Improved measurement of microbial activity in deep-sea sediments at in situ pressure and methane concentration

A robust and convenient method was developed to evaluate rates of microbial activity in gas-charged deep-sea sediments at in situ hydrostatic pressure, temperature, and gas concentration. The method used a hydrostatic chamber to maintain high pressures of 50 bar and greater, and a modified Hungate tube and plunger to contain samples. This technique can be easily applied to quantifying rates of microbial processes sensitive to dissolved gas concentration (e. g., sulfate reduction, anaerobic methane oxidation, and methanogenesis) in high pressure, gas-rich environments. Here, the method was used to determine rates of microbial activity in cold seep sediments from the Gulf of Mexico and the California coast. We measured rates of sulfate reduction (SR), anaerobic oxidation of methane (AOM), and methanogenesis (MOG) using radiotracer techniques. We compared in situ pressure and elevated methane concentration assays to traditional ex situ incubations at 1 bar using recovered (degassed) sediment. Methane concentrations were elevated in ex situ incubations to assess a substrate level effect. Upon the application of pressure, rates of AOM, SR, and MOG increased. AOM and MOG rates were more sensitive to methane addition than SR. Our results show that measuring rates of deep-sea microbial activity at in situ pressure and gas concentrations is essential to quantify carbon flow in these environments accurately.