Fluctuating salinity is symptomatic of climate change challenging aquatic species. The melting of polar ice, rising sea levels, coastal surface and groundwater salinization, and increased evaporation in arid habitats alter salinity world-wide. Moreover, the frequency and intensity of extreme weather events such as rainstorms and floods increase, causing rapid shifts in brackish and coastal habitat salinity. Such salinity alterations disrupt homeostasis, and ultimately diminish fitness, of aquatic organisms by interfering with metabolism, reproduction, immunity, and other critical aspects of physiology. Proteins are central for these physiological mechanisms. They represent the molecular building blocks of phenotypes that govern organismal responses to environmental challenges. Environmental cues regulate proteins in concerted fashion, necessitating holistic analyses of proteomes for comprehending salinity stress responses. Proteomics approaches reveal molecular causes of population declines and enable holistic bioindication geared towards timely interventions to prevent local extinctions. Proteomics analyses of salinity effects on aquatic organisms have been performed since the mid-1990s, propelled by the invention of two-dimensional protein gels, soft ionization techniques for mass spectrometry, and nano-liquid chromatography in the 1970s and 1980s. This review summarizes the current knowledge on salinity regulation of proteomes from aquatic organisms, including key methodological advances over the past decades.

All organisms encounter environmental changes that lead to physiological adjustments and drive evolutionary adaptations. These, in turn, induce behavioral, physiological and molecular changes that affect each other. Deciphering the role of molecular adjustments in physiological changes will help to understand how multiple levels of biological organization are synchronized during adaptations. Transmembrane transporters are prime targets for molecular studies of environmental effects, as they facilitate the ability of cells to interact with the external surrounding. Fish are subjected to fluctuations of environmental factors of their aquatic surrounding and exhibit different coping mechanisms. To study the molecular adjustments of fish proteins to their unique external surrounding, suitable experimental systems must be established. Mozambique tilapia (Oreochromis mossambicus) is an excellent model for environmental stress studies due to its extreme osmotolerance. We established a homologues cellular-based expression system, and an uptake assay, that allowed us to study effects of environmental conditions on transmembrane transport. We applied it to study the effects of environmental conditions on the activity of PepT2, a widely studied transporter due to its importance in absorption of dietary peptides and drugs. We created a stable, modified fish cell-line, exogenously expressing the tilapia PepT2 and tested the effects of temperature and water salinity on the uptake of fluorescent di-peptide, β-Ala-Lys-AMCA. While temperature affected the Vmax of the transport, salinity affected both the Vmax and the Km. These assays demonstrate the importance of suitable experimental systems for fish ecophysiology studies. The presented tools and methods can be adapted to study other transporters in-vitro.