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Water activity (Aw) is an important factor that limits and affects microbial growth in foods and soils; as such, the effect of Aw on bacterial growth has been investigated extensively in the food industry and in soil and plant ecosystems (Stark and Firestone, 1995; Fontana and Campbell, 2007; Welti-Chanes et al., 2007), as well as in habitability assessments of extraterrestrial environments (Tosca et al., 2008). Aw is a measure of how efficiently pore water facilitates the biogeochemical reactions that support microbial growth in solid–fluid systems. Specifically, Aw is defined as the ratio of the vapor pressure of a core sample (p) to that of pure water (p0) and is expressed as follows (e.g., Grant, 2004):

Aw = p/p0 = RH/100,


where RH (%) is the relative humidity of air and Aw ranges from 0 (no water) to 1 (pure water). A high Aw implies that an environment is more habitable to microbes; most microbes cannot proliferate at water activity values below 0.9, and even extremophiles, such as xerophilic fungi, cannot survive at water activities activity values of ~0.6 (Grant, 2004; Williams and Hallsworth, 2009; Stevenson et al., 2015).

In the last two decades, scientific ocean drilling programs have discovered microbial communities in deep subseafloor sediments (Parkes et al., 1994; Lipp et al., 2008; Kallmeyer et al., 2012). The number of microbial cells in these deep ocean floor sediments typically decreases with depth, and the microbial populations and the limit of microbial life are probably limited by a combination of chemical, physical, and geological conditions, such as temperature, pore water chemistry, and pH, which also limit microbial survival in soil and food (Sutherland et al., 1994; Jenkins et al., 2002; Hinrichs and Inagaki, 2012); however, the effect of Aw on microorganism growth in deep subseafloor sediments remains unclarified. Aw also influences the rate of chemical reactions (Bell and Hageman, 1994; Vittadini and Chinachoti, 2003), which implies that it could be used as an index for mass transport in deep subseafloor environments. Although the physicochemical properties of subseafloor sediments are measured routinely on scientific ocean drilling surveys, the Aw characteristics of subseafloor sediments have never been examined, even at shallow depths. Most pores of subseabed sediments are filled with seawater, and high Aw close to 1 can be expected. However, Aw depends on solute concentration, composition, and temperature (Starzak and Mathlouthi, 2006). Pore size distribution and the fraction of clay minerals that adsorbed water (Beckett and Augarde, 2013) may also influence Aw. Therefore, Aw is likely to be lower than expected. The relationship between Aw and the sediment water content, which is referred to as the soil–water characteristic curve (SWCC), is not only useful for estimating unsaturated sediment behavior, but also the pore structure and size distribution, both of which affect permeability (Fredlund and Xing, 1994; Aung et al., 2001).

In this paper, we report Aw measurements conducted on core samples collected from Integrated Ocean Drilling Program (IODP) Expedition 337 (see the “Site C0020” chapter [Expedition 337 Scientists, 2013b]). The effect of lithology, porosity, pore water chemistry, and water content on Aw for core sediments was examined.