White Paper Title: 
Microbial adaptation to intense physicochemical gradients

  A conspicuous inhabitant of certain high temperature (40-105°C) deep-sea (> 2500 m) hydrothermal vent habitats is the polychaetous annelid Alvinella pompejana A. pompejana is found on the walls of high temperature vent chimneys from 13° N to 32° S along the Eastern Pacific Rise (EPR) and has been described by a number of research groups as one of the most thermotolerant and eurythermal metazoans on this planet.  Adult worms are found on black smoker chimneys, where they construct tubes directly on the chimney wall. Temperature probe measurements taken inside actively venting worm-inhabited tubes document the potential for worms to be frequently exposed to steep thermal gradients, with a temperature differential up to 60°C or more along the worm’s body.  The environment of diffuse flow habitats surrounding the worms’ tubes has been characterized not only by its temperature, but also by high levels of hydrogen sulfide (>1 mM) and high concentrations of heavy metals (0.3-200μM). These diffuse flow sites consist of areas of intense mixing of end-member type fluids with ambient seawater creating thermochemical gradients that are unmatched anywhere else on the planet.  

The degree to which A. pompejana is ‘adapted to’ or merely ‘tolerant of’ short-term high temperature exposures is unknown. While A. pompejana appears to flourish in this extreme environment, worms inhabiting low temperature chimneys are more exposed to predation, suggesting that the high temperature chimneys may be a thermal refuge for A. pompejana, with associated costs for living under elevated temperatures.  Indeed, analysis of mRNA transcript abundance for several heat shock proteins (HSPs) in a large EST library suggests a higher level of stress for worms inhabiting sites with elevated temperatures compared to those at lower temperatures.  HSPs are expressed under stress and aid in folding proteins, consuming ATP in the process.  They are regulated at the level of gene transcription, so that transcript abundance may be used as an indicator of heat stress. Analysis of HSP transcript abundance in A. pompejana demonstrates up to 3-fold higher expression of HSPs in posterior vs. anterior sections of worms as well as in worms inhabiting high temperature tubes vs. low temperature tubes. This suggests the cost of living at elevated temperatures may be much higher than living in cooler temperatures for A. pompejana, and that this species may seek out high temperature environments not out of preference, but as a refuge from predation. 

We have been conducting an interdisciplinary study to investigate the metabolic ecology and adaptation strategies associated with living in a thermal gradient.  For this project, we seek to answer the following questions:1. What is the range of temperatures for habitat selection for A. pompejana? , 2. What are the metabolic consequences of living in a thermal gradient for A. pompejana? 3.  What temperature-adaptive strategies (at both the cellular and genetic level) are employed by A. pompejana?  New tools have been developed to more accurately measure the sustained physicochemical gradients in the tubes of individual A. pompejana and to successfully preserve the mRNA of individual specimens on the seafloor.  These will be coupled with down stream comparative transcriptome analysis to resolve the genetics of thermal adaptation.  Results of this investigation will provide insight into metabolic and physiological constraints that influence habitat selection in ecosystems characterized by thermal gradients.   In addition, the identification and elucidation of temperature-adaptive strategies will advance our understanding of the evolution, physiology and ecology of organisms living in extreme environments.