The origins of oyster color: a PhD student's emotional journey

The festive season is (almost) upon us, a special occasion to celebrate and share the oyster on the festive menu. Adorned with its coat and colorful shell, the oyster leaves no one indifferent and still hides many secrets. This mollusc even manages to invite itself onto researchers' benches.

Michel Bonnard, University of MontpellierBruno Boury, University of Montpellier and Isabelle Parrot, University of Montpellier

This article is published as part of the Fête de la science, which takes place from October¹ to 11, 2021 in mainland France and from November 5 to 15, 2021 overseas and internationally, and of which The Conversation France is a partner. The theme of this year's event is "Eureka! The thrill of discovery". Find all the events in your region on the Fetedelascience.fr website.

This is particularly the case at the University of Montpellier, where a team of researchers in the chemistry of natural substances, of which I am a member, recently investigated the molecular origin of the pink coloration of the shell of the edible oyster (Pacific oyster or Crassostrea gigas).

So how did the oyster arouse the interest of these researchers to the point of leading them to initiate and develop a PhD subject leading to an unexpected scientific discovery?

Originally an intriguing purple-pink color

It all started with an observation. Pink to purple patterns cover part or all of the outer surface of oyster shell specimens. The scar of the adductor muscle, found inside the shell, is itself sometimes colored pink, purple or black.

Faced with this intriguing crimson-pink color, which is not widespread among bivalve mollusc shells, a number of hypotheses can be envisaged: could this pink be common to other biological models, such as flamingos, whose color is acquired thanks to a diet based on pink algae rich in carotenoids, the same family of pigments found in carrots and shrimp?

Or is this color due to a particular nanoscale structuring of the shell, as in the case of the iridescence of mother-of-pearl? Could it be a combination of several pigments associated with a particular structure? And what applications could such a colorful material have?

While the scientific community has been interested in the description and composition of mollusc shells colored gold, green or blue, pink has not been the subject of any published studies in oysters. A PhD student - myself - has been recruited to further investigate the subject and provide answers to this set of questions.

Colouring and structuring, first experiments

Once the excitement of discovering the project and meeting the team had passed, it was time for me to study the state of scientific knowledge.

If this bibliographical study doesn't turn out to be a real trail to the stars, the structuring and implementation of the scientific approach, the first experiments and the discovery of techniques and know-how quickly give way to the first results, which enable us to confront the initial hypotheses with experimental reality.

Elemental and structural analyses of solid samples fail to show a link between color and shell structure.

This color is therefore probably linked to the presence of a very small quantity of one or more pigments, the precise identification of which requires selective extraction from the shell.

Pigments and extraction, a time for questioning

The first challenge is to extract these pigments from the 98% calcium carbonate content of the shell. Methods of dissolution, extraction, separation and analysis were specifically developed. In science, nothing happens quite the way you'd expect - no weariness, no humming.

The first difficulties arise, and the choices made don't always pay off in the face of negative results. The methods tested are not all reproducible, and the project stagnates, requiring perseverance and rigor to establish a reliable methodology. It's a time for questioning oneself in the face of failures and the fear of not being up to scratch, of not fitting in.

We look for other experimental solutions, take a step back and discuss the project with other researchers, until we are surprised by an unexpected observation.

A faded sample, hope reborn

A sample left exposed to sunlight became discolored. Could this be the manifestation of a property specific to certain natural substances? Hope is reborn, but this empirical fact needs to be reproduced and mastered.

The back-and-forth between bibliographic studies and the bench is beginning to bear fruit, and is giving us a glimpse of an answer to the initial hypotheses.

Everything accelerated, tests multiplied and the results resonated: it was the euphoria of discovery. Illuminated under black light, this pink is due, in part, to the presence of a family of photoluminescent pigments: porphyrins.

These porphyrins are systematically present in the pink patterns on the outer surface of the shells and the adductor muscle scar, in contrast to the uncolored areas.

Pink shell samples are taken and dissolved in acidic aqueous media. After filtration, the solution obtained is pink-purple. Exposed to black light at around 400 nm, the solution emits a pinkish-red color characteristic of certain porphyrins, including turacin.

New horizons explored, hard work pays off

This discovery has led our research group to explore new horizons beyond those initially envisaged. Oyster organs are being investigated in order to link the presence of these compounds to a biological function.

The mantle, a contractile organ responsible for mineralizing the shell, is found to accumulate the same porphyrins as those identified in the shell.

After a few weeks, the endogenous origin of these compounds was established, marking a biological pathway dedicated to cellular respiration, similar to that leading to hemoglobin synthesis in humans.

The joy and excitement are fleeting, and you have to stay focused to quickly publish your results; a rival laboratory is on the trail left by the photoluminescence of these compounds.

From publication to defense, the accomplishment

Publication, the doctoral student's Holy Grail, which in reality is akin to the work of a scientific writer. How do you relate and discuss facts in a historical context and in a foreign language? How do you select the most convincing evidence and combine it? Which journal to publish in? These are just some of the questions that doctoral students are not really prepared to answer.

After a dozen drafts and peer review, the article was finally published: "Chemical evidence of rare porphyrins in purple shells of Crassostrea gigas oyster".

Finally comes the home stretch and those sleepless nights dedicated to writing the dissertation. Isolation, irritability, obsession - my emotions weren't exactly positive during this period. At the end, the apathy is total , but the manuscript arrives safely and can be assessed by the defense jury, made up of my peers.

A final remobilization is necessary in the run-up to the presentation: you have to defend your work in front of the jury. After many rehearsals, it's D-day. The oral presentation is fluid, the exchanges constructive and the work recognized. It's time to savor the moment and hand over the baton.

Looking back, two emotions predominate in my mind: a sense of accomplishment for having contributed to the development of fundamental knowledge within the scientific community, and the feeling of having progressed to a certain level of expertise.

Michel Bonnard, Post-doctoral fellow in "description, identification and exploitation of natural substances", University of MontpellierBruno Boury, Researcher, University of Montpellier and Isabelle Parrot, Teacher-Researcher, University of Montpellier

This article is republished from The Conversation under a Creative Commons license. Read theoriginal article.