Edouard Kraffe

In a previous study we compared lipid composition and functional parameters of circulating cells from Cerastoderma edule affected or not by disseminated neoplasia (neoplastic cells vs hemocytes) (Le Grand et al. Chem Phys Lipids 167:9–20 ). Neoplastic cells presented morpho‐functional modifications concomitant to striking membrane lipid alterations: the proportion of particular plasmalogen molecular species was drastically decreased. We wanted to test whether this pattern was representative of bivalve neoplastic cells. For the purpose, a similar study was conducted on another bivalve species affected by disseminated neoplasia, the soft‐shell clam (Mya arenaria). Although total reactive oxygen species production was unaffected, M. arenaria neoplastic cells presented some functional alterations: phagocytosis activity was reduced by 33 %. However, lipid compositions were not drastically altered. Particularly, sterol and plasmalogen levels did not differ between both cell types (about 43 % of membrane lipids and 35 % of phospholipids, respectively in hemocytes and neoplastic cells). This could be related to the fact that disseminated neoplasia was not related to hemolymph cell proliferation in M. arenaria (0.9 ± 0.2 10⁶cell mL⁻¹, considering both healthy and neoplastic clams, n = 6). Nevertheless this study highlighted minor but specific alterations of membrane lipid composition in M. arenaria neoplastic cells. The only phospholipid subclass in which the fatty acid profile strongly differed between both cell types was serine plasmalogen (PlsSer), with neoplastic cells presenting lower specific enrichment of 20:1n‐11 in PlsSer. Such specific alteration of membrane lipid composition strengthened the assumption of an implication of key plasmalogen molecular species in this leukemia‐like disease in bivalves.

Several parameters can affect membrane lipid composition in bivalves, including diet. Although two fatty acids (FA) 22:6n-3 and 20:5n-3 are essential membrane components, they are sparingly synthesized by bivalves and must be obtained from their diet. Here, effects of dietary modifications of membrane lipid composition were studied at both cellular and subcellular levels in the oyster Crassostrea gigas. To this end, we compared oysters fed two monoalgal diets that differed markedly in their FA composition and a mix of both. As expected, algae impacted phospholipids, in particular 22:6n-3 and 20:5n-3, reflecting differences of dietary microalgae FA composition. Meantime, total saturated FA, total monounsaturated FA, total polyunsaturated FA and total non-methylene-interrupted FA varied little and phospholipid class composition was only slightly affected by diets. Measures made in hemocytes indicated that only mitochondrial membrane potential was affected by diets. Total ROS production as well as mitochondrial superoxide production did not differ with diet. There was no difference in phosphorylating (state 3) and non-phosphorylating (state 4) rates of oxygen consumption rates or in cytochrome c oxidase activity of mitochondria isolated from gills between the three diets. Similarly, neither cytochromes a, b, c or c ₁ content nor citrate synthase activities were changed, suggesting that number and morphology of mitochondria were not affected by dietary treatment. These results suggest that oysters could possess high homeostatic capabilities, at both cellular and subcellular levels, to minimize the effect of dietary FA and related membrane lipid FA modifications on mitochondrial functions. These capabilities could be a means to face variations in diet composition in their natural environment and to preserve important oyster physiological functions such as growth and reproduction.

Sessile animals that live on the foreshore undergo tidal cycles, and have to face variations in physical and chemical parameters such as oxygen concentration. During emersion, availability of dissolved oxygen can be lowered for bivalves, which have only a small reserve of seawater inside their closed shell. Differences in oxygen concentration are thus expected to lead to modifications of the metabolism, including changes in mitochondrial activity. Previous studies investigated air exposure under extreme conditions, which do not always reflect environmental conditions these invertebrates have to cope with. In this study, oxidative capacities of gill mitochondria of the oyster Crassostrea gigas were studied during a tidal cycle period, by comparing oysters collected after emersion and immersion. Only minor differences were found in state 3 (oxidative phosphorylation) or state 4 (non-phosphorylating oxygen consumption) rates between the two conditions. Similarly, no difference was observed in cytochrome c oxidase activity or in oxygen consumption related to maximal electron flux through complexes I-IV, II-IV and IV. While capacities of substrate oxidation were maintained in both emersion and immersion conditions, capacity of mitochondria to produce adenosine triphosphate (ATP) was significantly lower in oysters sampled during emersion. These results suggest that although C. gigas could maintain aerobic metabolism during emersion period within a tidal cycle in its environment, energy producing mechanisms are affected.

Summary: As oxygen concentrations in marine coastal habitats can fluctuate rapidly and drastically, sessile marine organisms like the oyster Crassostrea gigas can experience marked and rapid oxygen variations. In this study, we investigated the responses of oyster gill mitochondria to short-term hypoxia (3 h and 12 h, at 1.7 mg O2 L-1) and subsequent re-oxygenation. Mitochondrial respiratory rates (state 3 and 4 stimulated by glutamate), phosphorylation efficiency (RCR ratio and ADP/O) were measured. Cytochrome c oxidase activity (CCO) and cytochrome concentrations (a, b, c1, c) were measured to investigate the rearrangements of respiratory chain subunits. The potential implication of an alternative oxidase (AOX) was investigated using an inhibitor of the respiratory chain (antimycin A) and through gene expression analysis in gills and digestive gland. Results indicate a down regulation of mitochondrial capacity, with 60% inhibition of respiratory rates after 12 h of hypoxia. RCR ratio remained stable, while ADP/O increased after 12 h of hypoxia and 1 h of re-oxygenation, suggesting increased phosphorylation efficiency. CCO showed a fast and remarkable increase of its catalytic activity only after 3 h of hypoxia. AOX mRNA levels showed similar patterns in gill and digestive gland, and were up regulated after 12 and 24 h of hypoxia and during re-oxygenation. Results suggest a set of controls in regulating mitochondrial functions in response to oxygen fluctuations and demonstrate the fast and extreme plasticity of oyster mitochondria in response to oxygen variations.

The present study aimed to elucidate the effective phylogenetic specificity of distribution of a cis‐4,7,10, trans‐13‐22:4 (22:4(n‐9)Δ13trans) among pectinids. For this purpose, we extended the analysis of membrane glycerophospholipids FA composition to 13 species of scallops, covering 11 genera and 7 tribes representatives of the three subfamilies Chlamydinae, Palliolinae and Pectininae and the subgroup Aequipecten. In species belonging to the subfamily Pectininae and the Aequipecten subgroup, 22:4(n‐9)Δ13trans was found in substantial amounts, but it was absent in other species belonging to the subfamilies Chlamydinae and Palliolinae. Homologous non‐methylene‐interrupted (NMI) FA, also hypothesized to differ along phylogenetic lines in bivalves, were totally absent or present only in trace amounts in representatives of the Aequipecten subgroup but ranged from 0.3 to 4.5% of the total FA in Pectinidae, Chlamydinae, and Palliolinae subfamilies. The species‐specific occurrence of NMI and 22:4(n‐9)Δ13trans FA in membrane lipids of pectinids agrees with the most recent phylogenies based on shell morphology and molecular characteristics. We examined the potential timing of the appearance of 22:4(n‐9)Δ13trans in pectinids on a geologic time scale.