Modelling interactions of acid-base balance and respiratory status in the toxicity of metal mixtures in the American oyster Crassostrea virginica

Examining the relationship between metal exposure (Cd and Hg), subcellular accumulation, and physiology of juvenile Crassostrea virginica

To assess the toxicity and accumulation (total and subcellular partitioning) of cadmium (Cd) and mercury (Hg), juvenile eastern oysters, Crassostrea virginica, were exposed for 4 weeks to a range of concentrations (Control, Low (1×), and High (4×)). Despite the 4-fold increase in metal concentrations, oysters from the High-Cd treatment (2.4 μM Cd) attained a body burden that was only 2.4-fold greater than that of the Low-Cd treatment (0.6 μM Cd), while oysters from the High-Hg treatment (0.056 μM Hg) accumulated 8.9-fold more Hg than those from the Low-Hg treatment (0.014 μM Hg). This fold difference in total Cd burdens was, in general, mirrored at the subcellular level, though binding to heat-denatured proteins in the High-Cd treatment was depressed (only 1.6-fold higher than the Low-Cd treatment). Mercury did not appear to appreciably partition to the subcellular fractions examined in this study, with the fold difference in accumulation between the Low- and High-Hg treatments ranging from 1.5-fold (heat-stable proteins) to 4.6-fold (organelles). Differences in toxicological impairments (reductions in condition index, protein content, and ETS activity) exhibited by oysters from the High-Cd treatment may be partially due to the nature of how different metals partition to subcellular components in the oysters, though exact mechanisms will require further examination.

Effects of cadmium accumulation from suspended sediments and phytoplankton on the Oyster Saccostrea glomerata

Metals are accumulated by filter feeding organisms via water, ingestion of suspended sediments or food. The uptake pathway can affect metal toxicity. Saccostrea glomerata were exposed to cadmium through cadmium-spiked suspended sediments (19 and 93μg/g dry mass) and cadmium-enriched phytoplankton (1.6-3μg/g dry mass) and cadmium uptake and effects measured. Oysters accumulated appreciable amounts of cadmium from both low and high cadmium spiked suspended sediment treatments (5.9±0.4μg/g and 23±2μg/g respectively compared to controls 0.97±0.05μg/g dry mass). Only a small amount of cadmium was accumulated by ingestion of cadmium-enriched phytoplankton (1.9±0.1μg/g compared to controls 1.2±0.1μg/g). In the cadmium spiked suspended sediment experiments, most cadmium was desorbed from sediments and cadmium concentrations in S. glomerata were significantly related to dissolved cadmium concentrations (4-21μg/L) in the overlying water. In the phytoplankton feeding experiment cadmium concentrations in overlying water were <0.01μg/L. In both exposure experiments, cadmium-exposed oysters showed a significant reduction in total antioxidant capacity and significantly increased lipid peroxidation and percentage of destabilised lysosomes. Destabilised lysosomes in the suspended sediments experiments also resulted from stress of exposure to the suspended sediments. The study demonstrated that exposure to cadmium via suspended sediments and to low concentrations of cadmium through the ingestion of phytoplankton, can cause sublethal stress to S. glomerata.

The combinational effects between sulfonamides and metals on nematode Caenorhabditis elegans

As emerging pollutants, antibiotic sulfonamides are continuously emitted into the environment and encounter those already-existing contaminants, e.g., heavy metals, which may cause toxicity interactions in polluted habitats. So far, the sulfonamide mixture effects and the combinational effects between sulfonamides and metals have been seldom studied. In this study, lifespan, lethality (24 and 120 h), locomotion behavior and growth (96 h) of Caenorhabditis elegans were measured after exposure to mixtures containing sulfonamides (sulfadiazine, sulfapyridine, sulfamethoxazole and sulfamethazine as representatives) and/or metals (cadmium, copper, lead and zinc as representatives) at environmental concentrations. Results showed that sulfonamides did not cause acute (24 h) lethality at chosen concentrations, but they decreased the lifespan in a concentration dependent fashion. Moreover, sulfonamide mixtures caused synergisms at higher concentrations but antagonisms at lower concentrations on the subacute (120 h) lethal effects. The toxicity interactions of sulfonamide mixtures were addition action on body bending frequency, and antagonism on reversal movement and body length. In sulfonamide and metal mixtures, the toxicity interactions were different in acute and subacute lethal results, indicating the influence of the exposure time. According to the comparison among effects of mixtures containing sulfonamides and/or metals, subacute lethality of sulfonamides was enhanced by metals based on the synergistic mixture effects, while their inhibitions on the growth and behavior were weakened by metals based on the antagonistic mixture effects. Our findings highlighted studies on combinational effects between emerging and common contaminants for more accurate environmental risk evaluation, and also urged further mechanism studies.

Microarray analysis highlights immune response of pacific oysters as a determinant of resistance to summer mortality

Summer mortality of Crassostrea gigas is the result of a complex interaction between oysters, their environment, and pathogens. A high heritability was estimated for resistance to summer mortality, which provided an opportunity to develop lines of oysters that were resistant (R) or susceptible (S) to summer mortality. Previous genome-wide expression profiling study of R and S oyster gonads highlighted reproduction and antioxidant defense as constitutive pathways that operate differentially between these two lines. Here, we show that signaling in innate immunity also operates differentially between these lines, and we hypothesize that this is at the main determinant of their difference in survival in the field. A reanalysis of our published microarray data using separate ANOVAs at each sampling date revealed a specific "immune" profile at the date preceding the mortality. In addition, we conducted additional microarray profiling of two other tissues, gills, and muscle, and both showed an overrepresentation of immune genes (46%) among those that are differentially expressed between the two lines. Eleven genes were pinpointed to be simultaneously differentially expressed between R and S lines in the three tissues. Among them, ten are related to "Immune Response." For these genes, the kinetics of R mRNA levels between sampling dates appeared different just before the morality peak and suggests that under field conditions, R oysters had the capacity to modulate signaling in innate immunity whereas S oysters did not. This study enhances our understanding of the complex summer mortality syndrome and provides candidates of interest for further functional and genetics studies.

The transcriptomic responses of the eastern oyster, Crassostrea virginica, to environmental conditions

Understanding the mechanisms by which organisms adapt to environmental conditions is a fundamental question for ecology and evolution. In this study, we evaluate changes in gene expression of a marine mollusc, the eastern oyster Crassostrea virginica, associated with the physico-chemical conditions and the levels of metals and other contaminants in their environment. The results indicate that transcript signatures can effectively disentangle the complex interactive gene expression responses to the environment and are also capable of disentangling the complex dynamic effects of environmental factors on gene expression. In this context, the mapping of environment to gene and gene to environment is reciprocal and mutually reinforcing. In general, the response of transcripts to the environment is driven by major factors known to affect oyster physiology such as temperature, pH, salinity, and dissolved oxygen, with pollutant levels playing a relatively small role, at least within the range of concentrations found in the studied oyster habitats. Further, the two environmental factors that dominate these effects (temperature and pH) interact in a dynamic and nonlinear fashion to impact gene expression. Transcriptomic data obtained in our study provide insights into the mechanisms of physiological responses to temperature and pH in oysters that are consistent with the known effects of these factors on physiological functions of ectotherms and indicate important linkages between transcriptomics and physiological outcomes. Should these linkages hold in further studies and in other organisms, they may provide a novel integrated approach for assessing the impacts of climate change, ocean acidification and anthropogenic contaminants on aquatic organisms via relatively inexpensive microarray platforms.