The origins of oyster color: a doctoral student's emotional journey
The end-of-year celebrations are (almost) upon us, a special occasion to celebrate and share oysters as part of the festivities. With its colorful shell and mantle, the oyster leaves no one indifferent and still hides many secrets. This mollusk has even found its way into researchers' laboratories.
Michel Bonnard, University of Montpellier; Bruno Boury, University of Montpellier and Isabelle Parrot, University of Montpellier
This article is published as part of the Fête de la science (Science Festival), which takes place from October1 to 11, 2021 in mainland France and from November 5 to 15, 2021 in overseas territories and internationally, and of which The Conversation France is a partner. The theme of this year's festival 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 recently, a team of researchers in natural substance chemistry, of which I am a member, has been studying the molecular origin of the pink coloration of the shell of the edible oyster (Pacific oyster or Crassostrea gigas).
So how did the oyster spark the interest of these researchers to such an extent that they initiated and developed a doctoral thesis leading to an unexpected scientific discovery?
Originally, an intriguing pinkish-purple color
It all started with an observation. Pink to purple patterns partially or completely cover the outer surface of oyster shell specimens. The adductor muscle scar, found inside the shell, is itself sometimes colored pink, purple, or black.
Faced with this intriguing purple-pink color, which is uncommon among bivalve mollusk shells, a number of hypotheses can be considered: could this pink color be common to other biological models, such as flamingos, which acquire their color from a diet based on pink algae rich in carotenoids, the same family of pigments found in carrots and shrimp?
Or could this color be due to a particular nanoscale structure of the shell, as is the case with the iridescence of mother-of-pearl? Could it be a combination of several pigments associated with a particular structure? And what could be the applications of a material with such a color?
While the scientific community has taken an interest in describing and analyzing the composition of mollusk shells colored gold, green, or blue, no published studies have examined pink shells in oysters. To explore this topic further and find answers to these questions, a doctoral student was recruited—myself.
Coloring and structuring, initial experiments
Now that the excitement of discovering the project and meeting the supervisory team has passed, it is time for me to study the state of scientific knowledge.
Although this bibliographic study did not prove to be a real path to stardom, the structuring and implementation of the scientific approach, the initial experiments, and the discovery of techniques and know-how quickly gave way to the first results, which made it possible to compare the initial hypotheses with the experimental reality.
Elemental and structural analyses of solid samples do not reveal any link between the coloration and structure of the shell.
This color is therefore likely to be linked to the presence of a very small amount of one or more pigments, the precise identification of which requires selective extraction from the shell.
Pigments and extraction: a time for rethinking
The first challenge is to successfully extract these pigments from the 98% calcium carbonate that makes up the shell. Methods for dissolution, extraction, separation, and analysis are specifically developed. In science, nothing ever happens exactly as imagined; there is no weariness, no humdrum routine.
The first difficulties arise, and choices do not always pay off in the face of negative results. Not all of the methods tested are reproducible, and the project stagnates. Perseverance and rigor are required to establish a reliable methodology. It is time to question oneself in the face of failure and the fear of not being up to the task, of not belonging.
We look for other experimental solutions, take a step back, discuss the project with other researchers until we are surprised by an unexpected observation.
A discolored sample, hope is reborn
A sample left exposed to sunlight has discolored. Could this be evidence of a property specific to certain natural substances? Hope is renewed, but this empirical fact must be reproduced and verified.
The back-and-forth between bibliographic studies and lab work is beginning to bear fruit and is allowing us to glimpse the outlines of an answer to the initial hypotheses.
Everything is accelerating, tests are multiplying, and the results are resonating: it's the euphoria of discovery. Illuminated under black light, this pink color is due, in part, to the presence of a family of photoluminescent pigments: porphyrins.
These porphyrins are consistently present in the pink patterns on the outer surface of the shells and the adductor muscle scar, unlike the uncolored areas.
Pink shell samples are collected and dissolved in acidic aqueous media. After filtration, the resulting solution is purple-pink. When exposed to black light at approximately 400 nm, the solution emits a red-pink color characteristic of certain porphyrins, including turacine.
New horizons explored, hard work pays off
This discovery has led our group of researchers to explore other avenues than those initially envisaged. The oyster's organs are being investigated in order to link the presence of these compounds to a biological function.
The mantle, a contractile organ responsible for shell mineralization, accumulates the same porphyrins as those identified in the shell.
After a few weeks, the endogenous origin of these compounds is established, markers of a biological pathway dedicated to cellular respiration, similar to that leading to hemoglobin synthesis in humans.
The joy and excitement are fleeting; you have to stay focused to quickly publish your results. A competing laboratory is hot on the trail of the photoluminescence of these compounds.
From publication to defense, the accomplishment
Publication, the Holy Grail for doctoral students, is in reality akin to the work of a scientific writer. How can facts be recounted and discussed in a historical context and in a foreign language? How can the most compelling evidence be selected and combined? Which journal should be used for publication? These are just a few examples of questions for which doctoral students are not really prepared.
After a dozen drafts and peer review, the article is finally published: "Chemical evidence of rare porphyrins in purple shells of Crassostrea gigas oysters."
Finally, the home stretch arrives, along with sleepless nights spent writing my thesis. Isolation, irritability, obsession—my emotions are not exactly positive during this period. In the end, I feel completely apathetic, but the manuscript arrives safely and can be evaluated by the defense committee, made up of my peers.
One last push is needed in preparation for the defense: you have to defend your work in front of the jury. After numerous rehearsals, the big day arrives. The oral presentation goes smoothly, the discussions are constructive, and your work is recognized. It's time to savor the moment and pass the baton.
Looking back, two emotions stand out for me: a sense of accomplishment for having contributed to advancing fundamental knowledge within the scientific community, and the feeling of having progressed to a certain level of expertise.
Michel Bonnard, Postdoctoral researcher, “Description, identification, and exploitation of natural substances,” University of Montpellier; Bruno Boury, Researcher, University of Montpellier and Isabelle Parrot, Lecturer – Researcher, University of Montpellier
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