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Children chasing fireflies on a serene summer’s eve might not care how the brown bugs light up the sky. But researchers have studied for years how they create these pulses. Now, using a massive particle accelerator to peer deeply into the tiniest firefly tracheoles, scientists have confirmed just how these insects switch their lanterns on and off.
The firefly lantern is in the abdomen; there, the oxygen-carrying trachea branches into smaller tracheoles, which are surrounded by light-producing photocytes. Mitochondria pack the area of the photocyte closest to the tracheole, while peroxisomes (the organelles in which the light is actually generated) linger further inside the cell. The peroxisomes carry luciferin, the enzyme luciferase, and ATP. Add oxygen to this mix, and light is produced. But the light is not always on. Evidence suggests that under normal conditions, oxygen coming from the tracheoles is taken up by the mitochondria and cannot get to the peroxisomes. But when nitric oxide (NO) saturates the mitochondria near the tracheole, it slows their activity and allows the oxygen to pass through to the peroxisomes, generating light.
However, the evidence could also support another theory: fluid in the tracheole regulates oxygen availability to the photocyte.
To figure out which theory is right, team leader Yeukuang Hwu at the Academia Sinica in Taiwan and his group needed three numbers: the total oxygen diffusion capacity into the photocytes from the tracheoles, the amount of oxygen the mitochondria in these small regions use, and the amount of oxygen the lanterns use while flashing. They wanted to use Henry’s Law to figure out these values, but to do so, they needed to know the length and width of the tracheoles. Unfortunately, the tracheoles were so small that regular instruments couldn’t measure them.
The team turned to Taiwan’s National Synchrotron Radiation Research Center to image the tiny organs with synchrotron phase contrast microtomography and transmission X-ray microscopy. “The time [on the machine] is always limited, which requires a lot of preparation work... It is always stressful, but this time everything worked well,” said co-author Giorgio Margaritondo of the Ecole Polytechnique Federale de Lausanne. “We were particularly pleased that we could detect all the smallest tubes, as required.”
They found the tracheole lengths and diameters in two different firefly species, and then determined that mitochondria use so much oxygen that they have to be tamped down in order for oxygen to be funneled to the peroxisomes. The NO theory was correct.
The new imaging techniques may be applied to other projects, such as visualizing tiny vessels in tumors, perhaps detecting cancer at a very early stage.
It’s also just really cool to learn how fireflies brighten the night.
“As a byproduct of the work, we confirmed that the flashing mechanism is extremely effective,” said Margaritondo. “Imagine one tiny thing that must keep flying and at the same time keeps emitting powerful flashes of light that everyone can see!”
Tsai YL, Li CW, Hong TM, Ho JZ, Yang EC, Wu WY, Margaritondo G, Hsu ST, Ong EB, Hwu Y. Firefly Light Flashing: Oxygen Supply Mechanism. Phys Rev Lett. 2014 Dec 19;113(25):258103.
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