7.1 Introduction

Since the discovery of hydrothermal vents and cold seeps in the 1980s (Corliss et al. 1979; Paull et al. 1984), the widespread chemosynthetic ecosystems on the seabed have revolutionized our understanding of life on Earth (Fisher 1995; Dubilier et al. 2008). Within chemosynthetic environments, the high abundance of reduced compounds, such as hydrogen sulfide (H2S) and methane (CH4), serve as energy sources for carbon-fixing microbes. These microbes, in turn, form the base of the food web of dense biological communities and make significant contributions to primary production in the deep sea, deep-marine biodiversity, and marine geochemical cycles (Baker et al. 2010).

Deep-sea cold seeps are chemosynthetic ecosystems that host highly productive communities and thus enhance regional diversity on continental margins (Levin et al. 2016). Due to their ubiquity and dominance in cold seep ecosystems, chemosymbiotic bivalves—in particular bathymodiolin mussels and vesicomyid clams—have attracted the attention of researchers since the discoveries of these ecosystems. They were first investigated in the Gulf of Mexico with respect to their biogeochemical behavior (Paull et al. 1985; Childress et al. 1986; Cordes et al. 2009) and were subsequently found and studied at virtually all active cold seeps worldwide (Nankai accretionary wedge: Fiala-Médioni et al. 1993; Håkon Mosby mud volcano: Gebruk et al. 2003; pockmarks on the West African margin: Olu-Le Roy et al. 2007; Concepción methane seep area off Chile: Sellanes et al. 2008; New Zealand: Thurber et al. 2010; Marmara Sea: Ritt et al. 2012; North Atlantic and Mediterranean Sea: Duperron et al. 2013; Gulf of Cadiz: Rodrigues et al. 2013; Pacific coast of Costa Rica: Levin et al. 2015; Guaymas Basin: Portail et al. 2015; Arctic: Åström et al. 2019; Krishna-Godavari Basin: Peketi et al. 2022). After nearly 40 years of research, the understanding of the unique symbiotic relationships between seep-inhabiting bivalves and chemotrophic bacteria and the related biogeochemical processes has grown considerably (Fisher 1995; Dubilier et al. 2008; Petersen and Dubilier 2009; Lorion et al. 2013; Decker et al. 2014; Petersen et al. 2016).

Encouraging progress has also been made in discovering and understanding seep-inhabiting bivalves in the South China Sea (SCS) in the last 20 years through a large amount of investment in capital and technology. To date, three active cold seep regions (Site F, Haima and Yam) with seabed manifestations of significant gas seepage and active biological activity have been identified in the SCS, and bivalves represent the dominant macrofauna at these sites. The species, their trophic modes, and their distribution in the SCS are shown in Table 7.1 and Fig. 7.1. Among the chemosymbiotic bivalves, there are five mussel species (Bathymodiolus aduloides and Gigantidas platifrons, Hashimoto and Okutani 1994; Gigantidas securiformis, Okutani et al. 2003; Gigantidas haimaensis, Xu et al. 2019; Nypamodiolus samadiae, Lin et al. 2022), and one clam species (Archivesica marissinica, Chen et al. 2018) has been found. In addition, seven bivalve species with heterotrophic or unknown feeding strategies were found in (or near) site F and Haima seep (Han et al. 2008; Jiang et al. 2019; Ke et al. 2022).

Table 7.1 Bivalve species from the cold seep ecosystems in the South China Sea
Fig. 7.1
A map of South China Sea for bivalves in cold seep ecosystems. It has photos of G platifrons, B.aduloides, G haimaensis, and A marissinica over it. Haima has G platifrons, G haimaensis, B aduloides, N samadiae, A marissinica, V rhombica, S xui, Malletia s p, and Propeamussium s p. Site F has G platifrons, B aduloides, Acharax s p, Conchocele s p, and Lucinoma s p. Yam has G platifrons and G securiformis.

Distribution of known bivalves in cold seep ecosystems in the South China Sea thus far. a The active seep sites—Site F, Haima and Yam—are highlighted by red circles. Representative seep bivalves are shown in b Gigantidas platifrons; c Bathymodiolus aduloides (modified from Feng et al. 2015); d Gigantidas haimaensis; and e Archivesica marissinica (modified from Wang et al. 2022a, b, c). Scale bars in be are 3 cm. See Table 7.1 for the abbreviations

Anatomically, chemosymbiotic bivalves are composed of soft tissue (gill, mantle, foot, siphons, etc.) and a calcareous shell, making them a unique archive for a diversity of research questions (Fig. 7.2). Soft tissue preserves information of the microenvironment that the bivalve inhabits and allows the reconstruction of biogeochemical processes, but it only provides an “instantaneous snapshot” of the brief time immediately before sampling. In contrast, the calcareous shell can provide insights into various environmental conditions during the entire lifetime of the bivalve and can potentially preserve this information for a long time and thus provide insights into evolutionary processes over geological timescales. Therefore, chemosymbiotic bivalves are excellent model organisms—their soft tissues and calcareous shell can be used to explore current and past biogeochemical processes, respectively (Wang et al. 2022a, b).

Fig. 7.2
A photograph of Gigantidas haimaensis. The labeled parts are, Calcareous shell, Gill, Foot, and Mantle. The stretch between the two sides of the shell is 2 centimeters.

Anatomy of Gigantidas haimaensis. Typical features of bivalves include the calcareous shell and soft tissues, such as the gill, mantle and foot

In this chapter, we compile the latest elemental and isotopic data from soft tissues and calcareous shells of diverse seep bivalves in the SCS. The trace element contents and stable isotope compositions of bivalves are outstanding indicators for evaluating their living environment, physiological functions, pathways of energy, and food sources. Together with the understanding of important biogeochemical processes of deep-sea cold seeps, we discuss the variations in geochemical composition in seep-inhabiting bivalves and provide new insights based on recent findings.

7.2 Tissue Element Compositions

Only limited studies have reported trace elements in the gill, mantle and shell of the bathymodiolin mussel G. platifrons (Wang et al. 2017; Zhou et al. 2020), systematic comparative studies (inter- and intraspecific comparison and comparison between soft tissues and shells) are still sparse. Generally, trace element contents are ranked in decreasing order, Zn > Fe > Cu > Sr > Ag > As > Al > Mn > Ba > Pb ≈ Cd > Ni > Co, and it seems that the contents of transition metal elements (e.g., Zn, Fe, Cu) in soft tissues are much higher than those in corresponding shells, indicating that bivalves may have strong physiological requirements for these elements (Fig. 7.3). Undoubtedly, through more measurements over a larger geographic area, longer timespan, and more specimens than species, the interactions among trace elements, the local environments and the physiology of bivalves may be further verified.

Fig. 7.3
A set of two diagrams for general overview of trace elements in the shell and soft tissue of seep bivalves. Calcareous shell has S r and M g for mineralogy, B a, Z n, M n, C u, N i, and L i for mineralogy physiology, T i C o, Z r, M o, and U for physiology environment, and T h for environment. Soft tissue has Z n, F e, C u, S r, A g, A s, A l, M n, B a, P b, C d, N i, and C o for physiology-environment-toxicity.

General overview of trace elements analyzed in the shell and soft tissue of seep bivalves, their approximate concentrations, and the factors potentially controlling their behavior (summarized based on data from Wang et al. 2022a)

In contrast, the contents of rare earth elements (REEs) in seep bivalve soft tissues of the SCS have been well studied (Wang et al. 2020). The aerobic oxidation of methane is thought to be largely dependent upon the use of REEs, but to date, the effects of this process on their abundance in bacteria or organisms living in symbiosis with methanotrophs remain to be evaluated. Wang et al. (2020) selected G. platifrons (methanotroph) and B. aduloides (thiotroph) from Site F and G. haimaensis (methanotroph) and A. marissinica (thiotroph) from the Haima seep site to compare the effects of different regions and different trophic modes on the contents and patterns of REEs in seep bivalves. They showed that the soft tissues of all methanotrophic bivalves had significant lanthanum (La) enrichment, whereas such a pattern was not observed in thiotrophic bivalves (Fig. 7.4). These results demonstrate that methanotrophic bivalves prospering at methane seeps display distinctive La enrichments linked to the enzymatic activities of their symbionts and that methanotrophic bacteria efficiently fractionates REE distributions in organisms and possibly in the environment.

Fig. 7.4
4 multiline graphs, a to d. A, Site F G platifrons, Gill, Mantle, Shell, and Foot have a declining trend. B, Site F B aduloides, Mantle and Gill, and Shell and Site F sediment have an inclining and declining trend, respectively. C, Haima G Haimaensis, Gill, Mantle, Shell, and Foot have a declining trend. D, Haima A marissinica, Mantle, Gill, Haima sediment, and Shell have a declining trend.

REE + Y patterns normalized to the Post-Archean Australian Shale (PAAS; Pourmand et al. 2012) for seep bivalves in the South China Sea (data from Wang et al. 2020). Abbreviations: (1) G. platifrons = Gigantidas platifrons; (2) B. aduloides = Bathymodiolus aduloides; (3) G. haimaensis = Gigantidas haimaensis; (4) A. marissinica = Archivesica marissinica

7.3 Tissue Isotope Compositions

It is well recognized that measuring the carbon, nitrogen, and sulfur isotope compositions of the macrofauna inhabiting cold seeps is one of the most powerful approaches to identifying their food and energy sources, as well as the determination of complex trophic interactions, including symbiosis and heterotrophy (Paull et al. 1985; Childress et al. 1986; Brooks et al. 1987; Carlier et al. 2010; Decker and Olu 2012; Demopoulos et al. 2019; Ke et al. 2022). Indeed, it is possible to distinguish consumers that assimilate carbon through chemosynthesis (more 13C- and 15N-depleted) from those that rely on photosynthetic primary production (Paull et al. 1985; Brooks et al. 1987). This is because methane is much more 13C-depleted (usually δ13CCH4 < −40‰; Whiticar 1999) than dissolved inorganic carbon (DIC) in the water column (δ13CDIC ≈ 0‰), and methane is assimilated by bacteria with only a small carbon isotopic fractionation (Alperin et al. 1988). Within cold seep areas, methane is mainly consumed in the surface sediment by anaerobic oxidation of methane (AOM) coupled with sulfate reduction and produces DIC and sulfide (Boetius et al. 2000), which generates a high microbial biomass (usually a consortium of anaerobic methanotrophic archaea and sulfate-reducing bacteria) and thus can provide a significant supply of methane-derived carbon to heterotrophic bacteria and higher-level consumers (Boetius et al. 2000). The δ13C signatures of symbiotic and heterotrophic seep-related metazoans indicate whether they preferentially rely on methanotrophically or thiotrophically derived carbon (Childress et al. 1986; Brooks et al. 1987) because the net δ13C fractionation between a consumer and its diet is small (typically < 1‰; Vander Zanden and Rasmussen 2001; McCutchan et al. 2003). Methanotrophic bacteria are usually more 13C-depleted than thiotrophic bacteria that fix DIC from the water column (Paull et al. 1985). However, δ13C values of symbiotic species that exclusively rely on thiotrophic bacteria may also be strongly modified when the DIC originates from a diversity of sources, including byproducts of methane oxidation (Lösekann et al. 2008).

Sulfur stable isotopes are useful in discriminating organic matter (OM) from the water column (by phytoplankton) or from reduced sediments (by chemosynthetic microorganisms). In general, the δ34S values of seawater sulfate are quite homogeneous at ~+21‰. Carbon fixation by phytoplankton results in only a small negative fractionation of sulfur isotopes, resulting in a narrow range in the sulfur isotope composition of oceanic particulate OM (+17‰ < δ34S < +21‰; Peterson and Fry 1987). Therefore, benthic organisms that depend exclusively on phytoplanktonic production show δ34S values with similar signatures because sulfur isotopes do not significantly fractionate between trophic levels (McCutchan et al. 2003). In contrast, dissimilatory sulfate reduction by bacteria within the sediment results in strong fractionation (−25‰ < δ34S < +5‰; Carlier et al. 2010). Consequently, organisms that assimilate these reduced compounds exhibit low δ34S values as well. To some extent, additional δ34S analyses may allow differentiation between the input of photosynthetic and chemosynthetic material for seep-related organisms (Brooks et al. 1987; MacAvoy et al. 2005).

The carbon, nitrogen, and sulfur isotopes of G. platifrons, B. aduloides and A. marissinica from cold seeps in the SCS have been investigated (Fig. 7.5; Feng et al. 2015; Zhao et al. 2020; Ke et al. 2022). According to the observations of the available data, some general patterns emerge:

Fig. 7.5
3 positive-negative bar graphs plot uppercase delta lowercase delta versus G platifrons 1 to 13, B aduloides 1 to 4, and A marissinica 1 to 5. The first graph has the highest positive bar for 3 G platifrons and the lowest negative bar for 4 G platifrons. The second graph has the highest positive bar for 2 G platifrons and lowest negative bar for 2 A marissinica. The third graph has the highest positive bar for 1 B aduloides and lowest negative bar for 10 G platifrons.

Differences (Δ) in δ13C, δ15N, and δ34S values among the gill, mantle and foot of seep bivalves (data from Feng et al. 2015, 2018 and unpublished). Blue rectangle: δxxC/N/Sgill-mantle; orange rectangle: δxxC/N/Sgill-foot. Abbreviations: G. platifrons = Gigantidas platifrons; B. aduloides = Bathymodiolus aduloides

  1. (1)

    Carbon and sulfur isotopic compositions of seep bivalve soft tissues are related to their respective trophic modes. The carbon isotope values of methanotrophic bivalves (−77.6‰ to −51.2‰) are obviously lower than those of thiotrophic bivalves (−37.7‰ to −33.0‰), while the sulfur isotope values show the opposite trend (methanotrophic bivalves: +1.4‰ to +19.3‰; thiotrophic bivalves: −12.9‰ to +14.6‰).

  2. (2)

    The characteristics of the carbon isotope signatures of different soft tissue parts of methanotrophic bivalves are consistent (δ13CMantle < δ13CGill < δ13CFoot), which has implications for the carbon transmission process in G. platifrons.

  3. (3)

    It is suggested that different nitrogen sources are used by bivalves, in which the main nitrogen source of B. aduloides is organic nitrogen from the sediments, while G. platifrons mainly uses NH4+ (Sun et al. 2017; Wang et al. 2022a, b, c). However, it remains unclear whether different nitrogen sources are responsible for the rather stable nitrogen isotope signature of B. aduloides (+1.1‰ to +1.2‰) and the lower and highly variable nitrogen isotope signature of G. platifrons (−0.9‰ to +1.3‰). Obviously, the nitrogen isotope signature archived by seep bivalves should be further explored.

7.4 Element and Isotope Compositions of Shells

By comparison, prospecting and investigation of the calcareous shells of these bivalves lags behind that of their soft tissue. Generally, due to some inherent methodological and physiological limitations related to the shell (the record of the whole life process – years to decades of accumulation, the high degree of mixing with seawater, bulk shell sampling methods, etc.), it seems that the shell reflects some insignificant geochemical signals that inevitably blend the information of seawater and carbonate. For example, trace element concentrations ranked in decreasing order for calcareous shells of G. haimaensis and A. marissinica are Sr > Mg > Ba > Zn ≈ Mn > Cu > Ni > Li ≈ Ti > Co ≈ Zr ≈ Mo ≈ U > Th, obviously because carbonate is rich in trace elements, such as Sr, Mg and Ba (Fig. 7.3; Wang et al. 2022a). However, few studies have demonstrated that shells, similar to soft tissue, can potentially be used to reconstruct biogeochemical processes (Feng et al. 2018; Wang et al. 2020). Although the REE contents of the shells are lower than those of the corresponding soft tissues, characteristic of La enrichment is also observed in the methanotrophy-related shells (Fig. 7.4). In addition, generally consistent characteristics between the carbon, nitrogen and sulfur isotopic information contained in the shell organic matter (SOM) and its corresponding soft tissue mantle are present (Fig. 7.6). In contrast to the quickly degrading soft tissue, the bivalve shell has a high preservation potential, including its original mineralogy. Hence, the geochemical signal in the shell can be used as a fingerprint of food chains, as well as to track early evidence of microbial life and to reconstruct the chemosynthetic behavior among different types of ecosystems in the geologic past (Feng et al. 2018; Wang et al. 2020).

Fig. 7.6
A set of three positive-negative bar graphs. The graphs plot uppercase delta lowercase delta versus G platifrons 1 to 11, B aduloides 1 to 3, and A marissinica 1 to 5. The first graph has the highest positive bar for 5 G platifrons and the lowest negative bar for 2 B aduloides. The second graph has the highest positive bar for 3 B aduloides and lowest negative bar for 7 G platifrons. The third graph has the highest positive bar for 1 B aduloides and lowest negative bar for 4 A marissinica.

Differences (Δ) in δ13C, δ15N, and δ34S values between the organic shell matrix and mantle of seep bivalves (data from Feng et al. 2018). Abbreviations: G. platifrons = Gigantidas platifrons; B. aduloides = Bathymodiolus aduloides; A. marissinica = Archivesica marissinica

7.5 Summary and Perspective

Research on seep bivalves in the South China Sea (SCS) has acquired a series of profound understandings that have spurred the wave of the development of cold seeps in China in the last 20 years. Studies on the elemental geochemistry, especially rare earth elements of seep bivalves, have further verified the catalysis of light rare earth elements on the aerobic oxidation of methane and provided an excellent example for exploring the coupling (enzymatic) relationship between life activities and trace elements. At the same time, by investigating the carbon, nitrogen, and sulfur isotopes of seep bivalves, temporally and spatially integrated trophic estimates used to understand and define trophic linkages among species and communities are provided.

However, it must be admitted that our knowledge of the geochemical behavior of seep bivalves is restricted since the study of the seep bivalves of the SCS is still in the initial stage of discovery and research. The existing issues include but are not limited to the following: (1) there is a large gap in the research on different species of seep bivalves. Intensive studies on the geochemical characteristics of seep bivalves focus on G. platifrons, G. haimaensis and A. marissinica, while few studies have been conducted on other species. The trace element data of soft tissues are particularly scarce. (2) The mechanism that controls the behavior of carbon, nitrogen, and sulfur isotopes in chemosymbiotic bivalves remains understudied. In addition to the interaction between host and symbiotic bacteria, facultative filter feeding among seep bivalves remains a hard-to-define factor that may obstruct predictions of the behavior of carbon, nitrogen, and sulfur isotopes in these bivalves. (3) Due to the technical difficulties of sampling the deep sea, species of nondominant bivalves are typically found only as bycatch rather than as the result of focused and systematic sampling. Promising in this respect is the Haima cold seep as it covers a vast area, and Yam is a recently discovered seep area, allowing the discovery and research of new species of bivalves.

The exploration of seep bivalves is of great significance to both biology and geology because the combination of soft tissue and calcareous shells makes them excellent model organisms to bridge contemporary earth surface processes (present) and the sedimentary record (fossil). Future research on seep bivalves should strengthen the combination of biology and geology (especially geochemistry) and further apply this research approach to all seep macrofauna, which is expected to decode the ‘vital effect’ of symbiotic organisms and their overprint on geological records. We believe that future research on seep bivalves (and all seep macrofauna) in the South China Sea cold seeps will continue to expand our knowledge of the biological activities in this unique ecosystem, and we look forward to the advancement of knowledge in the fields of life evolution, symbiotic relationships and beyond.