The chilling science of reviving undead spores

Here’s a haunt Riddle: 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 see: no heartbeat, no gene expression. There’s nothing going on there,” he says.

But a spore might actually just be dormant — in a deep state of suspended animation designed to survive inhospitable conditions that can last millions of years, until the day the spore “wakes up,” zombie-like, ready to to grow. For years, the question has been how spores know when to revive and how they actually do it. A new paper in Science von Süel’s group helped fill in those gaps – and the answer could have implications for everything from the search for life on other planets to methods of combating dangerous spores, such as those that cause foodborne diseases.

Spores are typically single cells with densely packed innards that can give rise to new organisms. While many plants produce them to disperse their seeds, bacteria can also form spores during times of extreme temperatures, drought, or nutrient deficiencies. The spore cell then hibernates through hard times.

Süel’s group was intrigued by the concept of a “mostly dead” cell that would revive when the environment became more favorable for survival. “It was clear how spores come back to life when you pour some good stuff on them,” like large amounts of nutrients, Süel says. Likewise, spores simply will not germinate if the environment is extremely hostile (e.g. if water is not available). But most environments, the team recognized, 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 slumbering spore be able to sense and process such a subtle hint?

It is important for the spore to accurately grasp its environment, as it would be a waste to expend the energy required to wake up and germinate in an unfriendly environment. This could hinder successful growth or even lead to death. “You have to be brought back to life with good timing, otherwise you’re throwing away your beautiful dormant state,” says Kaito Kikuchi, a former student in Süel’s lab and co-author of the study. “You want to make sure you throw away your protective gear if and only if the environment is good enough.”

First, the scientists had to figure out what biological processes the spores might be using while they were still hibernating. These processes could not use ATP (adenosine triphosphate or cellular energy) or rely on cellular metabolism (e.g. breaking down sugars) since these mechanisms are switched off during rest.

But, the researchers hypothesized, there might be an alternative method: The spores might be able to sense small cumulative changes in their environment until enough signals build up to set off some kind of wake-up alarm. The mechanism that would bring about these changes would be the movement of ions out of the cell – specifically potassium ions.

These movements can be triggered by positive environmental cues, such as the presence of nutrients. As the ions migrate out of the cell, thanks to passive transport, they create 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 senses more and more positive signals, more ions would move out of the cell. This would also produce a corresponding drop in potassium levels as the ions exit. Eventually, the potassium levels in the spore would drop to a certain threshold, signaling that it is safe for the cell to wake up. That would trigger resuscitation and germination.

In other words, Süel says, the spore essentially behaves in a similar way to a capacitor, or a device that contains 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 with this, because the cell membrane is very thin.”