Subcellular metal distribution in two deep-sea mollusks: Insight of metal adaptation and detoxification near hydrothermal vents

Comparative study of integrated bio-responses in deep-sea and nearshore mussels upon abiotic condition changes: Insight into distinct regulation and adaptation

Deep-sea mussels, one of the dominant species in most deep-sea ecosystems, have long been used as model organisms to investigate the adaptations and symbiotic relationships of deep-sea macrofauna under laboratory conditions due to their ability to survive under atmospheric pressure. However, the impact of additional abiotic conditions beyond pressure, such as temperature and light, on their physiological characteristics remains unknown. In this study, deep-sea mussels (Gigantidas platifrons) from cold seep of the South China Sea, along with nearshore mussels (Mytilus coruscus) from the East China Sea, were reared in unfavorable abiotic conditions for up to 8 days. Integrated biochemical indexes including antioxidant defense, immune ability and energy metabolism were investigated in the gill and digestive gland, while cytotoxicity was determined in hemocytes of both types of mussels. The results revealed mild bio-responses in two types of mussels in the laboratory, represented by the effective antioxidant defense with constant total antioxidant capability level and malondialdehyde content. There were also disparate adaptations in deep-sea and nearshore mussels. In deep-sea mussels, significantly increased immune response and energy reservation were observed in gills, together with the elevated cytotoxicity in hemocytes, implying the more severe biological adaptation was required, mainly due to the symbiotic bacteria loss under laboratory conditions. On the contrary, insignificant biological responses were exhibited in nearshore mussels except for the increased energy consumption, indicating the trade-off strategy to use more energy to deal with potential stress. Overall, this comparative study highlights the basal bio-responses of deep-sea and nearshore mussels out of their native environments, providing evidence that short-term culture of both mussels under easily achievable laboratory conditions would not dramatically alter their biological status. This finding will assist in broadening the application of deep-sea mussels as model organism in future research regardless of the specialized research equipment.

Survival strategies in arsenic-contaminated environments: Comparative insights from native and exotic aquatic species

The aim of this work was to study the sublethal effects, biokinetics, subcellular partitioning and detoxification of arsenic in two native Chinses species, Bellamya quadrata and Cipangopaludina cathayensis, as well as an exotic South American species, Pomacea canaliculata. The exotic species exhibited higher tolerance than native species. Physiologically based pharmacokinetic model results showed that the exotic species P. canaliculata exhibited a lower bioaccumulation rate and a greater metabolism capacity of As. Subcellular partitioning of As revealed that P. canaliculata exhibits superior As tolerance compared to the native species B. quadrata and C. cathayensis. This is attributed to P. canaliculata effective management of the metal sensitive fraction and enhanced accumulation of As in the biologically detoxified metal fraction. Under As stress, the biochemical parameters (superoxide dismutase, malondialdehyde, glutathione and glutathione S-transferase) of the exotic species P. canaliculata changed less in the native species, and they returned to normal levels at the end of depuration period. Our study provides evidence of the superior survival capability of the exotic species P. canaliculata compared to the native species B. quadrata and C. cathayensis under environmentally relevant levels of As contamination.

Cytosolic distribution of copper in the gills of field-collected oysters with different copper bioaccumulation

Oysters can hyper-accumulate copper (Cu) without apparent toxicity, but the mechanism of sequestering excessive cytosolic Cu in oysters remains unclear. We here investigated the Cu distribution in the cytosolic proteins (CPs) in the gills of oysters (Crassostrea hongkongensis) through size-exclusion chromatography coupled to inductively coupled plasma mass spectrometry (SEC-ICP-MS). Oysters collected from the southern coast of China contained a gradient of gill Cu concentrations ranging from 132 to 3540 μg g-1 (dry weight), with 7-41 % of Cu distributed in the CPs fraction. The CPs-Cu concentrations were 8.6 times higher in oysters with high Cu concentrations compared to low concentrations. In the CPs, Cu was dispersed with a broad range of molecular weight, suggesting the involvement of various cytosolic proteins in Cu binding. Among the 10 major Cu peaks, peaks 2 (>600 kDa) and peak 8 (18 kDa) contained substantial Cu and showed obvious differences in response to the variation of CPs-Cu levels. Peak 8 contained metallothionein-like proteins that decreased their role in Cu binding as CPs-Cu concentrations increased. LC-MS/MS analysis revealed that peak 2 contained macromolecular protein complexes (MPCs), which played a critical role in binding excess Cu. The comparison with other bivalve species further suggested that sequestering excess CPs-Cu in MPCs was a special strategy employed by oysters in response to high Cu accumulation. This study provides valuable insights into the mechanism of hyper-accumulation and sequestration of Cu in oysters and helps to better understand Cu biomonitoring by oysters.

Genetic adaptations of sea anemone to hydrothermal environment

Hydrothermal vent habitats are characterized by high hydrostatic pressure, darkness, and the continuous release of toxic metal ions into the surrounding environment where sea anemones and other invertebrates thrive. Nevertheless, the understanding of metazoan metal ion tolerances and environmental adaptations remains limited. We assembled a chromosome-level genome for the vent sea anemone, Alvinactis idsseensis sp. nov. Comparative genomic analyses revealed gene family expansions and gene innovations in A. idsseensis sp. nov. as a response to high concentrations of metal ions. Impressively, the metal tolerance proteins MTPs is a unique evolutionary response to the high concentrations of Fe2+ and Mn2+ present in the environments of these anemones. We also found genes associated with high concentrations of polyunsaturated fatty acids that may respond to high hydrostatic pressure and found sensory and circadian rhythm-regulated genes that were essential for adaptations to darkness. Overall, our results provide insights into metazoan adaptation to metal ions, high pressure, and darkness in hydrothermal vents.

Cadmium bioaccumulation and distribution in the freshwater bivalve Anodonta woodiana exposed to environmentally relevant Cd levels

Anodonta woodiana is a globally distributed freshwater bivalve, which is a unique bioindicator in the "Freshwater Mussel Watch" project. Numerous countries have used A. woodiana for biomonitoring the aqueous cadmium (Cd) contamination. However, the bioaccumulation and distribution characteristics of environmentally relevant Cd concentrations in the bivalve were unknown. In this study, A. woodiana was exposed to aqueous Cd concentrations (1.1, 2.6, and 5.5 μg/L) for 30 days. The concentrations of Cd in the whole soft tissues were linearly and positively correlated with the aqueous Cd concentrations and exposure time (P < 0.05). Analysis of the organic bioaccumulation and distribution showed that Cd concentrations and proportions in the gills and mantle were linearly and positively correlated with aqueous Cd concentrations and with the Cd concentrations in the whole soft tissues (P < 0.05). Analyses of the subcellular fractions showed that Cd concentrations and proportions in the metal-rich granule (MRG) were linearly and positively correlated with aqueous Cd concentrations and with the Cd concentrations in the whole soft tissues (P < 0.05). This suggests that the gills and mantle could be the main target organs for Cd bioaccumulation, and that the MRG could be the major site for Cd bioaccumulation and distribution. These results improve our understanding of the bioaccumulation and distribution mechanisms of Cd in bivalves.

Zinc source differentiation in hydrothermal vent mollusks: Insight from Zn isotope ratios

Hydrothermal vent represents an extreme environment where metal-enriched fluids are in contact with chemosymbiotic animals. In the present study, Zn isotopic compositions were determined in multiple tissues of three dominant hydrothermal vent mollusks (the mussel Bathymodiolus marisindicus and two gastropods Chrysomallon squamiferum and Gigantopelta aegis) collected from a hydrothermal vent field (Southwest Indian Ridge in the Indian Ocean). We found approximately 1.78‰ differences in the δ⁶⁶Zn values among the three vent mollusks despite of their similar range of Zn concentrations. The significant variation in the δ⁶⁶Zn values was considered to be indicative of different Zn uptake sources among the three species as a result of their morphological adaptations. Zinc uptake associated with symbiotic activities may be more relevant in the vent gastropods, whereas Zn uptake from hydrothermal fluids during filter-feeding may also play a role in the vent mussels. However, no significant difference in δ⁶⁶Zn values was observed among tissues of any of the mollusks, showing the absence of Zn isotope fractionation during internal Zn transport. Our results demonstrated that variable Zn uptake pathways existed among different hydrothermal vent mollusks and could be differentiated by determining the Zn isotopic compositions in their tissues. We also highlight that Zn isotope ratios can be used to track Zn sources to the vent mollusks.

Size speciation of dissolved trace metals in hydrothermal plumes on the Southwest Indian Ridge

Determining the size speciation and chemical transformation of trace metals is of paramount in order to better assess the impact of hydrothermal activities on the ocean metal budgets. In this study, we examined the concentration and size speciation of dissolved trace metals (i.e. Mn, Fe, Cu, Ni, Mo, As, Pb, Cd) in the hydrothermal plumes of two vent fields (i.e. Longqi and Tiancheng) on the Southwest Indian Ridge. The majority of dissolved Mn (75-100%) in the buoyant and non-buoyant plumes were presented as soluble Mn (<1 kDa), while dissolved Fe in the buoyant plume contained considerable colloidal Fe (54-95%). More than 66% of hydrothermally dissolved Fe was removed in the buoyant plume within a short distance of dispersion. Except for the samples most proximal to the fluid source, concentrations of Cu, Ni and Mo in the plumes were comparable to those of the background seawater and independent of the plume dilution. Concentrations of dissolved As and Pb in the buoyant plume of the Tiancheng field were higher than those of the Longqi field, resulting from the scavenging of As in the Longqi field and the release of Pb from metal sulfide dissolution in the Tiancheng field. Concentrations of dissolved Cd in the non-buoyant plume were nearly identical to the background seawater and soluble Cd was dominant (75-92%) in the soluble phase. In contrast, 33-96% (or 0.024-0.085 μg/kg) of dissolved Cd was removed in the buoyant plume and the remaining dissolved Cd was mainly in colloidal phase (up to 96%), suggesting that hydrothermal plume was likely an important sink of oceanic Cd and colloidal ligands played an important role in the stabilization of hydrothermal Cd. Our study has demonstrated the very dynamic nature of trace metal speciation in hydrothermal vent fluids.