Genetic responses to metal contamination in two clams: Ruditapes decussatus and Ruditapes philippinarum

Coastal ecosystems are subjected to a wide variety of disturbances, including those due to xenobiotics of agricultural and industrial origin. These pollutants as heavy metals can modify the genetic diversity of populations by favouring or counter-selecting certain alleles or genotypes by differential mortality. In the present study, two genetic markers (phosphoglucomutase and glucosephosphate isomerase) and a protein marker (metallothionein) were monitored in order to determine the impact of heavy metals in different clam populations. Analysis of the genetic structure of the clam populations examined reveals that those inhabiting environments contaminated by heavy metals exhibit a higher allelic diversity and possess alleles at PGM loci that could be selected by the presence of heavy metals. The evaluation of metallothionein levels using a specific polyclonal antibody developed in the Pacific oyster (Crassostrea gigas) demonstrated the existence of a relationship between metallothionein concentrations and the level of metal pollution for clam populations sampled from different sites. An inter-specific difference was also detected between Ruditapes decussatus and Ruditapes philippinarum living in sympatry at the same site, suggesting a differential response of these two species upon exposure to an identical heavy metal concentration.     

Morphosedimentary evolution of the macrotidal Seine estuary subjected to human impact

Many estuaries in the world have been subjected to significant human impact since the 19th century. Natural infilling and human activities, including building of embankments, dikes and jetties have modified both the morphology of these estuaries and the distribution of sedimentary facies. The Seine, a macrotidal estuary, provides a good opportunity to study such modifications, as the latest morphosedimentary observations could be compared to old sediment maps as well as to geotechnical drilling data. In the mouth of the Seine, originally a sand/gravel system sediment distribution, has now been transformed into a muddy system. The result is a regressive sequence several meters thick. It is typically fining-up and corresponds to a shift from a distal term (pebbles, gravel, and coarse sands) at its base to a proximal term (mud and fine sands) at its top. Civil engineering works have reduced the available amount of space within the estuary, leading to an increase in the natural downstream shift of the depocenter of mud brought by winter river floods. The deposition area of the mud is today in the open marine zone, where waves and tidal currents render the balance of this process precarious.     

Implications of rapid footpoint motions of photospheric flux tubes for coronal heating

Some recent observations at Pic-du-Midi (Mulleret al., 1992a) suggest that the photospheric footpoints of coronal magnetic field lines occasionally move rapidly with typical velocities of the order 3 km s^–1 for about 3 or 4 min. We argue that such occasional rapid footpoint motions could have a profound impact on the heating of the quiet corona. Qualitative estimates indicate that these occasional rapid motions can account for the entire energy flux needed to heat the quiet corona. We therefore carry out a mathematical analysis to study in detail the response of a vertical thin flux tube to photospheric footpoint motions in terms of a superposition of linear kink modes for an isothermal atmosphere. We find the resulting total energy that is asymptotically injected into an isothermal atmosphere (i.e., an atmosphere without any back reflection). By using typical parameter values for fast and slow footpoint motions, we show that, even if the footpoints spend only 2.5% of the time undergoing rapid motions, still these rapid motions could be more efficient in transporting energy to the corona than the slow motions that take place most of the time.     

Quaternary sedimentary regime on the Berthois Spur (Bay of Biscay)

A study of cores from the Celtic Margin shows that high terrigenous accumulation rates occurred at the end of isotopic stage 3 and during isotopic stage 2. These high rates prevailed during rapid changes from relatively warm conditions (Älesund interstadial) to maximum glacial condition. The input of terrigenous material may have culminated with the blockage of the North Sea by an ice sheet and the diversion of NW European flumes toward the Celtic Margin. The uppermost sediments are in a state of overconsolidation as the consequence of erosional processes following the episod of high sediment supply.     

Xenobiotic-induced immunomodulation in the European flat oyster,Ostrea edulis

The presence of chemical contaminants in marine coastal waters is a major subject of concern since many molecules are potentially immunotoxic, even at low concentration. During the last decade, studies in sentinel species, such as the blue mussel, Mytilus edulis, or the Pacific oyster, Crassostrea gigas, have revealed that immunosuppressive responses can be related to xenobiotic exposure, in the laboratory and in the field. In the present study, European flat oysters were experimentally exposed to heavy metals, to investigate possible alterations of their immune function. Several hematological and functional parameters of hemocytes were measured by flow cytometry, a technique allowing rapid, sensitive, cell-by-cell measurements in large cell populations. Results reveal a depression of phagocytosis and several subcellular, physiological changes in oysters exposed to cadmium alone or to cadmium and copper, suggesting an overall alteration of the phagocytic function.