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Ecology and Evolution
Vol. xx, No.
xx, 2012; Pages: xxx - xxx

Microbial ecology of deep-sea hydrothermal vents: physiology and cell-to-cell communication in anaerobic chemosynthetic bacteria

Pérez-Rodríguez, Ileana



Abstract

The global influence of mid-oceanic ridges (MOR) first became apparent through continental drifting–its immanent force easily appreciated in today’s resulting continents. The role of MORs as a source of global-ocean chemistry is less apparent but equally immense. Key to these processes is fluid-rock reactions between circulating seawater and hot new basalt. With the discovery of hydrothermal vent ecosystems in the 1970’s, yet another important consequence of rock-fluid interaction was established in chemosynthesis. Early photographic descriptions of “frosted white and yellow precipitates” covering basalt rocks close to discharged hydrothermal fluids, with benthic communities emerging from them, referred to the now known chemosynthetic biofilms that interact with hydrothermal fluids. These microorganisms have a pivotal role in transforming the geochemistry of Earth’s oceans. The main objectives of this dissertation are to study anaerobic chemosynthetic vent microorganisms, and to explore the molecular ecology of these biofilm communities. Initial approaches included isolation of anaerobic chemosynthetic microorganisms resulting in the description of two novel bacterial species: the epsilonproteobacterium Nautilia nitratireducens strain MB-1T, and Phorcys thermohydrogeniphilus strain HB-8T, a new genus in the Aquificales. Both bacteria are obligate thermophilic anaerobes, capable of hydrogen oxidation coupled to sulfur- and nitrate-reduction. Further investigation focused on mechanisms regulating vent biofilms, the dominant growth strategy in vent microbial communities. Quorum-sensing (QS), a mechanism relying on cell density and the production of extracellular signals for cell-cell communication, is used by many microbial species to regulate biofilm formation. One QS signal is Autoinducer-2, whose precursor is synthesized by the LuxS enzyme. To study QS in vent environments, Caminibacter mediatlanticus and Sulfurovum lithotrophicum, cultured members of the well represented Epsilonproteobacteria, were used as model systems. The luxS gene and transcripts were detected in their genomes and during growth, respectively; these luxS-expressing cultures induced bioluminescence, a QS response, in a Vibrio harveyi reporter strain. Detection of luxS transcripts in-situ, also indicated that QS is likely occurring in natural vent biofilms. This data demonstrates that vent Epsilonproteobacteria posses the luxS/AI-2 system for cell-cell communication. This work is relevant to our overall understanding of microbial phenotypic plasticity in response to environmental factors.

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