"Most of the air is nitrogen, so why isn't it used?"
Kana asked while looking outside.
Rei answered. "Nitrogen doesn't want to move."
"Doesn't want to move?"
"The triple bond is too strong."
Milia drew a structural formula. "N≡N. Two nitrogen atoms connected by three bonds."
"What's the problem with being strong?" Kana asked.
"It doesn't break. Doesn't participate in chemical reactions," Rei explained.
"So it's inert?"
"Yes. 78 percent of air is nitrogen, but most is chemically asleep."
Kana wrote in her notebook. "But life needs nitrogen, right?"
"Amino acids, nucleobases, all contain nitrogen," Milia answered.
"Then how do we use it?"
"Nitrogen fixation. Breaking N≡N and converting to reactive forms."
Rei continued. "In nature, lightning, or nitrogen-fixing bacteria."
"Lightning?" Toma was surprised.
"High energy breaks the triple bond. N₂ + O₂ → 2NO"
Milia supplemented. "But inefficient. Bacteria are the main players."
"What kind of bacteria?"
"Rhizobia, cyanobacteria. They have an enzyme called nitrogenase."
Kana showed interest. "How does that enzyme break the triple bond?"
"Complex metal cluster. Iron and molybdenum assembly," Rei explained.
"It requires tremendous energy, right?"
"Exactly. Consumes 16 ATP molecules to finally fix one N₂ molecule."
Milia showed numbers. "N₂ + 8H⁺ + 8e⁻ + 16ATP → 2NH₃ + H₂ + 16ADP"
"Becomes ammonia?" Kana confirmed.
"Yes. Ammonia is highly reactive. From here, amino acids are made."
Rei drew a diagram. "NH₃ → glutamine → other amino acids"
"The starting point of all proteins."
Kana thought. "What about organisms that can't fix nitrogen?"
"Most plants can't do it themselves. So they form symbioses," Milia answered.
"Symbiosis?"
"Leguminous plants and rhizobia. Bacteria fix nitrogen and supply it to plants."
Rei continued. "In return, plants provide sugars. Mutualistic symbiosis."
Kana murmured. "Cooperation around nitrogen."
"Humans also depend on nitrogen fixation," Milia said.
"How?"
"Haber-Bosch process. Industrial nitrogen fixation."
Rei explained. "N₂ + 3H₂ → 2NH₃. Forced reaction under high temperature and pressure."
"This becomes fertilizer?"
"Yes. Supporting world food production."
Kana was surprised. "Without nitrogen fixation?"
"Half the population couldn't be fed," Milia said quietly.
"That important..."
Rei continued. "But environmental burden is also large. Excess nitrogen pollutes water."
"Balance is difficult?"
"Yes. Nitrogen is essential, but too much is also a problem."
Kana looked outside. "Nitrogen in the air waits quietly."
"Protected by triple bonds, motionless."
Milia added. "But when awakened by special power, it becomes life's material."
Rei nodded. "The reason nitrogen doesn't move is because it's stable. When it moves, it has great meaning."
Kana stared at the test tube. "Quiet nitrogen creates life."
"That's nitrogen's mystery," Rei concluded.
The three continued their experiment, feeling nitrogen's invisible quietness.