With no natural light and hundreds of meters below sea level, the Earth’s crust is alive both geologically and biologically. Deep sea geological structures, such as hydrothermal vents, teem with life. Similar stunning islands of life are found in other deep sea formations called marine hydrocarbon seeps. These seeps spew gaseous and liquid hydrocarbons, compounds that support a rich microbial and benthic faunal population.
Marine hydrocarbon seeps exude gases, such as methane (CH4), C2-C5 alkanes, and other heavier liquid hydrocarbons. Methane is a potent greenhouse gas and has been implicated in global-warming. Marine seeps account for nearly 20-30% of the natural fossil CH4 emissions to the atmosphere. Thus, significant research has been done on microbial methane cycling in seep environments, focusing primarily on the anaerobic oxidation of methane (AOM), which is a major CH4 sink. Meanwhile, petroleum contamination of marine waters has fueled research on the anaerobic degradation of heavier hydrocarbons. However, very little is known regarding anaerobic C2-C5 alkane degradation, although stable isotope data clearly indicates the occurrence of such processes in marine seeps. Short-chain alkanes comprise a substantial portion of the non-methane, lower molecular weight hydrocarbons released from marine seeps. Additionally, recent studies have implicated a connection between C2-C5 alkane oxidation and AOM bringing this ill-addressed issue to the attention of both geologists and microbiologists.
In the Girguis lab, we are using a multidisciplinary approach to understand the ecophysiology and biogeochemistry of anaerobic methane and other short-chain alkane oxidation. Using microcosm experiments with sediments collected from marine hydrocarbon seeps across the world, we are trying to understand the basic mechanisms that might occur simultaneously with alkane oxidation. Unique incubation systems have also be designed and built to suit mimic in situ conditions. Determination of microbial community structure in conjunction with geochemical measurements will shed light on these complex microbial processes.
Figure 1. A hydrocarbon cold seep in the Monterey Canyon, 950 meters water depth. These seeps are typically covered by lush mats of sulfur-oxidizing bacteria such as Beggiatoa.
Figure 2. Cold seeps like this one in the Monterey Canyon are often modest in size. The seep shown here is about 4 meters in diameter. In other environments, such as the Gulf of Mexico, seeps can be significantly larger in size, and the associated organisms carpet the seafloor in great expanses.