The effect of prolonged starvation on the relative free amino acid composition of the extracellular body fluids and protein bound amino acids in the oyster, Crassostrea gigas

The role of the balance between energy production and ammonia detoxification mediated by key amino acids in divergent hypersaline adaptation among crassostrea oysters

Global ocean salinity is changing under rapid climate change and intensified anthropogenic activity. Increased differences in salinity threaten marine biodiversity, organismal survival, and evolution, particularly sessile invertebrates dwelling in highly fluctuating intertidal and estuarine environments. Comparing the responses of closely related species to salinity changes can provide insights into the adaptive mechanisms underlying inter- and intraspecific divergence in salinity tolerance, but are poorly understood in marine bivalves. We collected wild individuals of four Crassostrea species, in addition to two populations of the same species from their native habitats and determined the dynamics of hydrolyzed amino acids (HAAs) and transcriptional responses to hypersaline stress. In response to hypersaline stress, species/populations inhabiting natural high-salinity sea environments showed higher survival and less decline in HAAs than that of congeners inhabiting low-salinity estuaries. Thus, native environmental salinity shapes oyster tolerance. Notably, a strong negative correlation between the decline in HAAs and survival indicated that the HAAs pool could predict tolerance to hypersaline challenge. Four HAAs, including glutamine (Glu), aspartic acid (Asp), alanine (Ala) and glycine (Gly), were identified as key amino acids that contributed substantially to the emergency response to hypersaline stress. High-salinity-adapted oyster species only induced substantial decreases in Glu and Asp, whereas low-salinity-adapted congeners further incresaed Ala and Gly metabolism under hypersaline stress. The dynamics of the content and gene expression responsible for key amino acids pathways revealed the importance of maintaining the balance between energy production and ammonia detoxification in divergent hypersaline responses among oyster species/populations. High constructive or plastic expression of evolutionarily expanded gene copies in high-salinity-adapted species may contribute to their greater hypersaline tolerance. Our findings reveal the adaptive mechanism of key amino acids in salinity adaptation in marine bivalves and provide new avenues for the prediction of adaptive potential and aquaculture with high-salinity tolerant germplasms.

Protein turnover, physiological energetics and heterozygosity in the blue mussel, Mytilus edulis : the basis of variable age-specific growth

Genotype-dependent changes in whole-body protein turnover are shown to be of significance to metabolic efficiency, energetic maintenance requirements and associated rates of dietary absorption in Mytilus edulis . Faster growth rates were, on average, evident in more heterozygous mussels. Such variation did not stem from different efficiencies of nitrogen conversion, which remained high and independent of either genotype or age. Rather, faster growth derived from decreased energy requirements for maintenance. This decrease was associated with greater efficiencies of protein synthesis [(net protein balance/gross protein synthesis) x 100] and lower intensities of nitrogen metabolism. Energy ‘saved’ by virtue of higher metabolic efficiency was used to effect increases of both ingestion and absorption efficiency. Alternatively, reduced synthetic efficiencies among more homozygous mussels necessitated elevated protein synthesis per unit deposition, high efficiencies of nitrogen conversion being sustained by a greater proportional recycling of breakdown products. Energetic costs were thus enhanced, with a consequent reduction in growth. Efficiencies of protein synthesis declined with age, whereas maintenance requirements grew. These findings are discussed in relation to individual fitness, and proposed as a mechanism by which genotype may mediate intraspecific variations of growth among animals generally.

High taurine levels in the Solemya velum symbiosis

1. To compare biochemical differences between bivalves with and without endosymbiotic chemoautotrophic bacteria, specimens of Solemya velum, a bivalve species known to contain bacterial endosymbionts, and the symbiont-free soft-shelled clam Mya arenaria, were collected from the same subtidal reducing sediments during October and November 1988. 2. Total and free amino acid compositions were determined for both species. Protein-bound amino acids were calculated as the difference between total and free amino acids. In addition, stable isotope ratios of the total and free amino acids of each species were measured to determine potential sources for these molecules. 3. Both species had similar total hydrolyzable- and protein-bound amino acid compositions; approximately 50% of the protein-bound amino acids were essential amino acids. In S. velum, the small size of the digestive system suggests that these amino acids are probably synthesized by the endosymbiotic bacteria and translocated to the animal tissue. The delta 13C and delta 15N ratios of the amino acids are very similar to the isotope ratios previously found in both the endosymbionts and whole tissues of S. velum. The relative and absolute amounts of free amino acids are very different in the two species. In S. velum, the absolute concentrations of taurine, a sulfur-containing amino acid, were greater than the total free amino acid concentrations found in other bivalves. 4. The delta 34S ratios of the free amino acids of S. velum, which were predominantly composed of taurine, were extremely negative (-17.2/1000) suggesting that taurine is synthesized using sulfur originally derived from external reduced sulfur sources, such as pore water sulfides. The possible roles for taurine in this animal-bacteria symbiosis are discussed.