The Cells That Breathe Two Ways

Summary

Take a deep breath. A flow of air has rushed into your lungs, where the oxygen moves into your bloodstream, fueling metabolic fires in cells throughout your body. You, being an aerobic organism, use oxygen as the cellular spark that frees molecular energy from the food you eat. But not all organisms on the planet live or breathe this way. Instead of using oxygen to harvest energy, many single-celled life-forms that live in environments far from oxygen’s reach, such as deep-sea hydrothermal vents or stygian crevices in the soil, wield other elements to respire and unlock energy. This physical separation of the oxygen-rich and oxygen-free worlds is not merely a matter of life utilizing available resources; it’s a biochemical necessity. Oxygen doesn’t play nice with the metabolic pathways that make it possible to respire with the use of other elements, such as sulfur or manganese. It gives aerobes like us life, but for many anaerobes, or creatures that respire without oxygen, oxygen is a toxin that reacts with and damages their specialized molecular machinery. “Oxygen — we love it, of course,” said Courtney Stairs, an evolutionary biologist at Lund University in Sweden. “But it’s actually a pretty harmful molecule for most of life on our planet, and even ourselves. We have ways to mitigate the negative effects of oxygen. So we can’t imagine life without it, but life is actually quite hard with it.” For the first couple billion years of life on Earth, organisms avoided this predicament altogether. Back then, the air and oceans were mostly devoid of oxygen, so life was almost entirely anaerobic, or non-oxygen-breathing. Then, around 2.7 billion years ago, the seas filled with industrious, photosynthetic cyanobacteria. They had invented a way to turn sunlight into sugar and oxygen, and they flourished. Over hundreds of millions of years, their accumulated breathing filled the atmosphere and oceans with oxygen. This so-called Great Oxidation Event was a pivotal transformat...