Lipid Biomarker Patterns Reflect Nutritional Strategies of Seep-Dwelling Bivalves From the South China Sea

Abstract

Invertebrates living at methane seeps such as mussels and clams gain nutrition through symbiosis with chemosynthetic, chiefly methanotrophic and thiotrophic bacteria. Lipid biomarkers, including their compound-specific carbon stable isotope compositions, extracted from the host tissues are predestined for deciphering the various sources of diets and the associations among varying environments, endosymbionts, and hosts. Here, we investigated lipid inventories of soft tissues of two bathymodiolin mussel species hosting aerobic methanotrophic bacteria (Gigantidas platifrons from Site F and Gigantidas haimaensis from Haima seeps), one bathymodiolin mussel with thiotrophic bacteria (Bathymodiolus aduloides from Haima seeps), and one vesicomyid clam (Archivesica marissinica from Haima seeps) from the South China Sea. The gills of mussels hosting methanotrophic symbionts were found to contain high amounts of lipids of aerobic methanotrophic bacteria, such as the 4,4-dimethyl lanosterol, and other 4-methyl sterols, and the type I methanotroph-specific monounsaturated fatty acids (MUFAs) C16:1ω9 and C16:1ω8. Production of methyl-sterols is favored over fatty acids at low oxygen concentrations, as demonstrated in culture experiments with Methylococcus capsulatus. Since lesser fatty acids and abundant sterols are found in G. haimaensis compared to G. platifrons, G. haimaensis apparently lived at very low oxygen levels. Extremely high levels of MUFAs C16:1ω7 and C18:1ω7 were found in gill tissue of both B. aduloides and the vesicomyid clam A. marissinica. Given the absence of ω8 fatty acids, both B. aduloides and the vesicomyid clam contain thiotrophic bacteria only. The occurrence of 13C-enriched 24-methylenecholesterol in B. aduloides indicates that the animal complemented its diet by filter-feeding (ca. 3% of the total sterol inventory) on photosynthetically derived carbon, whereas the majority of sterols are pointing to a diet relying on endosymbionts. Different types of 4-methyl sterols were observed between the thiotroph-containing mussel and methanotroph-containing mussels, suggesting different biosynthetic steps are present from lanosterol to cholesterol between animal hosts and aerobic methanotrophs. Among the four bivalve species, specific lipid biomarker patterns diagnostic for either the symbionts or the hosts yielded similar δ13C values in each species, indicating that the host obtained its nutrition either directly from the symbionts or derived at least most of its carbon in this way. The information derived from lipid biomarkers of bivalves and their corresponding symbionts in modern environments is vital to interpret data from the rock record, where most other methods to study microbial community composition are not applicable.

Insights into symbiotic interactions from metatranscriptome analysis of deep‐sea mussel Gigantidas platifrons under long‐term laboratory maintenance

“…However, few DEGs were detected between the MS and MA groups (Figure 2c), indicating that the effect of symbiont loss on the mussel transcriptome after long-term laboratory maintenance was much higher than the effect of the difference in abundance of symbionts between the two groups. In addition, despite their ability to filter-feed on organic particulate matter as a supplement of nutrients (Piquet et al, 2022), deep-sea mussels living in natural environments derived the majority of nutrients from their endosymbionts (Guan et al, 2022). In our experiments, mussels were kept unfed under laboratory conditions, and the nutrients supplied by filtration were limited.…”

A new species of the genus Catillopecten (Bivalvia: Pectinoidea: Propeamussiidae): morphology, mitochondrial genome, and phylogenetic relationship

“…In this seep area, bathymodioline mussel Gigantidas haimaensis Xu et al, 2019, and siboglinid tubeworms Paraescarpia echinospica Southward et al, 2002(Sun et al, 2021 and Sclerolinum annulatum Xu et al, 2022 form huge colonies on the sea bed. Other low-density populations of bivalves were also reported in this area, including Gigantidas platifrons Hashimoto & Okutani, 1994, "Bathymodiolus" aduloides Hashimoto and Okutani, 1994, Nypamodiolus samadiae Lin et al, 2022, and other undetermined species of Malletia and Acharax (Xu et al, 2019;Guan et al, 2022;Ke et al, 2022). These siboglinid and bivalve species are endosymbiotic with chemosynthetic bacteria in their trophosome and gill epithelial cells, respectively, to provide energy and materials for the holobiont (Ip et al, 2021;Sun et al, 2021;Lin et al, 2023).…”

Deciphering deep-sea chemosynthetic symbiosis by single-nucleus RNA-sequencing

“…Among the proteases, cathepsins were thought to be evolutionary conserved molecular tools that host utilized to control the residence of their symbiont microbes 66 . They were also highly expressed in symbiotic tissue of other deep-sea chemosynthetic animals, such as vesicomyid clams and vestimen-tiferan tubeworms 67–69 . Bacteriocytes also expressed genes encoding cellular vesicle transports (kinesins, Bpl_scaf_14819-0.11, Bpl_scaf_54265-1.13, and Bpl_scaf_4784-1.40) 53 , potential amino acid transporter 57 (Bpl_scaf_36159-5.5), and genes involved in intracellular vesicle transport, such as the FYVE and coiled-coil domain-containing protein 1 58 (Bpl_scaf_33726-5.7).…”

“…The symbiont of G. platifrons belongs to type I methanotrophy, of which the core metabolic function is linked with the development of intracytoplasmic membranes leading to a high lipid/biomass content 70,71 . Recent lipid biomarker analyses showed that the gill of G. platifrons contains a high amount of bacterial lipids, which are directly utilised by the host to synthesise most of its lipid contents 67 . Downstream of the bacteriocyte’s metabolic cascade, genes encoding proteins that may be involved in fatty acid/lipid metabolism, such as perilipin2 (Bpl_scaf_27158-3.8), which is the critical protein to form intracellular lipid droplets 72 , and a variety of fatty acid metabolism enzymes (acetyl carboxylase 2, Bpl_scaf_55250-5.11 73 ; fatty acid desaturase 1-like isoform X1, Bpl_scaf_35916-0.6 74,75 ; long-chain fatty acid-ligase ACSBG2-like isoform X1, Bpl_scaf_28862-1.5 76,77 ), were up-regulated, suggesting that the fatty acid could be a major form of nutrients passing from the symbiont to the host mussel.…”