Scaling the invisible wall: Molecular acclimation of a salinity-tolerant diatom to freshwater.

In aquatic ecosystems, marine and freshwater environments are separated by steep salinity gradients. The osmotic stress induced by this 'invisible wall' forms an insurmountable barrier for many aquatic lifeforms, including bacteria, algae and animals. Because the osmotic differences when transiting a salinity divide are so hard to overcome, most species have adapted exclusively to a marine or a freshwater lifestyle. A major consequence of this physiological specialization into marine and freshwater organisms is that transitions are relatively rare, impeding regular contact and colonization. While some animals use specialized organs or behaviour to cope with unfavourable salinity levels, unicellular algae such as diatoms are completely dependent on cellular mechanisms to mitigate salinity stress. In this issue of Molecular Ecology, Downey and colleagues investigate the transcriptomic response of a salinity-tolerant diatom to a shock treatment with freshwater (Molecular Ecology, 2023). Through frequent sampling and integration of existing RNA sequencing data, a fine-grained model of the acclimation to hypo-osmotic stress emerges. Deciphering the pathways that drive the acute and long-term acclimation to freshwater has major implications for diatom ecology, diversification and resilience to global change.