Written by Federico Citterich
Conceived and reviewed by Alessandro Rossetta
From glowing jellyfish to fluorescent minerals, light emission after energy absorption — known as fluorescence — reveals hidden properties of both the biological and physical world. In this first article of our four-part series, we explore what fluorescence is, how it works, and where it naturally occurs.
Why do some creatures glow under ultraviolet (UV) light? The answer lies in a dazzling dance between energy and molecules — and it’s called fluorescence. But what exactly is fluorescence? Well, it all starts with energy.
When matter absorbs energy — typically from light — the molecules within it are temporarily pushed into what’s called an excited state. Think of it like giving the molecule a jolt that lifts some of its electrons- in the atoms that make it up – out of their resting energy level. But nature doesn’t let them stay excited for long. These molecules want to return to their stable, ground state, and the journey back can take different paths.
As one of these paths, molecules can release energy as heat, through vibrations or collisions with surrounding molecules. Or again, excited molecules can transfer their energy directly to a nearby molecule instead of emitting it out again. In some cases, the absorbed energy can even trigger a chemical reaction, permanently altering the molecule.
Sometimes, as the molecule relaxes, it releases some of the absorbed energy in the form of visible light by emitting photons. The result? A glow that’s often colorful, brief, and beautiful. This is called fluorescence.
Schematic of the process of fluorescence
This is why “there are no free meals in nature”. Even when we get beautiful fluorescence, we always pay a little energy tax – it’s never 100% efficient.
This is also described by a key feature of fluorescence known as the Stokes shift: the emitted light always has a longer wavelength (and so lower energy) than the absorbed light. This is because energy and wavelength are inversely related. This means that shorter wavelengths correspond to higher energy, while longer wavelengths carry less — and that’s exactly why fluorescence always comes with an energy cost: the light emitted is lower in energy, with a longer wavelength than the light originally absorbed. For instance, a molecule that absorbs high-energy UV light (short wavelength) often emits lower-energy visible light (longer wavelength) when it fluoresces, such as light in the blue or green range.
FLUORESCENCE IN LIVING ORGANISMS: JELLYFISH AND THE DISCOVERY OF GFP
You’ve probably seen this in documentaries: a jellyfish under ultraviolet light, suddenly glowing a ghostly blue-green.That’s one of the clearest examples of fluorescence in nature – and one that has revolutionized medicine and even earned a Nobel Prize!
It all started when in 1960 Osamu Shimomura was studying the crystal jelly Aequorea victoria – a jellyfish found in the Pacific Ocean – trying to answer one question: what made it glow green when agitated?
His attempts to isolate a glowing enzyme weren’t going well. Despite multiple adjustments in the lab, the samples produced only a faint light. Frustrated, he poured the solution into the sink to clean up — and that’s when a sudden, bright blue flash lit up the drain.
Aequorea victoria
Shimomura soon realized it wasn’t the sink causing the flash – it was the seawater. More precisely, the calcium in the seawater was reacting with the jellyfish extract to produce the blue light. But that discovery raised a new question: crystal jellies glow green, not blue.
That’s when Shimomura hypothesized that the jellyfish contained another molecule – one that absorbed the blue light and re-emitted it as green. The glowing compound turned out to be a unique protein, which he named Green Fluorescent Protein, or GFP.
Osamu Shimomura
Today, GFP is used all over the world in biology and medicine as a fluorescent marker – helping scientists track gene expression, visualize cells in real time, and even study how diseases progress inside living organisms.
In 2008, Shimomura’s groundbreaking discovery earned him, along with Martin Chalfie and Roger Tsien, the Nobel Prize in Chemistry, marking GFP as one of the most transformative tools in modern science.
Today we don’t really know why jellyfish fluoresce. Potential hypotheses and evolutionary reasons include camouflage, counter-illumination, communication, and luring prey. However, these are only possibilities, and the exact evolutionary function remains uncertain.
FLUORESCENCE IN LIVING ORGANISMS: PLANTS
Also some plants exhibit natural fluorescence, although it’s often invisible to the naked eye and only detectable under specific circumstances or with special imaging equipment.
This fluorescence typically comes from specific pigments and metabolites within plant tissues, including:
- Chlorophyll: fluoresces red under UV or blue light;
- Flavonoids and phenolic compounds: fluoresce blue or green;
- Ligninin cell walls: weak fluorescence in some cases.
Fluorescence in plants is mostly a byproduct of energy processing, especially in photosynthesis, and can be used to monitor plant health. In fact, stressed or diseased plants often show different fluorescent patterns.
Researchers also use chlorophyll fluorescence imaging to assess photosynthetic efficiency, water stress, and other physiological states – valuable in agriculture, climate studies, and plant biology.
FLUORESCENCE OUTSIDE THE LIVING WORLD
Fluorescence can be found even outside the living world. In abiotic nature, one of the most fascinating examples of fluorescence is in minerals.
Some minerals naturally fluoresce under UV light, emitting bright colors like green, blue, red, or orange. This is due to trace impurities or defects in the crystal structure – often involving elements like manganese, uranium, chromium, or rare earth metals (e.g. europium, terbium). Some common fluorescent minerals include fluorite, calcite, willemite, and scheelite.
A fluorescent mineral
Fluorescence in minerals is obviously incidental – it’s a physical property related to how the crystal absorbs and releases energy. In geology and gemology, it’s used to identify minerals, detect synthetics, and explore ore deposits.
Fluorescence is not only a fascinating natural phenomenon, but also a valuable tool in biochemical and biomedical research. Its ability to reveal molecular interactions and track biological processes has made it indispensable in fields ranging from cell biology to diagnostics. In the next articles in this series, we will explore how fluorescence is detected, how it’s applied in biomedical research, and how it contributes to the development of innovative diagnostic strategies.
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