Abstract
Springs hosted in ophiolites are often affected by serpentinization processes. The characteristically low DIC and high CH4 and H2 gas concentrations of serpentinizing ecosystems have led to interest in hydrogen based metabolisms in these subsurface biomes. However, a true subsurface signature can be difficult to identify in surface expressions such as serpentinizing springs. Here, we explore carbon and nitrogen resources in serpentinization impacted springs in the tropical climate of the Zambales and Palawan ophiolites in the Philippines, with a focus on surface vs. subsurface processes and exogenous vs. endogenous nutrient input. Isotopic signatures in spring fluids, biomass, and carbonates were examined to identify sources and sinks of carbon and nitrogen, carbonate geochemistry, and the effect of seasonal precipitation. Seasonality affected biomass production in both low flow and high flow spring systems. Changes in meteorological precipitation affected δ13CDIC and δ13CDOC values of the spring fluids, which reflected seasonal gain/loss of atmospheric influence and changes in exogenous DOC input. The primary carbon source in high flow systems was variable, with DOC contributing to biomass in many springs, and a mix of DIC and carbonates contributing to biomass in select locations. However, primary carbon resources in low flow systems may depend more on endogenous than exogenous carbon, even in high precipitation seasons. Isotopic evidence for nitrogen fixation was identified, with seasonal influence only seen in low flow systems. Carbonate formation was found to occur as a mixture of recrystallization/recycling of older carbonates and rapid mineral precipitation (depending on the system), with highly δ13C and δ18O depleted carbonates occurring in many locations. Subsurface signatures (e.g., low DOC influence on Cbiomass) were most apparent in the driest seasons and lowest flow systems, indicating locations where metabolic processes divorced from surface influences (including hydrogen based metabolisms) are most likely to be occurring.
Highlights
Recent interest in the terrestrial deep biosphere has been fueled by estimates of biomass (Kallmeyer et al, 2012; Lomstein et al, 2012; McMahon and Parnell, 2014) and reports of unique microbial communities and ecosystem functions (Biddle et al, 2012; Parkes et al, 2014; Solden et al, 2016)
The ratio of dissolved organic carbon (DOC):Dissolved Inorganic Carbon (DIC) at each major field area is considered in Supplementary Figure S4
We are lacking δ13CDIC from precipitation during the time periods that we were in the field, so estimates of input from this source of DIC were not considered
Summary
Recent interest in the terrestrial deep biosphere has been fueled by estimates of biomass (Kallmeyer et al, 2012; Lomstein et al, 2012; McMahon and Parnell, 2014) and reports of unique microbial communities and ecosystem functions (Biddle et al, 2012; Parkes et al, 2014; Solden et al, 2016). Surface connected expressions of the terrestrial deep biosphere such as caves, wells, and springs are convenient and less expensive (compared to drilling based endeavors), but elicit questions about authenticity of a subsurface signature. The most success has come from studies with direct access to subsurface sampling. These works have relied on comparative metagenomics (e.g., Lau et al, 2014), geochemical modeling integrated with statistical analyses (e.g., Osburn et al, 2014), and innovative culturing techniques (e.g., Rowe et al, 2017) to distinguish subsurface contributions to nutrients, energy, diversity, and biomass. Isolating a subsurface signature when samples are obtained within the surface biome (i.e., springs and seeps) poses additional challenge. The widespread ability to fix nitrogen (Hamilton et al, 2011) or carbon (Osburn et al, 2011; Urschel et al, 2015) using non-photosynthetic pathways in terrestrial hydrothermal systems suggests these traits are maintained due to a general lack of reliable nutrient input from the surface biome
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