Here is a scary riddle: is a spore alive or dead?
Gürol Süel, a biologist at the University of California at San Diego, would not blame you if you voted for death: “There is nothing to detect: no heartbeat, no genetic expression. Nothing is happening,” he said.
But a spore may in fact simply be dormant – in a deep state of suspended animation meant to survive inhospitable conditions that may persist for millions of years, until one day the spore “wakes up”, similar to a zombie, ready to grow. For years, questions of how spores know when to reanimate, and how they actually do so, have been open. A new paper in Science by Süel’s group has helped fill those gaps – and the answer could have ramifications for everything from the search for life on other planets to methods of dealing with dangerous spores, such as those that cause ailments. food origin.
Spores are usually 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 times of extreme temperatures, drought, or nutrient deficiency. The spore cell then essentially hibernates during tough times.
Süel’s group was intrigued by the concept of a “mostly dead” cell that resurrects when the surrounding environment becomes more conducive to survival. “It was clear that the spores would come back to life if you dumped a bunch of good stuff on them,” like large amounts of nutrients, Süel says. Likewise, when the environment is extremely hostile (for example, if there is no water 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 sleeping spore be able to detect and process such a subtle clue?
Getting an accurate reading of its environment is important for the spore, as it would be a waste of energy to wake up and germinate in a hostile environment. This could hamper successful growth, or even lead to death. “You have to come back to life with good timing, otherwise you throw away your beautiful dormancy,” says Kaito Kikuchi, a former student in Süel’s lab and co-author of the study. “You want to make sure that you shed your protections when, and only when, the environment is good enough.”
First, the scientists had to identify the biological processes that the spores could use while still hibernating. These processes could not use ATP (adenosine triphosphate or cellular energy) or rely on cellular metabolism (eg, the breakdown of sugars), because these mechanisms are shut down during dormancy.
But, the researchers speculated that there was an alternative method: the spores might be able to detect small, cumulative changes in their environment, until enough signals accumulate to trigger some kind of alarm. of awakening. The mechanism that would induce these changes would be the movement of ions out of the cell, in particular potassium ions.
These movements can be triggered by positive environmental cues, such as the presence of nutrients. When ions leave the cell through 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 detect more positive signals, more ions would flow out of the cell. This would also create a corresponding drop in potassium levels, as the ions exit. Eventually, the potassium content of the spore would decrease to a certain threshold, signaling that it is safe for the cell to wake up. This would trigger resuscitation and germination.
In other words, says Süel, the spore essentially acts as a capacitor or device that contains electrical energy. “A capacitor is essentially an insulator separating the concentration gradient of charges,” he says. “You can really store a lot of energy this way, because the cell membrane is very thin.”
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