Chemical evidence of rare porphyrins in purple shells of Crassostrea gigas oyster

Abstract

The colour of oyster shells is a very diverse characteristic morphotype, forming intriguing vivid patterns both on the inside and outside of the shell. In the present study, we have identified for the first time, the presence of several porphyrins as constituents of the shell pigmentation of the Crassostrea gigas oyster consumed worldwide. The precise molecular structures of halochromic, fluorescent and acid-soluble porphyrins, such as uroporphyrin and turacin, are unambiguously determined by reverse phase liquid chromatography combined with high resolution mass spectrometry. their presence account for the purple colouration of shells but also for the dark colouration of adductor muscle scars. We have also defined the endogenous origin of these porphyrins, specifically secreted or accumulated by the shell forming tissue. These findings are pioneering analytical proofs of the existence of the haem pathway in the edible oyster Crassostrea gigas, evidenced by the chemical identification of haem side-products and supported by the recent publication of the corresponding oyster genome. Shells are the armour of many molluscs. Their diversities of shape, ornamentation and colour have always attracted humans throughout the ages. In bivalves, related to the requirements of the pearl farming industry, shell morphotypes and colours of pearl oysters (e.g. Pinctada margaritifera) have been extensively studied compared to their edible cousins (e.g. Crassostrea gigas) 1. Prized by gourmets, the aesthetic appeal of shellfish as a gastronomic showcase is arousing growing interest. However, the accumulation of wasted shells becomes a real environmental problem, predominantly in oyster farming or tourist areas 1. At a time when sustainable and intelligent uses of renewable aquatic resources are needed, the transformation of this calcareous by-product into a value-added material is a relevant challenge to be taken up. While the use or transformation of calcium carbonate or its organic matrix is the first-line source for recycling edible oyster shell, its colouration is still unwell understood and therefore little valued 1. Despite the functional roles of shell colours in camouflage, thermoregulation or immunity, the molecular mechanisms related to shell colouration remain poorly documented 2. This gap of knowledge is partly due to the lack of elucidation of the chemical nature of shell pigments, particularly in the Crassostrea genus. By combining the identification of shell pigments with corresponding biosynthetic pathways and associated genes, key information on the relation between shell colouration, mineralization or other molecular mechanisms can be obtained as previously reported in marine snails 3. Therefore, the identification of shell pigments appears to be a possible gateway to the recovery of natural dyes from wasted oyster shells for a broad range of applications in health or material science, helping to contribute to a sustainable future for shellfish farming industry 1. In the Pteriomorphia group, including edible and pe...

First Evidence of Ommochromes in Bivalves: the Case of the Edible Oyster Crassostrea Gigas

“…Among the six major signals observed on the chromatogram at 405 nm, the UV-vis absorption pro le of the compound eluted at 30.08 min is characteristic of uroporphyrin I or III on the basis of its Soret and Q bands (Fig. 1) 7 . The major compounds eluted from 12 to 23 min have a broad absorption band from approximately 430 to 600 nm, with λ max ranging from 530 to 486 nm (Fig.…”

“…For instance, the black colour of the adductor muscle scar of shells of the edible oyster Crassostrea gigas, initially hypothesized as a contribution of melanins by S. Hao et al 6 , was subsequently ruled out without resolving the nature of this colour. We also recently identi ed uroporphyrin and derivatives in the mantle of C. gigas and the purple and dark patterns of its shell 7 . These porphyrins, considered as side-products of the highly conserved haem biosynthetic pathway, constitute an evidence of the haem-based cellular respiration of C. gigas 8 .…”

RNA Interference by Ingested Dsrna-Expressing Bacteria to Study Porphyrin Pigmentation in Crassostrea gigas

“…The bacteria-feeding RNAi method used in this study revealed the function of genes related to uroporphyrin pigmentation in C. gigas . The effect of porphyrin pigments on shell color has been reported in mollusks, and uroporphyrin I and III in oyster shells have also been identified [ 3 , 17 , 18 ]. Genes associated with the uroporphyrinogen synthesis have been characterized in marine snails and pacific oysters [ 9 ].…”

“…Porphyrins are a widespread group of pigments in nature which are believed to contribute to shell colors in mollusks [ 14 ], often leading to red, brown or purple shell pigmentation [ 3 ]. Among porphyrins, the uroporphyrin I and III are found in mollusks [ 15 , 16 , 17 , 18 ]. It has been suggested that porphyrin in shell pigmentation is produced de novo by the animal through the haem pathway, the production of uroporphyrin is derived from the oxidation of uroporphyrinogen I and III which belong to non-enzymatic side paths [ 17 , 19 ].…”

“…Although the complete C. gigas genome sequence is available (PRJEB35351), due to the lack of molecular techniques to manipulate gene expression, the molecular mechanism of oyster porphyrin pigmentation remains unclear. Studies in purple-shell oysters revealed the presence of uroporphyrin in the shell and mantle of C. gigas [ 18 ]. Through selective breeding, mutant oysters possessing pure orange shells without any other pigmentation was obtained [ 35 ].…”