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A species of bacteria from the coastal waters of Antarctica is unable to evolve to tolerate temperatures much higher than it can currently survive. The finding suggests there is a hard limit to organisms鈥� ability to evolve adaptations to heat waves and long-term higher temperatures driven by climate change.

Bacteria are the Houdinis of evolution, escaping even the most challenging of environmental pressures. That is thanks to their sheer numbers and the fact that new generations grow so rapidly that they can quickly evolve adaptations. But they have their limits.

聽at ETH Zurich in Switzerland and her colleagues subjected genetically homogeneous colonies of a marine bacterium called Pseudoalteromonas haloplanktis to gradually increasing heat, starting at 15掳C and increasing to 30掳C over 900 generations.

P. haloplanktis is regularly able to grow at temperatures as low as -2.5掳C and as high as 29掳C, but it grows fastest around 15掳C. The gap between the temperature at which the bacteria thrive and at which they die leaves headroom for evolution to occur in between, says Toll-Riera. If anything could rapidly evolve a way to deal with heat, P. haloplanktis was it.

After between 70 and 270 generations at 30掳C, the bacteria were able to grow well. They evolved to survive 1掳C above their usual maximum of 29掳C. But they hit a limit. When the researchers exposed the bacteria to temperatures beyond 30掳C, they could barely grow, and they couldn鈥檛 grow at all above 32掳C. Even for this microbial escape artist 鈥渋t seems that it鈥檚 not so easy to evolve鈥�, says Toll-Riera.

The finding adds 鈥渁 note of realism鈥� to the notion that many species can survive rapid environmental change through evolution, says at McGill University in Canada. 鈥淎 hard limit applies even in an organism which is evolving as rapidly as any cellular organism can,鈥� he says.

The study could also help explain why such a limit exists. The researchers found many of the same genetic variants occurred in populations evolving in parallel. Several variants made proteins more stable; others enhanced an enzyme that destroys misfolded proteins. It could be that protein misfolding imposes a limit on adaptation to heat, says Toll-Riera.

Toll-Riera is cautious about generalising far beyond bacteria in a lab but says other research has found 鈥渉ard limits鈥� to adaptations to increasing temperatures in laboratory studies of tropical fish.

鈥淲e鈥檙e already seeing these mass mortality events when heat waves hit,鈥� says at the University of Bergen in Norway. 鈥淵ou can already see that the thermal limits are being exceeded for many species.鈥�

Evolution isn鈥檛 the only route to survival. 鈥淓volutionary rescue is hard, but that doesn鈥檛 say that organisms cannot respond to heat waves,鈥� says Toll-Riera. Species can migrate or adapt in ways that don鈥檛 involve genetic change. But evolution alone probably isn鈥檛 enough.