The high energy of a lightning strike creates an unusual form of phosphorus once common on primordial Earth and still used by many microbes today.
Phosphorus forms the DNA molecule's spine, enrobes every living cell as a constituent of their membranes, and is a key component of bones and teeth. Author Isaac Asimov once called phosphorus "life's bottleneck," because it makes up 1 percent of an organism but is only present in 0.1 percent of minerals on Earth.
Today, most phosphorus enters the world's ecosystems as granite and other rocks weather over time, releasing the molecule orthophosphate: a phosphorus atom linked to a hydrogen and four oxygens. But with little oxygen in the early atmosphere, ancient microbes evolved a very different chemical pathway to break down orthophosphate into phosphite, a reduced form of phosphorus that has just three oxygens. Because little phosphite exists in nature today, scientists have long wondered why many microbes still make the enzymes needed to digest it.
Geologist Matthew Pasek of the University of Arizona in Tucson, who previously studied the possibility that early life used phosphorus from iron meteorites, knew that cloud-to-ground lightning was one of the few natural phenomena that could produce enough energy to create phosphite. When a bolt strikes the soil it burns a molten, hollow tube called a fulgurite that Pasek says looks like a "mini volcano with glass splattered around it."
These fulgurites, which range from a few grams up to tens of kilograms, are not easy to find, and meteorite dealers often pay nomads in the Sahara Desert to collect them during their travels. For a couple hundred dollars each, Pasek purchased fulgurites from around the world on eBay, at the Tucson Gem Show, and through other sources
Then, he crushed the rocks up to analyze what type of phosphorus molecules they contained: Fulgurites found in organic soils contained about 22 percent of their phosphorus as phosphite, whereas other fulgurites taken from soil lacking organic material had 37 to 68 percent of their phosphorus in another oxygen-poor form linked to iron. The results were published this week in Nature Geoscience. (Scientific American is part of the Nature Publishing Group.)
"When lightning strikes, it acts like a mini smelter, and the organic molecules strip off oxygen from the phosphorus," Pasek says.
But just how significant is lightning to the global phosphorus cycle? Lightning strikes Earth 44 times every second, which means it could produce just 2,000 to 3,000 kilograms of phosphite yearly. Today, humans introduce thousands of times that amount of phosphite through the corrosion of steel and rusting of old cars.
For that reason, John Quinn, a microbiologist at Queen's University Belfast in Northern Ireland, says that Pasek's findings are "more interesting than important." Even so, he is impressed by the study and hopes it spurs geologists to look for more significant sources of phosphite in the environment.
So far, Pasek is the only taker. "There have been a lot of biologists saying there has to be a geological source of reduced phosphorus, and geologists say phosphorus is kind of boring," he says. If fulgurites aren't the only source of reduced phosphorus, they are at least the most striking one.