbesen, 1996). To address this question, it is necessary to understand the process of silicification and the degree to which the biogenic signatures observed in the fossils reflect the living microbial communities from which they derive. The aim of this research was to provide the first step in this interpretation, by providing a detailed characterisation of the extant microbial communities of two modern New Zealand hydrothermal systems, Kuirau Park and Orakei Korako. New Zealand’s North Island is rich in geothermal activity and offers an ideal environment to resolve this problem. Sites ranging in age from modern to over 10 million years provide a unique geological timeline of sinter deposits that enable the investigation of bio-signatures through all the key states of mineralogical maturation, from opal to quartz. A number of previous studies, based largely on microscopic observations, have suggested that distinctive relationships exist between water temperature, mat colour, silica sinter texture and the microorganisms present (Copeland, 1936; Walter, 1976b; Cady & Farmer, 1996). However, it is now well established that these methods do not necessarily provide an accurate indication of microbial diversity or community structures. This research used 16S rRNAfull cycle approach to characterise the microbial communities from two New Zealand hydrothermal systems. Microbial mats were sampled over a wide temperature gradient (24-80°C) and included a variety of distinctive biofilm structures and geological features. The predominant bacterial species in each zone were identified by 16S rRNA sequencing methods. Fluorescence in situ Hybridisation (FISH) was used to validate the presence and frequency of those microorganisms identified by 16S rRNA analysis. Analysis of 16S clone libraries indicated that all mats comprised a diverse bacterial community with greater than 10 taxa detected in all samples. This result suggests that these communities are far more complex than previously estimated by microscopic methods. All communities below 65°C and a small number above were found to be dominated by cyanobacteria. Specific genera of cyanobacteria were found to be abundant in each of the temperature zones. Chlorogloeopsis dominated the majority of high-temperature (55-70°C) communities. The communities of mats found in the mid-temperature (40-55°C) ranges at Kuirau Park were dominated by Synechococcus while those of Orakei Korako were dominated by Leptolyngbya. Calothrix dominated low-temperature (< 40°C) brown mats. Scanning Electron Microscopy (SEM) was carried out to look at the silicification of microorganisms within these communities and indicated a silicification bias towards filamentous organisms. Though rod-shaped bacteria were found throughout the samples and possibly contribute to the diversity of communities, the biogenic signal of these organisms is likely to be lost. Collectively the results of this study suggest that the dominant filamentous organisms seen in microbial mats of modern systems are likely candidates for preservation in the fossil record. However, the biogenic signals of less morphologically conspicuous organisms, which may represent much of the microbial diversity within these mat communities, are lost. Though further studies are required to determine the functional relevance of these organisms, the findings of this study provide an important starting point for accurate interpretation of the fossil record.