Here’s a spooky conundrum: is a spore alive or dead?
Gürol Süel, a biologist at the University of California, San Diego, wouldn’t blame you if you voted for death: “There’s nothing to detect: no heartbeat, no gene expression. Nothing happens,” he says.
But in reality, a spore might lie dormant, in a deep state of suspended animation destined to survive inhospitable conditions that may persist for millions of years, until the day the spore “awakens,” like a zombie, ready to grow For years, questions about how spores know when to revive, and how they actually do it, have been open. A new paper in Science from Süel’s group has helped fill in those blanks, and the answer could have ramifications for everything from the search for life on other planets to methods of fighting dangerous spores like those that cause foodborne illness.
Spores are usually single cells with tightly packed insides that can create new organisms. Although many plants produce them to spread their seeds, the bacteria can also form spores during periods of extreme temperatures, dryness or nutrient deficiency. The spore cell then essentially hibernates during the rough times.
Süel’s group was intrigued by the concept of a “mostly dead” cell that revives when the surrounding environment becomes more conducive to survival. “It was clear how the spores come back to life if you throw in a bunch of good stuff,” such as large amounts of nutrients, Süel says. Similarly, when the environment is extremely hostile (for example, if no water is available), the spores simply will not germinate. But most environments, the team realized, aren’t so black and white. For example, “good” signals, such as the presence of the nutrient L-alanine, may appear intermittently and then disappear. Would a dormant spore be able to detect and process such a subtle clue?
Getting an accurate read of its environment is important for the spore, because it would be a waste to expend the energy needed to wake up and germinate in a hostile environment. This could prevent successful growth or even lead to death. “You have to come back to life with good timing, because otherwise your good latency will be thrown away,” says Kaito Kikuchi, a former student in Süel’s lab and co-author of the study. “You want to make sure you throw your protections when, and only when, the environment is good enough.”
First, the scientists had to identify what biological processes the spores could use while they were still hibernating. These processes could not use ATP (adenosine triphosphate or cellular energy) or depend on cellular metabolism (eg, breaking down sugars), as these mechanisms shut down during dormancy.
But, the researchers hypothesized, there was an alternative method: The spores might be able to detect small cumulative changes in their environment, until enough signals accumulate to set off a kind of wake-up alarm. The mechanism that would induce these changes would be the movement of ions outside the cell, specifically, potassium ions.
These movements can be triggered by positive environmental signals, such as the presence of nutrients. When ions travel out of the cell thanks to passive transport, they generate a difference in potassium concentration inside and outside the cell. This difference in concentration allows the spore to store potential energy. Over time, as the spore continues to sense more positive signals, more ions would leave the cell. This would also create a corresponding drop in potassium levels, as the ions leave. Eventually, the potassium content of the spore would drop to a certain threshold, indicating that it is safe for the cell to wake up. This would cause reanimation and germination.
In other words, Süel says, the spore acts essentially like a capacitor or a device that holds electrical energy. “A capacitor is basically an insulator that separates the concentration gradient of charges,” he says. “You can really store a lot of energy that way, because the cell membrane is so thin.”