The isotopic biosignatures of photo‐ vs. thiotrophic bivalves: are they preserved in fossil shells?

Abstract

Symbiont-bearing and non-symbiotic marine bivalves were used as model organisms to establish biosignatures for the detection of distinctive symbioses in ancient bivalves. For this purpose, the isotopic composition of lipids (δ13C) and bulk organic shell matrix (δ13C, δ34S, δ15N) from shells of several thiotrophic, phototrophic, or non-symbiotic bivalves were compared (phototrophic: Fragum fragum, Fragum unedo, Tridacna maxima; thiotrophic: Codakia tigerina, Fimbria fimbriata, Anodontia sp.; non-symbiotic: Tapes dorsatus, Vasticardium vertebratum, Scutarcopagia sp.). ∆13C values of bulk organic shell matrices, most likely representing mainly original shell protein/chitin biomass, were depleted in thio- and phototrophic bivalves compared to non-symbiotic bivalves. As the bulk organic shell matrix also showed a major depletion of δ15N (down to -2.2 ‰) for thiotrophic bivalves, combined δ13C and δ15N values are useful to differentiate between thio-, phototrophic, and non-symbiotic lifestyles. However, the use of these isotopic signatures for the study of ancient bivalves is limited by the preservation of the bulk organic shell matrix in fossils. Substantial alteration was clearly shown by detailed microscopic analyses of fossil (late Pleistocene) T. maxima and Trachycardium lacunosum shell, demonstrating a severe loss of quantity and quality of bulk organic shell matrix with time. Likewise, the composition and δ13C-values of lipids from empty shells indicated that a large part of these compounds derived from prokaryotic decomposers. The use of lipids from ancient shells for the reconstruction of the bivalve's life style therefore appears to be restricted.

The giants of the phylum Brachiopoda: a matter of diet?

“…In a recent study, Dreier et al . () suggested that bulk organic shell matrices in Recent phototrophic and thiotrophic bivalves have lower δ 13 C org values (ranging from −31.0‰ to −25.5‰) than those of filter‐feeding non‐symbiotic bivalves where the latter have δ 13 C org values from −22.0‰ to −17.8‰. Comparable δ 15 N org values are −2.2‰ to +0.1‰ for thiotrophic bivalves, +2.6‰ to +2.9‰ for phototrophic bivalves and +3.1‰ to +4.3‰ for filter‐feeders.…”

“…Correlation of the δ 13 C org and δ 15 N org values (Dreier et al . , fig. 1) suggests that the chemosymbiotic lifestyle is characterized by low δ 13 C org /low δ 15 N org , the non‐symbiotic lifestyle by high δ 13 C org /high δ 15 N org and the phototrophic lifestyle by low δ 13 C org /intermediate δ 15 N org .…”

“…; Dreier et al . , ), such as the carbon and nitrogen isotope composition (δ 13 C org , δ 15 N org ) of the primary organic fraction occluded in the shell, after the evaluation of its degree of preservation. To discriminate between a suspension‐feeding lifestyle vs a symbiotic lifestyle, we compared different species of Gigantoproductus occurring together in different palaeoenvironmental settings (Nolan et al .…”

Characterizing Photosymbiosis Between Fraginae Bivalves and Symbiodinium Using Phylogenetics and Stable Isotopes

“…Currently known Fraginae-associated symbiont phylogenetic diversity based on our results and five other molecular studies (Carlos et al, 1999(Carlos et al, , 2000Baillie et al, 2000;LaJeunesse, 2001;Dreier et al, 2014) are summarized ( Table 3, Supplementary Table 2). All freshly isolated symbiont communities (i.e., non-cultured) were from clade C, except for one individual of Fragum unedo, which harbored clade A symbionts .…”

“…Their symbionts are mostly placed into Symbiodinium clade A, C, and occasionally D (Kirkendale and Paulay, 2017; see comprehensive literature list in Supplementary Table 2) and a single host individual can sometimes possess multiple symbiont lineages (DeBoer et al, 2012). Within Fraginae, the symbiont community from four species have been examined to date using molecular markers: Fragum fragum (Linnaeus, 1758), Fragum unedo (Linnaeus, 1758), Corculum cardissa, and Corculum monstrosum (Gmelin, 1791) (Carlos et al, 1999(Carlos et al, , 2000Baillie et al, 2000;LaJeunesse, 2001;Dreier et al, 2014). Symbionts freshly isolated from the hosts mostly belong to Symbiodinium clade C, while symbionts cultured from host tissues are from clade A (Carlos et al, 1999).…”