Dynamic Regulation of Ions and Amino Acids in Adult Asian Hard Clams Meretrix lusoria Upon Hyperosmotic Salinity

The dynamic regulation of ions and amino acids in the gills and mantle of the Asian hard clam, Meretrix lusoria, following the exposure to a hyperosmotic environment was hitherto unclear. The present study revealed that the osmolality as well as the Na+ and Cl– concentrations in the hemolymph were significantly increased 3 h after transferring the clams from an environment with the salinity of their natural habitat (brackish water; BW; 20‰) to one with hyperosmotic salinity (seawater; 35‰). In addition, we found that the specific activities of Na+/K+-ATPase, a key enzyme that plays a significant role in cell osmoregulation, in the gills and mantle of clams were significantly increased at 72 and 12 h post-transfer, respectively, during acclimation to hyperosmotic salinity. Similarly, the contents of free amino acids (FAAs) such as taurine, alanine, and glycine were significantly elevated during hyperosmotic salinity acclimation. Previous research indicates that taurine is the most abundant FAA in the gills and mantles of Asian hard clams and that the taurine transporter (TAUT) plays an important role in taurine accumulation. The present study showed that TAUT mRNA and protein expression were significantly and transiently increased in the mantle of Asian hard clams following exposure to seawater; although the expression of TAUT mRNA in the gills of Asian hard clams was also transiently stimulated by exposure to hyperosmotic salinity, the relative TAUT protein abundance decreased only at later stages. Accordingly, the findings of this study improve our understanding of the dynamic processes of ion and amino acid regulation in the peripheral tissues of bivalves under hyperosmotic stress.

The time course of molecular acclimation to seawater in a euryhaline fish

The Arabian pupfish, Aphanius dispar, is a euryhaline fish inhabiting both inland nearly-freshwater desert ponds and highly saline Red Sea coastal lagoons of the Arabian Peninsula. Desert ponds and coastal lagoons, located respectively upstream and at the mouths of dry riverbeds (“wadies”), have been found to potentially become connected during periods of intense rainfall, which could allow the fish to migrate between these different habitats. Flash floods would therefore flush Arabian pupfish out to sea, requiring a rapid acclimation to a greater than 40 ppt change in salinity. To investigate the molecular pathways of salinity acclimation during such events, a Red Sea coastal lagoon and a desert pond population were sampled, with the latter exposed to a rapid increase in water salinity. Changes in branchial gene expression were investigated via genome-wide transcriptome measurements over time from 6 h to 21 days. The two natural populations displayed basal differences in genes related to ion transport, osmoregulation and immune system functions. These mechanisms were also differentially regulated in seawater transferred fish, revealing their crucial role in long-term adaptation. Other processes were only transiently activated shortly after the salinity exposure, including cellular stress response mechanisms, such as molecular chaperone synthesis and apoptosis. Tissue remodelling processes were also identified as transient, but took place later in the timeline, suggesting their importance to long-term acclimation as they likely equip the fish with lasting adaptations to their new environment. The alterations in branchial functional pathways displayed by Arabian pupfish in response to salinity increases are diverse. These reveal a large toolkit of molecular processes important for adaptation to hyperosmolarity that allow for successful colonization to a wide variety of different habitats.

Influence of Salinity on the Survival and Growth of Juvenile Coregonus Ussuriensis Berg

To explore the suitable salinity range of Coregonus ussuriensis Berg, we investigated the effect of induced salinity change in captivity on C. ussuriensis with an initial body weight of 35 ± 1.5 g. After 30 days of salinity acclimation, the survival, growth performance, blood biochemical profiles, antioxidative capacity, and tissue structure of juveniles under four salinity conditions (8‰, 16‰, 24‰, and 32‰) were investigated. Our results revealed that serum penetration, blood glucose, and serum Na+, Cl−, and Mg2+ gradually increased with increasing salinity until 32‰ salinity, when a significant difference was observed, whereas the K+ concentration showed a downward trend. The tissue sections showed that under high salinity (32‰), the liver and gill tissues of the fish were severely damaged and the vacuolation was serious. The levels of superoxide dismutase, glutathione peroxidase, and serum cortisol gradually increased with increasing salinity. A gene expression analysis showed that the increase in salinity induced higher expression of stress-, growth-, and inflammation-related genes (HSP70, Gh and Igf-1, and IL-1β, respectively). The downregulation of stress-related gene expression at 32‰ salinity may indicate that this level of salinity exceeded the regulatory capacity of C. ussuriensis. We concluded that C. ussuriensis may survive in an estuary under 0–24‰ salinity. Our findings provide insights into the physiological adaptation of C. ussuriensis to salinity change. These results could improve our knowledge of the stress response and resilience of estuarine fish to hyposalinity and hypersalinity stress.

The time course of molecular acclimation to seawater in a euryhaline fish

The Arabian pupfish, Aphanius dispar, is a euryhaline fish inhabiting both inland nearly-freshwater desert ponds and highly saline Red Sea coastal lagoons of the Arabian Peninsula. Red Sea populations have been found to receive migrants from desert ponds that are flushed out to sea during flash floods, requiring rapid acclimation to a greater than 40 ppt change in salinity. To investigate the molecular pathways of salinity acclimation during such colonization events, a Red Sea coastal lagoon and a desert pond population were sampled, with the latter exposed to a rapid increase in water salinity. Changes in branchial gene expression were investigated via genome-wide transcriptome measurements over time from 6 hours to 21 days. The two natural populations displayed basal differences in genes related to ion transport, osmoregulation and immune system functions. These mechanisms were also differentially regulated in seawater transferred fish, revealing their crucial role in long-term adaptation. Other processes were only transiently activated shortly after the salinity exposure, including cellular stress response mechanisms, such as molecular chaperone synthesis and apoptosis. Tissue remodeling processes were also identified as transient, but took place later in the timeline, suggesting their importance to long-term acclimation as they likely equip the fish with lasting adaptations to their new environment. The alterations in branchial functional pathways displayed by Arabian pupfish in response to salinity increases are diverse. These reveal a large toolkit of molecular processes important for adaptation to hyperosmolarity that allow for successful colonization to a wide variety of different habitats.

Molecular and physiological characterization of a crustacean cardioactive signaling system in a lophotrochozoan – the pacific oyster (Crassostrea gigas): a role in reproduction and salinity acclimation

The crustacean cardioactive peptide (CCAP) is an important neuropeptide involved in the regulation of a variety of physiological processes in arthropods. Although this family of peptides has an ancestral origin, its function remains poorly understood among protostome species – apart from arthropods. We functionally characterized three G protein-coupled receptors (GPCRs) in the oyster Crassostrea gigas, phylogenetically related to ecdysozoan CCAP receptors (CCAPRs) and to chordate neuropeptide S receptors (NPSRs). Cragi-CCAPR1 and Cragi-CCAPR2 were specifically activated by the Cragi-CCAP1 and Cragi-CCAP2 peptides, respectively, both derived from the same CCAP precursor. In contrast, Cragi-CCAPR3 was only partially activated by CCAP1 and CCAP2 at high concentrations. The Cragi-CCAPR1 and Cragi-CCAPR2 genes were expressed in various adult tissues. They are both most expressed in the gills, while Cragi-CCAPR3 is mainly expressed in the visceral ganglia (VG). Cragi-CCAP precursor transcripts are higher in the VG, the labial palps and the gills. Receptor and ligand-encoding transcripts are more abundantly expressed in the gonads in the first stages of gametogenesis, while the Cragi-CCAP precursor is up-regulated in the VG in the last stages of gametogenesis. This suggests a role of the CCAP signaling system in the regulation of reproductive processes. A role in water and ionic regulation is also supported considering the differential expression of the CCAP signaling components in oysters exposed to brackish water.

Changing microbial activities during low salinity acclimation in the brown alga Ectocarpus subulatus

SummaryEctocarpus subulatus is one of the few brown algae found in river habitats. Its ability to tolerate freshwater is due, in part, to its uncultivated microbiome. We investigated this phenomenon by modifying the microbiome of laboratory-grown E. subulatus using mild antibiotic treatments, which affected its ability to grow in low salinity. The acclimation to low salinity of fresh water-tolerant and intolerant holobionts was then compared. Salinity had a significant impact on bacterial gene expression as well as the expression of algae- and bacteria-associated viruses in all holobionts, albeit in different ways for each holobiont. On the other hand, gene expression of the algal host and metabolite profiles were affected almost exclusively in the fresh water intolerant holobiont. We found no evidence of bacterial protein production that would directly improve algal stress tolerance. However, we identified vitamin K synthesis as one possible bacterial service missing specifically in the fresh water-intolerant holobiont in low salinity.We also noticed an increase in bacterial transcriptomic activity and the induction of microbial genes involved in the biosynthesis of the autoinducer AI-1, a compound that regulates quorum sensing. This could have caused a shift in bacterial behavior in the intolerant holobiont, resulting in virulence or dysbiosis.Originality-Significance StatementThe importance of symbiotic microbes for the health and stress resistance of multicellular eukaryotes is widely acknowledged, but understanding the mechanisms underlying these interactions is challenging. They are especially difficult to separate in systems with one or more uncultivable components. We bridge the gap between fully controlled, cultivable model systems and purely environmental studies through the use of a multi-omics approach and metabolic models on experimentally modified “holobiont” systems. This allows us to generate two promising working hypotheses on the mechanisms by which uncultivated bacteria influence their brown algal host’s fresh water tolerance.