Exploring the impact of temperature on dissolved organic matter and life in the deep biosphere off Muroto, Nankai Trough (IODP Expedition 370)

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IODP Expedition 370 - Temperature Limit of the Deep Biosphere off Muroto (T-Limit) - (September 10 - November 23, 2016), aimed to explore the limits of life in the deep subseafloor biosphere at a location where temperature increases with depth at an intermediate rate and exceeds the known temperature maximum of microbial life (~120°C) at the sediment/basement interface. Such conditions are met in the protothrust zone of the Nankai Trough off Cape Muroto, Japan, where Site C0023 was established in the vicinity of ODP Sites 808 and 1174 at a water depth of 4776 m using DV Chikyu. Hole C0023A was advanced across the sediment/basement interface down to a total depth of 1180 meters below seafloor, 112 cores were retrieved, and long-term temperature observations were started (see also: http://www.jamstec.go.jp/chikyu/e/exp370/index.html). The primary scientific objectives of Expedition 370 are (a) to detect and investigate the presence or absence of life and biological processes at the biotic-abiotic transition of the deep subseafloor with unprecedented analytical sensitivity and precision; (b) to comprehensively study the factors that control biomass, activity, and diversity of microbial communities; and (c) to elucidate if continuous or episodic flow of fluids containing thermogenic and/or geogenic nutrients and energy substrates support subseafloor microbial communities in the Nankai Trough accretionary complex. This project will address the overarching expedition objectives through in-depth investigation of dissolved organic matter (DOM) in a set of 112 interstitial water samples retrieved from Hole C0023A: (1) Based on high-resolution depth profiles for the concentration and carbon isotopic composition of organic metabolites, such as acetate and other volatile fatty acids (VFAs), methanol and ethanol, we aim to gain insights into microbial activity and carbon flow. (2) Using high-resolution depth profiles for major fluorescent DOM components, such as humic-like DOM, protein-like DOM and subgroups thereof, will seek to investigate the bioavailability and alteration of organic matter during sediment burial and heating, and its impact on microbial abundance and activity. (3) Finally, investigation of selected samples with ultrahigh resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS) will allow us to characterize the elemental composition of DOM on a molecular level, to gain mechanistic insights into pathways of organic matter alteration during burial and heating, and to trace the put of DOM with fluids from deeper, hotter sources.