We humans tend to cringe at winter temperatures. We put on extra layers, crank up the thermostat, and wait impatiently for the tell-tale drip of spring thaw. However, there are plenty of tiny organisms all around us that aren’t just biding their time; they’re thriving in the bitter cold. If you could listen to as well as watch them under a microscope, you wouldn't hear a single complaint about the temperature.
Psychrophiles, literally “cold lovers,” are organisms adapted to live at extremely cold temperatures. These are single-celled life forms, most often bacteria, but also blue green algae, yeasts, and fungi that can grow at temperatures as low as -13 degrees.
Why not pack it up and move to Florida? By adapting to low temperatures, psychrophiles have freed themselves from limits that hold back other kinds of life. Over 80% of the planet’s biosphere never gets above 41 degrees; psychrophiles have moved into all these places, from high peaks to extreme ocean depths.
Researchers are constantly looking for new species that can survive lower and lower temperatures in remote parts of Antarctica or the deep ocean floor, so you might think these organisms are strange and exotic. But in fact we're surrounded by them.
Many are decomposers that live in the soil, eating and growing even through the winter. They'll keep working on a compost pile even in January. Unfortunately, they also like dairy products. When your milk goes rancid, you have a psychrophile to thank. In the freezer, a slowly growing population of psychrophiles is chomping on your ice cream.
While most live quiet lives, the group does have notorious members. Consider for example Pseudogymnoascus destructans, the fungus associated with the white nose syndrome that has been killing off bats.
If you've ever found a jug of bad milk in the back of your fridge, you might be thinking we’d be better off without psychrophiles, but think again. As decomposers, they make up the very base of the food web, recycling nutrients so other kinds of life can use them. It's a good thing for the rest of us that they've figured out how to thrive when the cold has us inside huddled under blankets.
It’s not easy to stay alive in the cold. First, the lack of heat usually causes cells to wind down like mechanical toys with drained batteries because normal cell proteins will crumple into unusable shapes in the cold. Cells have fatty outer membranes that need to be fluid to do their job, and at freezing temperatures, membranes become hard – think how difficult it is to spread butter right out of the fridge. And finally, if ice crystals form inside a cell, they can cut the cell to ribbons.
To overcome all this, psychrophiles have developed several clever evolutionary tricks. For starters, they have special flexible proteins that won’t warp into unusable lumps. They also use more unsaturated fats to make up their cell membranes than do most other organisms. Unsaturated fats stay loose and fluid even in the cold. If our membranes are like butter – which gets hard in the fridge – then their membranes are like margarine, which has higher amounts of unsaturated fats and stays spreadable even when chilled. As for the risk of death by ice cuts, they have antifreeze that prevents ice crystals from forming and shredding their cells.
Psychrophiles have one last trick up their sleeve: they do everything really slowly. They don't need fast metabolisms because they don't live fast lives. They can divide just six times a year, in contrast to E. coli, who love the same comfortably warm temperatures that we do, and divide every 20 minutes. We can learn from these tiny organisms and slow down the hectic pace of our busy lives; it may make the cold of winter just a little more bearable.