Substituent effect in unsymmetrical lutetium bisphthalocyanines: a DFT analysis

International audienceDFT calculations have been carried out on a series of unsymmetrical bisphthalocyanine lutetium complexes in which one of the ligands is substituted by 8 or 16 chlorine atoms. It is shown that their unpaired electron is predominantly localized on the non-substituted ligand. An orbital explanation is provided to rationalize this effect. A good agreement is found between the computed and experimental ionization potentials and electroaffinities and the redox potentials of a closely related series, as well as between their TDDFT-computed and experimental UV-visible transitions which are analysed

Carbon, nitrogen and phosphorus elemental stoichiometry in aquacultured and wild-caught fish and consequences for pelagic nutrient dynamics

International audienceThe elemental carbon (C), nitrogen (N) and phosphorus (P) compositions of the whole-body and gut content of wild marine fish inhabiting the Bay of Biscay (Northeast Atlantic) were studied. Furthermore, the literature was examined for studies of aquacultured fish, reporting the elemental composition of the whole-body fish, that of their food, and nutrient assimilation and gross growth efficiencies (GGE). In both wild-caught and aquacultured fish, significant differences in C, N and P elemental composition were found between species, with P being the most variable component. Differences among species in terms of C, N and P content could be explained by varying proportions of storage compounds in whole-body fish, and varying degrees of ossification. Aquacultured fish feces were found to be P-rich, because of a lower P assimilation efficiency, compared to C or N assimilation efficiencies. Examination of aquacultured fish literature also revealed that C, N and P GGE and nutrient resupply ratios agreed with basic principles of homeostatic regulation of whole-body fish elemental composition. Extrapolation of the results to broader marine systems indicated that fish may be important for conveying nutrients toward the ocean interior

Response of a phytoplankton community to increased nutrient inputs: A mesocosm experiment in the Bay of Brest (France)

International audienceIn the Bay of Brest (France), benthic fluxes of silicic acid (DSi) are thought to be strongly involved in diatom dominance and their intensity depends on the density of an invasive gastropod, Crepidula fornicata. While the proliferation of this gastropod is likely to support diatom dominance, local fishermen have proposed its removal as it perturbs the development of the native Great Scallop, Pecten maximus. However, such a removal may be associated with potentially harmful algal blooms. The response of the phytoplankton community to modifications of the benthic fluxes of DSi, resulting from a further proliferation or the removal of C fornicata, was examined in a mesocosm nutrient enrichment experiment over a period of 16 days in the spring of 2004. Six mesocosms (5.4 m(3)) were filled using a natural planktonic assemblage from the Bay and received N, P and Si additions at the stoichiometric ratio of the in situ benthic fluxes (Si:N:P=16:16:1), with a gradient of enrichment ranging from 0 to almost 4 times the maximal benthic flux measured in the Bay. The diatom Chaetoceros (mainly C. debilis) rapidly outgrew non-siliceous species of microphytoplankton in all enclosures although kinetic experiments provided direct evidence of Si limitation. This dominance of diatoms at DSi concentrations <2 mu M highlighted that the residual DSi concentration measured daily in the mesocosms is not a reliable criterion to predict diatom dominance in marine ecosystems whereas the dependence of the Si uptake rate on fertilization rates suggests that emphasis should be placed on daily nutrient addition. Quantitative modifications of nutrient additions only changed the magnitude of the diatom bloom and high fertilization rates may lead to high sedimentation fluxes of organic material, which can strongly harm the entire benthic community. Finally the diatom dominance at low fertilization rates raised questions about the role of benthic fluxes in preventing harmful algal blooms within the Bay system