A Compilation of the Stable Isotopic Compositions of Carbon, Nitrogen, and Sulfur in Soft Body Parts of Animals Collected from Deep-Sea Hydrothermal Vent and Methane Seep Fields: Variations in Energy Source and Importance of Subsurface Microbial Processes in the Sediment-Hosted Systems

Abstract

The stable isotopic signatures of biophilic elements, such as carbon, nitrogen, and sulfur, exhibited in animal soft body parts are excellent indicators for evaluating the pathways of energy and food sources. Thioautotrophic and methanotrophic nutrition prevailed in deep-sea hydrothermal vent and methane seep areas results in sulfide-sulfur and methanecarbon isotopic ratios. In this study, we reevaluated the carbon, nitrogen, and sulfur isotope compositions of animals taken from deep-sea hydrothermal vents and methane seep areas in order to understand the detailed pathways of energy and food sources for the habitants. The results showed that most animals collected from sediment-starved hydrothermal areas rely on thioautotrophic nutrition, using hydrogen sulfide dissolved in venting fluids as the sole primary energy source. On the other hand, animals from sediment-covered hydrothermal vent and cold seep fields show some variations in energy sources, of both hydrothermal and microbial origins. Sediment-covered areas tend to be enriched in biomass and diversity relative to sediment-starved areas. The results suggest that fluid discharged through sediments to the seafloor are strongly affected by subsurface microbial processes and result in increased biomass and diversity of the seafloor animal community.

Hydrothermal activity lowers trophic diversity in Antarctic hydrothermal sediments

“…Diazotrophy has been detected previously in hydrothermal vents and cold seeps, typified by low δ 15 N values (e.g. Rau, 1981;Desai et al, 2013;Wu et al, 2014;Yamanaka et al, 2015). Diazotrophy in various reducing settings has been found associated with the anaerobic oxidation of methane (Dekas et al, 2009), methanotrophy (Mehta and Baross, 2006), and (in a non-marine cave) sulfate reduction (Desai et al, 2013).…”

“…Bulk faunal isotopic signatures are inadequate to determine which of these chemosynthesisrelated mechanisms is responsible for Siboglinum δ 15 N values, which would require analysis of the functional genes in the Siboglinum endosymbionts. The δ 15 N values for both siboglinids (δ 15 N Sclerolinum −5.3 ‰ ± 1.0, Siboglinum −8.9 ‰ ± 0.8) indicated reliance upon locally fixed N 2 (Rau, 1981;Dekas et al, 2009Dekas et al, , 2014Wu et al, 2014;Yamanaka et al, 2015) rather than utilisation of sediment organic nitrogen (δ 15 N = 5.7 ‰ ± 0.7). These values were also in contrast to the rest of the nonchemosynthetic obligate species, which generally had much heavier δ 15 N values.…”

“…δ 13 C values (mean −41.4, range −45.7 to −38.1 ‰, n = 8) corresponded very closely to published values of thermogenic methane (−43 to −38 ‰) from the Bransfield Strait (Whiticar and Suess, 1990), strongly suggesting that methanotrophy was the dominant carbon source for this species. Biogenic methane, although present in the Bransfield Strait, typically has much lower δ 13 C values (Whiticar, 1999;Yamanaka et al, 2015), indicating a hydrothermal and/or thermogenic source of methane in the Bransfield Strait (Whiticar and Suess, 1990). Sulfur isotopic signatures were also very low in the Siboglinum sp.…”

“…Mineral sulfide was present at Hook Ridge that ranged between −28.1 and +5.1 ‰ (Petersen et al, 2004), which is consistent with the relatively high δ 34 S variability in S. contortum. Alternatively, sulfide supplied as a result of microbial sulfate reduction (Canfield, 2001) may have been the primary source of organic sulfur, similar to that of solemyid bivalves in reducing sediments (mean δ 34 S of −30 to −20 ‰; Vetter and Fry (1998) and in cold seep settings (Yamanaka et al, 2015). Sulfate reduction can also be associated with the anaerobic oxidation of methane (Whiticar and Suess, 1990;Canfield, 2001;Dowell et al, 2016), suggesting that methanotrophic pathways could also have been important at Hook Ridge.…”

“…Microbial aggregations are commonly visible on the sediment surface in hydrothermal sediments (Levin et al, 2009;Sweetman et al, 2013;Dowell et al, 2016), but microbial activity also occurs throughout the underlying sediment, occupying a wide range of geochemical and thermal niches (reviewed by Teske et al, 2014). Sedimented chemosynthetic ecosystems may present several sources of organic matter to consumers (Bernardino et al, 2012;Sweetman et al, 2013;Yamanaka et al, 2015) and the diverse microbial assemblages can support a variety of reaction pathways, including methane oxidation, sulfide oxidation, sulfate reduction, and nitrogen fixation (Teske et al, 2002;Dekas et al, 2009;Jaeschke et al, 2014). Phospholipid fatty acid (PLFA) analysis can be used to describe recent microbial activity and δ 13 C signatures (Boschker and Middelburg, 2002;Yamanaka and Sakata, 2004;Colaço et al, 2007).…”