Molecular Ecology of Free-Living Chemoautotrophic Microbial Communities at a Shallow-sea Hydrothermal Vent

Abstract

Deep-sea hydrothermal systems are unique habitats for microbial life with primary production based on chemosynthesis. They are considered to be windows to the subsurface biosphere. Their far more accessible shallow-sea counterparts are valuable targets to study the effects of hydrothermal activity on geology, seawater chemistry and microorganisms. Such an area of shallow-sea hydrothermal venting is observed approximately 2.5 km east off Panarea Island (Sicily, Italy). This system is characterized by fluid temperatures of up to 135°C, gas emissions dominated by CO2 and precipitation of elemental sulfur on the seafloor. It is quite well studied, yet, only very few studies exist on its microbial ecology. This thesis is therefore targeting the microbiology of sediment cores as part of an interdisciplinary project which combines geological, geochemical, biomarker and molecular biological investigations. It was intended to correlate the environmental parameters with the taxonomic composition and the metagenomes of the microbial community thereby gaining insights into the interaction of geosphere and biosphere. All samples were taken at Hot Lake, an oval-shaped (~10 by 6 meters) shallow (~2.5 m deep) depression at 18 m below sea level. The sediments in this depression are strongly affected by hydrothermal activity. In situ temperatures at 10 cm below sea floor of 36°C and 74°C were measured at two different sites within Hot Lake. Based on the physico-chemical parameters, a thermodynamic modeling was performed which revealed sulfur oxidation and sulfur reduction to be exergonic at Hot Lake. Microbial community structures of different sediment layers were first screened by automated rRNA intergenic spacer analysis (ARISA). Based on the ARISA fingerprints, a total of eight bacterial and archaeal 16S rRNA gene libraries were constructed from surface to bottom layers of sediments to gain more insights into microbial diversity. Comparative sequence analyses revealed a dominance of sequences affiliated with Epsilonproteobacteria, Deltaproteobacteria and Bacteroidetes. In the surface sediments, sequences close to anoxygenic phototrophic Chlorobi were also detected. In the bottom sediments, thermophilic bacteria such as Thermodesulfobacteria spp. were found. Hyperthermophilic Archaea sequences related to Desulfurococcaceae and Korarchaeota were retrieved from 74°C hot sediment. Based on the most closely related cultured representatives, it could be deduced that the majority of microorganisms in Hot Lake sediments have a sulfur-dependent metabolism, including sulfide oxidation, sulfur reduction or sulfate reduction. Fluorescence in situ hybridization showed the dominance of Bacteria in all depths of sediments. With increasing depth and temperature, the abundance of Archaea increased relatively to that of Bacteria. Metagenomic analyses revealed that Epsilonproteobacteria were dominating surface sediments of Hot Lake where they gain energy from sulfur metabolism to fix CO2 by the reductive tricarboxylic acid (rTCA) cycle. This is consistent with findings reported from deep-sea hydrothermal vent systems. The results have led to the conclusion that mixing between hydrothermal fluids and seawater results in distinctly different temperature gradients and ecological niches in Hot Lake sediments. Overall, the correlation of geochemical profiles, IPL analyses, characterization of the microbiological community and metagenomic analyses provided strong evidence for a sulfur-dominated metabolism in the surface sediments of Hot Lake.