Understanding acclimation and adaptation to salinity changes in benthic osmoconformers

Salinity poses a limiting factor for the distribution of both marine and freshwater species. With ongoing climate change, a decrease in salinity is predicted for many coastal areas worldwide with likely consequences for the distribution of species, biodiversity and ecosystem functioning. This makes it urgent to understand the cellular basis for marine animal salinity tolerance. The Baltic Sea, with its natural salinity gradient, can thus serve as a time machine to study consequences of future desalination. Baltic Sea animals thus can serve as ideal case studies to understand local adaptation and physiological plasticity across a wide salinity gradient. The first part of this thesis consists of an extensive systematic review and meta-analysis of the salinity effect on osmolyte concentrations of benthic osmoconformers. In the second part the cellular osmolyte systems were quantified and related to growth, tissue hydration and mortality of six invertebrate model species acclimated to low salinity. The results confirm an important role of organic osmolytes, but also stress a previously largely neglected role of inorganic ions in facilitating acclimation to persistent low salinity. Critical salinity ranges were established for all species based on the findings. Thirdly, the transcriptomic response of two mussel populations locally adapted to diverging salinity regimes was analysed to shed further light on the molecular basis of salinity tolerance capacity. This thesis employs an integrative approach to measure phenotypic plasticity towards low salinity across a wide range of species from biochemical to transcriptomic levels. The study of the capacity for acclimation and adaptation to salinity changes in marine species will thus be a highly relevant field of research in the future to predict the effects of desalination on species, populations and ecosystems