Functional Dynamics and Biogeochemical Impact of Subseafloor Microbial Ecosystems at Axial Seamount, a Mid-Ocean Ridge Cabled Observatory (Christopher Algar – post-doctoral-scientist – advisor: Julie Huber)
Hydrothermal vents provide a window into the subseafloor environment. These environments are unique in that it is chemoautotrophy rather than photosynthesis that provides the energy to drive the primary production necessary to support higher trophic levels. This chemical energy results from the mixing of cold oxygenated seawater with hot hydrothermal fluids and provides a variety of reduced compounds that can serve as potential electron donors. Thermodynamic calculations suggest that diffuse flow vents contribute far greater primary biomass then other vent environments to the subseafloor. However to quantify the carbon and nutrient processing at these vents and place them in a larger global biogeochemical perspective requires the development of quantitative biogeochemical models that describe both the physical environment (the flow and mixing of hydrothermal fluids and sea water) and the functioning of the resident microbial ecosystem. I am currently working with Dr. Julie Huber and her team to develop such a model.
The model is being developed to describe the biogeochemistry of diffuse vents at Axial Seamount, an active hydrothermal system along the Juan de Fuca Ridge. This site is an ideal system to develop such a model because of the rich data set collected over the last decade that describes the fluid geochemistry, microbial population structure and seafloor volcanism of Axial. In particular recent advances in the use of genomic tools, such as functional gene surveys and shotgun metagenomics have provided insight into the functional potential of the microbial communities at these vents. Developing methods for incorporating this information into a predictive biogeochemical model is a major goal of our model development.
The model domain will be set up as shown in Figure 1. It will consist of a flow path along which hydrothermal fluid will travel from its source deep in the crust to the seafloor and along the way mix with seawater percolating through the crust. Coupled to this will be an ecosystem model. The ecosystem model will attempt to model the competition for resources (nutrients and energy) that occurs in the real hydrothermal system. This will be done by initially populating the model with a large number of potential organisms each with a different suite of life strategies, i.e. nutrient requirements and metabolic pathways. Properties will be assigned to organisms based upon a system of benefits and trade-offs that are consistent with our current knowledge of physiology. For example, organisms that can grow at low nutrient concentrations must invest a lot of energy into transporting nutrients into their cells and so should have low maximum growth rates. Once the model is populated with these different competing strategies, the model is propagated forward in time and strategies best representing the demands of the local environment will win out. It is hypothesized that the modeled ecosystem that develops will represent the functional potential that has been measured at Axial Seamount. Dr. Huber’s group has carried out functional gene surveys at Axial for the methyl coenzyme M reductase (mcrA/mrtA) gene and the sulfur oxidation (soxB) gene over a range of vent samples with varying chemical/pH conditions, these genes can serve as proxies for methanogenesis and sulfur oxidation. Changes in the presence or absence of these genes under different vent conditions should agree with the community function predicted by the model for these processes. In addition data from mRNA surveys and stable isotope pairing experiments carried out on the seafloor will help to identify the active chemoautotrophs and provide another check on the accuracy of the model.
The development of this biogeochemical vent model should prove to be a valuable tool in describing the biogeochemistry of Axial Seamount and could then be applied to other less studied vent sites around the world. In addition to quantifying carbon and nutrient cycling at vents, the predictive nature of the ecosystem model could allow it to be used as an exploratory tool by identifying potentially active metabolic pathways that may otherwise be missed. This could aid microbiologists in designing experiments ahead of time or suggest which genes should be targeted during field work.
A. Murat Eren
Woo Jun Sul