For many biologists, the intertidal zone is a captivating place. At low tide, seawater collects in depressions and crevasses along rocky shorelines, leaving behind tidepools teeming with life. Tidepools are often packed wall-to-wall with anemones, barnacles and snails, among other intriguing marine taxa trying to stay wet long after the tide recedes. However, not all intertidal residents are fortunate enough to secure a tidepool refuge during low tide, which is particularly problematic in temperate regions that experience bitter, cold winters. When intertidal animals are immersed in seawater, they are protected from sub-zero air temperatures. Without a tide pool refuge, however, air-exposed creatures can freeze when the tide goes out, thawing only upon immersion when the tide returns. Remarkably, many intertidal residents can survive the formation of ice crystals in their body, but how do they tolerate repeated freeze-thaw cycles? In a fascinating new study led by Lauren Gill at the University of British Columbia, a team of researchers discovered how bay mussels (Mytilus trossulus) survive repeated freezing and thawing during harsh Canadian winters.
Gill and her team first collected mussels from the rugged coastline in Vancouver, Canada, to investigate their survival rate under different freezing regimes: one continuous freeze lasting 8 h, two separate freezes lasting 4 h each or four separate freezes lasting 2 h each. The researchers induced freezing in the mussels by exposing them to air at –8°C. For mussels that underwent multiple freezing bouts, the team provided a 24 h recovery in 7°C seawater between each freeze to mimic the relief that they would experience with natural tides. The researchers predicted that repeated freezing and thawing would increase mussel mortality. However, much to their surprise, survival was almost 100% in mussels frozen four separate times (4×2 h) and a mere 25% in mussels frozen only once (1×8 h).
At this point, the researchers were keen to uncover the physiological mechanisms that led to such high survival in their repeatedly frozen mussels. They suspected that two different proteins could be playing an important role. The first, heat-shock protein 70 (HSP70), is known to help the body deal with extreme temperature stress. Extremely hot and cold temperatures can denature (i.e. damage) proteins in the body, but HSP70 can help protect proteins from denaturation. Sure enough, the team discovered that in repeatedly frozen mussels, the expression of HSP70 increased considerably after freezing, thereby providing mussels with protection against subsequent freezing events. The second protein is ubiquitin, which tracks down any damaged proteins in the body and flags them for disposal. Interestingly, the researchers found that in repeatedly frozen mussels, the number of proteins flagged by ubiquitin increased after freezing, indicating that the mussels were working hard to repair any molecular damage caused by freezing.
Taken together, Gill and her team found that mussels have a higher survival rate when they experience repeated freeze-thaw cycles compared with a single lengthy freeze of the same total duration. Furthermore, periods of thawing are critical for surviving sub-zero temperatures because they offer mussels an opportunity to make important proteins that protect against, and repair, molecular damage caused by freezing. So, the next time you’re out exploring the captivating tidepools of the intertidal, be sure to remember the creatures outside of these pools and the great lengths that they must go to in order to survive.