Constraining net chemoautotrophic primary productivity at hydrothermal vents

P. Girguis¹* & C. Meile²

Corresponding author: pgirguis@oeb.harvard.edu
¹Harvard University, Department of Organismic & Evolutionary Biology, Cambridge, MA 02138
²University of Georgia, Department of Marine Sciences, Athens, GA 30602

Abstract:
The high productivity of hydrothermal vent communities is well documented, but our understanding of how and to what extent physico-chemical conditions such as temperature and the geochemical milieu influence rates of net primary productivity among chemoautotrophic microbes (including symbionts) remains poorly constrained. Our work (Girguis and Childress 2006, J. Exp. Biol. 209:3516-3528) using high-pressure respirometry systems to examine the relationship between geochemistry and net primary productivity (net carbon fixation) documented that under environmentally relevant conditions Riftia tubeworms are among the most productive organisms studied to date. Such mass specific carbon fixation rates by individual organisms, however, do not reflect the rates of net productivity by communities in situ. To provide robust estimates of net primary productivity rates based on empirically determined geochemical parameters (and temperature), we are currently developing quantitative relationships between environmental conditions and net carbon respiration rates (R). Using extensive series of laboratory experiments under controlled and well characterized physico-chemical conditions, our approach entails the identification of qualitative functional dependencies of R on temperature, pH, sulfide and oxygen concentrations etc. and uses regression analysis to quantify these relationships. It is our goal to employ this model to better constrain the net rates of primary productivity by Riftia aggregations in situ.

Contributions to Integration and Synthesis:
The above exemplifies an approach that allows for a realistic in situ estimate of primary production in hydrothermal vent systems. It combines in situ chemical conditions with experimental data on tubeworm respiration rates, which to some extent is available from a variety of environments. Our work illustrates an integrative effort, combining the expertise of a physiologist with a computational biogeochemist. Crossing traditional disciplines, it follows the goal articulated in the R2K science plan to unravel and quantify relationships between chemical nature and biological activity driven by deep subsurface processes at ocean spreading centers. The Girguis group studies the physiology and biochemistry of deep-sea microorganisms, with an emphasis on carbon and nitrogen metabolism, to better understand their role in mediating local and global biogeochemical cycles. The group also investigates physiological relationships between microbes and animals in natural systems, in particular hydrothermal vents, applying are range of experimental techniques. The Meile lab performs reactive transport modeling, with one focus area on the role of burrowing macrofauna in early diagenesis and their impact on benthic-pelagic coupling. We are interested in exploring the possibilities of adapting the modeling approaches - which integrate (macro- and micro-) biological, chemical and physically process descriptions - to meet the conditions relevant at hydrothermal vents.