Microbial Cells and Aerobic Respiration from Seafloor to Basement in the South Pacific Gyre

Abstract:

The seafloor is broadly divided into two regions (Emerson et al., 1985): one where sedimentary microbial respiration is high and oxygen (O₂) penetrates only millimeters to centimeters into the sediment (Revsbech et al., 1980), and another where sedimentary respiration is low and O₂ penetrates much deeper (Murray& Grundmanis, 1980; D’Hondt et al., 2011; Røy et al, 2012; Orcutt et al., 2013). Active anaerobic microbial communities persist for hundreds of meters or more in subseafloor sediment of the high-respiration region. In the low-respiration region, the existence of microbial communities is previously unknown throughout most of the sedimentary sequence (Morita & Zobell, 1955; D’Hondt et al., 2009; Røy et al., 2012). Here we show that microbial cells and aerobic respiration persist through the entire sediment sequence (to depths of at least 75 m below seafloor) throughout the vast expanse of the oligotrophic South Pacific Gyre. This sediment and underlying basalt may be continuously exposed to O₂ for its entire history (up to 120 myrs at our sites). Redfield stoichiometry of dissolved O₂ and nitrate indicates that net sedimentary O₂ reduction is coupled to oxidation of marine organic matter. Oxygen and aerobic communities may occur throughout the entire sediment sequence in 15-44% of the Pacific and 9-37% of the global ocean. This result has major implications for the nature and distribution of subseafloor life. It may ultimately affect the chemical evolution of Earth’s mantle and subduction-related volcanic systems.

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