Project EARTH-09-TV1: Searching for the Molecular Skeletons of a Hydrothermal Deep Biosphere at Guaymas Basin in the Gulf of California

The deep biosphere extends across the globe with microbial communities living in marine sediments, the fractures of old oceanic crustal basalts, deep aquifers of granitic batholiths, in oil reservoirs deep within continental settings, and within the hydrothermal vents of mid-oceanic spreading ridges (Amend and Teske, 2005). This realm is estimated to be Earth's largest pool of organic carbon. Many of these microbes are among the earliest organisms to have evolved and thus represent a window into Earth's earliest biosphere. Yet almost nothing is known about how the deep biosphere has evolved over geologic time. This is because the techniques used to identify the microbes living within the subsurface do not work for ones that have long since passed away. Only two analytical techniques are capable of elucidating the microbial "biosignatures" of an ancient subsurface. The first approach includes the stable isotopic analysis of carbon and sulfur bound to minerals that formed within the subsurface. When the isotopic composition of these elements is similar to what is formed by modern-day metabolic processes, the organisms that use these processes are inferred to have been living in the sediment before the sediments became mineralized. The second is a molecular, organic geochemical approach. Many organic molecules are capable of surviving very long periods of time trapped in sediments. Some of these molecules have enough structural complexity that they can be traced to specific types of organisms. By comparing the diversity and abundance of these molecules the presence of a subsurface biosphere can sometimes be detected (Ventura et al., 2007). This studentship will use state of the art methods in comprehensive two-dimensional gas chromatography (GC×GC) and multi-collector inductively coupled mass spectrometry (MC-ICP-MS) to analyze sediment cores collected from a hydrothermal vent field at Guaymas Basin in the Gulf of California (Fig. 1). The data will be used to develop techniques capable of discriminating between the remains of organisms that have died at the Earth's surface and later became buried from contribution that were incorporated by the microbes living within the sediments. The results of this studentship will be compared to various parallel microbiological and geochemical experiments being conducted by Dr. Stefan Sievert and Dr. Jeffrey Seewald at Woods Hole Oceanographic Institution, USA (Fig. 2). The microbiological investigations include down core DNA and RNA genomic analyses, incubation experiments, and intact polar lipid surveys. Geochemical analyses are characterizing the vent fluid chemistry and carbon isotopic compositions of dissolved hydrocarbon gases trapped within the pore space of these sediments. The studentship will have the opportunity to compare the stratigraphic changes in the subsurface microbial community structure to changes in the hydrocarbon composition and trace metal concentrations of the sediments. This project marks the first time GC×GC has ever been used to disentangle a complex mixture of diagenetically transformed hydrocarbons from multiple pools of biologically sourced