Osmoionic homeostasis in bivalve mollusks from different osmotic niches: Physiological patterns and evolutionary perspectives

Biochemical and molecular responses to long-term salinity challenges in northern quahogs Mercenaria mercenaria

Salinity is a key environmental factor for aquatic organisms for survival, development, distribution, and physiological performance. Salinity fluctuation occurs often in estuary and coastal zones due to weather, tide, and freshwater inflow and thus heavily affects coastal marine aquaculture. The northern quahog Mercenaria mercenaria is an important aquaculture species along the Atlantic coast in the US, but information on the effects of salinity stress on physiological, immunological, and molecular responses is still scarce. The goal of this study was to investigate cellular and molecular responses through challenges of long-term hypo- and hyper-salinities in northern quahogs. The objectives were to: 1) measure the survival of market-sized quahogs under a three-month salinity challenge at 15 (hyposalinity), 25 (control), and 35 ppt (hypersalinity); 2) determine cellular changes of hemocytes through analysis of immune functions; 3) determine changes of the total free amino acid concentration in gills, and 4) evaluate the molecular responses in gills using RNAseq technology with qPCR verification. After a three-month salinity challenge, no mortality was observed, and increases in body weight were identified with a significantly higher increase in the hypersalinity group. Northern quahogs equilibrated their hemolymph osmolality with the ambient seawater and were verified to be osmoconformers. Significant differences were observed in total hemocyte concentration, lysosomal presence, ROS production, and phagocytic rate, but no differences were found in cell viability. The total free amino acid concentration within gills was positively correlated to water salinity, indicating amino acids were critical organic osmolytes. The transcriptome of gills using RNAseq revealed differential expression genes (DEG) encoding amino acid transporters (SLC6A3, SLC6A6, SLC6A13, SLC25A38), ion channel proteins (T38B1, GluCl, ATP2C1), and water channel protein (AQP8) in hyposalinity or/and hypersalinity groups, indicating these genes play critical roles in intracellular isosmotic regulation. Overall, the findings in this study provided new insights into osmoregulation in northern quahogs.

Inorganic osmolytes and enzymatic biomarkers from the manabi oyster (Crassostrea cf. corteziensis) in response to saline stress

Abrupt drops in salinity that occur in tropical estuaries during the equatorial rainy season led to hyposaline conditions which may reduce the populational density of oysters. To assess the effect of saline stress on physiological and metabolic responses of the Manabi oyster (Crassostrea cf. corteziensis) was exposed to 35, 30, 20,10 and 5‰ concentrations during 96 h. Inorganic osmolytes, pH, salinity, haemocyanin and protein concentration in the plasma as well as the number of oysters with closed valves were recorded. Aspartate aminotransferase (AST), alanine aminotransferase (ALT), lactate dehydrogenase (LDH), alkaline phosphatase (ALP) and catalase (CAT) activity were analysed. Inorganic osmolytes and internal salinity were elevated in oysters exposed to 35, 10 and 5‰. A significant number of oysters with valve closure was observed in 10 and 5‰, which coincided with a decline in physiological pH and changes in haemocyanin concentrations. AST activity and AST/ALT ratio were reduced under 35, 10 and 5‰, and CAT increased in oysters exposed to 35‰; but protein concentration, LDH and ALP did not show significant variations. Metabolic adjustment and behavior of the Manabi oyster could explain tolerance and survival (at least for a short term) to hyposaline stress in tropical estuarine ecosystems.

Short communication: ROS production and mitochondrial membrane potential in hemocytes of marine bivalves, Mytilus galloprovincialis and Magallana gigas, under hypoosmotic stress

Bivalve mollusks that inhabit low-depth coastal and estuarine areas frequently experience osmotic stress that may be also associated with alterations of antioxidant enzyme activities and markers of oxidative stress. Mitochondria are a major source of reactive oxygen species (ROS) in eucaryotic cells. Overpoduction of ROS induces oxidative stress leading to a damage of intracellular compartments and cell death. In euryhaline bivalves, information concerning cellular ROS production upon osmotic stress and changes in mitochondrial membrane potential is scarce. The present study investigates osmotic stability and hemocytes` regulatory volume decrease (RVD) of Mediterranean mussel (Mytilus galloprovincialis) and the Pacific oyster (Magallana gigas). We also studied dynamic changes in intracellular ROS levels and mitochondrial membrane potential in hemocytes undergoing the RVD response following hypoosmotic swelling. Our data revealed that osmotic stability of mussel and oyster hemocytes did not significantly differ. Loss of environmental osmolarity from 460.0 ± 2.0 mOsm l-1 to 216.0 ± 4.0 mOsm l-1 resulted in an increase of hemocyte volume by 60% of the initial cellular volume in mussels and by 28% in oysters. After rapid hypoosmotic swelling hemocytes of both species demonstrated the RVD response. At the end of 60 min exposure to hypoosmotic environment, hemocyte volume significantly decreased in both species by 10-12% compared to the maximal hemocyte volume. Hypoosmotic shock induced an increase of mitochondrial membrane potential in hemocytes of mussels and oysters. In mussels, increased mitochondrial membrane potential was accompanied with decreased ROS levels in hemocytes, whereas oyster hemocytes showed enhanced ROS production.

Multidimensional plasticity jointly contributes to rapid acclimation to environmental challenges during biological invasions

Rapid plastic response to environmental changes, which involves extremely complex underlying mechanisms, is crucial for organismal survival during many ecological and evolutionary processes such as those in global change and biological invasions. Gene expression is among the most studied molecular plasticity, while co- or posttranscriptional mechanisms are still largely unexplored. Using a model invasive ascidian Ciona savignyi, we studied multidimensional short-term plasticity in response to hyper- and hyposalinity stresses, covering the physiological adjustment, gene expression, alternative splicing (AS), and alternative polyadenylation (APA) regulations. Our results demonstrated that rapid plastic response varied with environmental context, timescales, and molecular regulatory levels. Gene expression, AS, and APA regulations independently acted on different gene sets and corresponding biological functions, highlighting their nonredundant roles in rapid environmental adaptation. Stress-induced gene expression changes illustrated the use of a strategy of accumulating free amino acids under high salinity and losing/reducing them during low salinity to maintain the osmotic homoeostasis. Genes with more exons were inclined to use AS regulations, and isoform switches in functional genes such as SLC2a5 and Cyb5r3 resulted in enhanced transporting activities by up-regulating the isoforms with more transmembrane regions. The extensive 3'-untranslated region (3'UTR) shortening through APA was induced by both salinity stresses, and APA regulation predominated transcriptomic changes at some stages of stress response. The findings here provide evidence for complex plastic mechanisms to environmental changes, and thereby highlight the importance of systemically integrating different levels of regulatory mechanisms in studying initial plasticity in evolutionary trajectories.

Cell volume maintenance capacity of the sea anemone Bunodosoma cangicum: the effect of copper

Cell volume regulation is an essential strategy for the maintenance of life under unfavorable osmotic conditions. Mechanisms aimed at minimizing the physiological challenges caused by environmental changes are crucial in anisosmotic environments. However, aquatic ecosystems experience multiple stressors, including variations in salinity and heavy metal pollution. The accumulation of heavy metals in aquatic ecosystems has a significant effect on the biota, leading to impaired function. The aim of this study was to investigate the capacity of volume regulation in isolated cells of the sea anemone Bunodosoma cangicum exposed to nominal copper (Cu) concentrations of 5 and 50 µg L^-1, associated or not with hypoosmotic (15‰) or hyperosmotic (45‰) shock for 15 min. In the absence of the metal, our results showed volume maintenance in all osmotic conditions. Our results showed that cell volume was maintained under all osmotic conditions in the absence of Cu. Similarly, no significant differences were observed in cell volumes under isosmotic and hyperosmotic conditions in the presence of both Cu concentrations. A similar homeostatic response was observed under the hypoosmotic condition with 5 µg L^-1 Cu. Our results showed an increase in cell volume with exposure of the cells to the hypoosmotic condition and 50 µg L^-1 Cu. The response could be associated with the increased bioavailability of Cu, reduced ability to resist multixenobiotics and their efflux pathways, and the impairment of water efflux in specialized transmembrane proteins. Therefore, B. cangicum pedal disk cells can tolerate osmotic variations in aquatic ecosystems. However, the capacity to regulate cell volume under hypoosmotic conditions can be affected by the presence of a metal contaminant (50 µg L^-1 Cu), which could be due to the inhibition of water channels.

Influence of pH on physiological and behavioral responses of Pomacea canaliculata

The physiological and behavioral responses of Pomacea canaliculata exposed to different pH values (4, 5, 6, 7, 8, 9, and 10) were evaluated. Survival, behavior (avoidance), metabolites (mantle), net ion fluxes (Na⁺ and K⁺), and ATPase activity (gills) were the parameters analyzed. The final survival rates were 100% (pH 4-9) and 90% (pH 10), and the groups did not differ significantly. Avoidance behavior was not identified in animals exposed to an extreme pH compared to pH 7. The main changes observed in the metabolites were in those exposed to an alkaline pH. Glucose (pH 9) and total protein (pH 9 and 10) levels increased, and lactate decreased (pH 9 and 10) compared to a neutral pH. There was an increase in Na⁺ efflux at pH 4, 5, and 8 and an influx at pH 9 and 10. Extreme pH values (4 and 10) also caused an increase in K⁺ efflux. At pH values outside the neutrality range (pH 7), there was a significant decrease in the activities of Na⁺/K⁺-ATPase (4, 5, 6, 9, and 10) and H⁺-ATPase (pH 4, 5, 9, and 10). Variations in environmental pH did not cause statistically significant mortality or avoidance behavior in P. canaliculata at the analyzed times. However, due to changes in energy metabolism (glucose and lactate, mainly) and ionoregulation, these can be considered sensitive biomarkers of stress in this species.

Studying cell volume beyond cell volume

The first part of the paper describes two simple microscopic techniques that we use in our laboratory. One measures cell volumes in adherent cultures and the other measures cell dry mass; both measurements are done on the same instrument (a standard bright-field transmission microscope with only one or two narrow-band color filters added) and on the same cells. The reason for combining cell volume with dry mass is that the ratio of the two-dry mass concentration (MC)-is an important and insufficiently utilized biological parameter. We then describe a few applications of MC. The available experimental data strongly suggest its critical role in biological processes, including cell volume regulation. For example, most eukaryotic cells have surprisingly similar values of MC. Moreover, MC (and not cell volume) is tightly controlled in growing cell cultures at highly variable external osmolarities. We review the results showing that elevation of MC is a direct cause of shrinkage-induced apoptosis. Also, by focusing on MC, one can study heterogenous processes, such as necrotic swelling, or discriminate between apoptotic dehydration and the loss of cell fragments.