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 dead: “There’s nothing to detect: no heartbeat, no gene expression. There’s nothing going on,” he says.
But a spore might actually just be dormant—in a deep state of suspended animation meant to outlast inhospitable conditions that can persist for millions of years, until the day the spore “wakes up,” zombie-like, ready to grow. For years, the questions of how spores know when to reanimate, and how they actually do it, have been open ones. A new paper in Science by 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, such as those that cause foodborne illness.
Spores are typically single cells with tightly packed innards that can create new organisms. While many plants produce them to spread their seeds, bacteria can also form spores during periods of extreme temperatures, dryness, or nutrient deficiency. The spore cell then essentially hibernates its way through tough times.
Süel’s group was intrigued by the concept of a “mostly dead” cell reviving when the surrounding environment becomes more conducive to survival. “It was clear how spores come back to life if you dump a bunch of good stuff on them,” like large quantities of nutrients, says Süel. Likewise, when the environment is extremely hostile (for example, if no water is available), spores will simply not germinate. But most environments, the team realized, are not so black and white. For instance, “ good” signals, like the presence of the nutrient L-alanine, might appear intermittently, then vanish. Would a slumbering spore be able to sense and process such a subtle hint?
Getting an accurate read on its surroundings is important for the spore, because it would be a waste to expend the energy needed to wake up and germinate in an unfriendly environment. That could stymie successful growth, or even lead to death. “You need to come back to life with nice timing, because otherwise you throw away your nice dormancy,” says Kaito Kikuchi, a previous student in Süel’s laboratory and a study coauthor. “You want to make sure you’re throwing away your protections when, and only when, the environment is good enough.”
First, the scientists needed to identify which biological processes the spores could use while they were still hibernating. These processes could not use ATP (adenosine triphosphate, or cellular energy) or rely on cellular metabolism (for example, breaking down sugars), since those mechanisms are shut down during dormancy.
But, the researchers hypothesized, there was an alternative method: The spores might be able to sense small cumulative changes in their environment, until enough signals build up to trigger a sort of wake-up alarm. The mechanism that would induce these changes would be the movement of ions out of the cell—specifically, potassium ions.
These movements can be triggered by positive environmental signals, like the presence of nutrients. When the ions travel out of the cell thanks to passive transport, they generate a difference in potassium concentration inside versus outside the cell. This concentration difference allows the spore to store potential energy. Over time, as the spore continues to sense more positive signals, more ions would move out of the cell. This would also create a corresponding drop in potassium levels, as the ions exit. Eventually, the potassium content in the spore would lower to a certain threshold, signaling that it is safe for the cell to wake up. That would trigger reanimation and germination.
In other words, says Süel, the spore essentially acts similar to a capacitor, or a device that holds electrical energy. “A capacitor is basically an insulator separating the concentration gradient of charges,” he says. “You can really store a lot of energy in this way, because the cell’s membrane is very thin.”