Fatal attraction: rare species in the spotlight

The exploitation of rare and endangered species can end in the species's extinction because the increased value people associate with rarity increases the economic incentive to exploit the last individuals, creating a positive feedback loop. This recently proposed concept, called the anthropogenic Allee effect (AAE), relies on the assumption that people do value rarity, but this remains to be established. Moreover, it also remains to be determined whether attraction to rarity is a trait confined to a minority of hobbyists (e.g. wildlife collectors, exotic pet owners) or characteristic of the general public. We estimated how much the general public valued rare species compared with common ones, using five different metrics related to personal investment: time spent, physical effort, unpleasantness, economic investment and risk. We surveyed the visitors of a zoo. To see the rare species, the visitors to the zoo invested more time in searching and contemplation, they were ready to expend more physical effort, they tolerated more unpleasant conditions, they were willing to pay more and, finally, they risked more to obtain (steal) a rare species. Our results provide substantial evidence of how the general public places more value on rare species, compared with common species. This confirms the AAE as an actual process, which in addition concerns a large part of the population. This has important consequences for the conservation of species that are rare now, or that could become so in the future

Nanogels de caséines réticulées: approches colloïdale et interfaciale

National audienceDans le lait, les molécules de caséines (25g/l) sont associées à du phosphate de calcium pour former des suprastructures appelées micelles de caséines. A pH 6,7, ces suprastructures correspondent à des colloïdes poreux, chargés négativement (-20 mV), très hydratés avec un diamètre moyen d’environ 150 nm [1, 2]. En ce sens, ces suprastructures peuvent être considérés comme des nanogels [3]. Cependant ces structures sont très dépendantes des conditions physico-chimiques et peuvent se dissocier notamment à des pH acides ou après l’addition de chélatants du calcium. Les objectifs de ce travail étaient de : 1. Réticuler les micelles de caséines afin de créer de nouveaux nanogels stabilisés. Pour les réticuler, la génipine, molécule extraite des fruits de Gardenia jasminoides a été utilisée car elle est décrite comme réactive avec les groupements aminés libres des protéines [4]. Dans cette étude, la génipine a été ajoutée à des suspensions micellaires (25 g/l à pH 7.15) à des concentrations finales de 5, 10 et 20 mM. La réaction était réalisée à 50°C pendant 24h. Un témoin (sans génipine) était traité dans les mêmes conditions ; 2. Caractériser physico-chimiquement ces nouvelles structures avec une attention particulière portée sur les acides aminés impliqués dans la réaction et les conséquences sur les propriétés physiques et interfaciales des micelles de caséines réticulées. Les acides aminés lysine et arginine étaient impliqués dans la réaction, induisant une réticulation intramicellaire sans pour autant modifier la taille micellaire (~160 nm de diamètre). Ces micelles réticulées étaient plus chargées négativement et moins hydratées. L’analyse microscopique de leur surface montra des aspects allant de rugueux (pour le témoin) à lisse (pour les micelles réticulées). Les micelles de caséines ainsi réticulées étaient résistantes à la dissociation notamment induite par le citrate et l'urée, suggérant la formation d’une suprastructure stable, contrairement aux micelles natives. En termes de propriétés physiques, la viscosité des suspensions étaient diminuées en fonction du degré de réticulation et probablement en relation avec la diminution de la voluminosité des micelles. Par ailleurs, les valeurs finales de tension de surface ainsi que le temps nécessaires pour atteindre ces valeurs augmentaient avec le degré de réticulation. La rhéologie interfaciale (liquide/air) montra que les micelles natives se réorganisaient et formaient une interface viscoélastique, tandis que les nanogels réticulés formaient des interfaces plus élastiques. Les perspectives de ce travail sont de poursuivre les études physiques de ces nanogels de caséines réticulés dans différentes conditions physico-chimiques. Les nanogels de caséines ainsi formés nous semblent posséder de nombreuses potentialités, notamment comme nanotransporteurs, qu’il est essentiel de tester dans de futures études

Physico-chemical characterization of casein micelles cross-linked by genipin

Modifications of casein micelles (CM) constitute a challenge to create new structures and functionalities. In this context, genipin, which is a cross linking agent, was used to modify CM and the objective was to evaluate their new physico-chemical properties. Genipin (5, 10 and 25 mM) was added to casein micelle suspensions (25 g.l-1) dispersed in a buffer system ( 25 mM HEPES and 2 mM CaCl2, pH 7.15). The reaction was carried out at 50 °C during 24 h. A control sample without genipin was treated in the same conditions. The genipin reaction was monitored by changes in UV/visible-spectra between 190-900 nm. The reacted products were evaluated by reversed-phase liquid chromatography (RP-LC) and sodium dodecyl sulfate – polyacrylamide gel electrophoresis (SDS-PAGE). The reaction level was estimated by the measurement of the available lysine and arginine. The consequences of the reaction on the physico-chemical characteristics of casein micelles were investigated by measuring their zeta potentials, size distributions; hydrations; shapes by scanning electron microscopy (SEM); viscosity, and surface tension by pendant drop method. The reaction between genipin and casein molecules was characterized by formation of blue pigmented products, presenting a maximum absorption at ~600 nm. Higher were the genipin concentrations, more intense was the UV absorption. The RP-LC profiles showed that above 5 mM of genipin, individual casein molecules were not separated. SDS-PAGE gels confirmed these results and revealed that casein molecules formed polymers with molecular weights greater than 200 kDa. The analysis of amino-acids highlighted that lysine was mainly involved in cross-linking, with a minor participation of arginine. By comparing to control sample, the percentage reduction in the concentrations of lysine and arginine reached 95 and 12 %, respectively, for 25 mM of genipin added. The zeta potential and hydration values of CM were gradually reduced, indicating changes in their surface properties. The size distribution and SEM did not revealed major changes regarding the average diameter and shape of the CM. Internal cross-linking was confirmed by submitting the CM to dissociation conditions, followed by ultracentrifugation and analyses of the mass of pellet obtained. Regarding the viscosity, all the samples behaved as Newtonian fluids, nevertheless the values decreased progressively according to cross-linking intensity. Contrarily, the final values of surface tension as well as the adsorption rates at air/water interfaces increased gradually. Thanks to this new reagent, it was possible to create originally modified CM. Further researches are necessary to elucidate the modifications in their internal structure and the consequences in their functional properties

Crosslinking of casein micelles by Genipin

Casein micelles (CM) are porous colloidal particles of about 150 nm of diameter, composed of different casein molecules and calcium phosphate. Their modifications, by technological treatments and/or addition of bio-chemical reagents, constitute a scientific challenge to create new structures and functionalities. In this work, modification of CM by a natural cross-linker, named genipin, was performed and their new physico-chemical properties were appreciated. Genipin (5, 10 and 20 mM) were added to micellar suspensions of casein at 25 g.l-1, previously dispersed in a buffer system consisting of 25 mM HEPES and 2 mM CaCl2 at pH 7.15. The reaction was carried out at 50 °C for 24 h. A control sample was treated in the same conditions. The experiment was done 3 times independently. The genipin reaction was monitored by changes in UV/visible-spectra between 190-900 nm during incubation at 50 °C. After that, the reacted products were evaluated by reversed-phase high pressure liquid chromatography (RP-HPLC) and sodium dodecyl sulfate – polyacrylamide gel electrophoresis (SDS-PAGE). Chromatography and electrophorese were performed under dissociation conditions. The reaction level was estimated by the measurement of the available lysine and arginine. The consequences of the cross-linking on the colloidal properties were investigated by measuring: zeta potential and size distribution by dynamic light scattering; hydration after pellet formation by ultracentrifugation; shape by scanning electron microscopy (SEM); viscosity at shear rates lower than 100 s-1; and surface tension by pendant drop method. The reaction between genipin and primary amines was characterized by formation of blue pigmented products, presenting a maximum absorption at ~600 nm. Higher were the genipin concentrations, more intense was the light absorption. The RP-HPLC profiles showed that above 5 mM of genipin, it was not possible to observe individual casein molecules. SDS-PAGE gels confirmed these results and revealed that casein molecules formed polymers with molecular weights greater than 200 kDa. The analysis of amino-acids highlighted that lysine was mainly involved in cross-linking, with a minor participation of arginine. By comparing to control sample, the percentage reduction in the concentrations of lysine and arginine reached 95 and 12 %, respectively, for 20 mM of genipin. The zeta potential and hydration values of CM were gradually reduced, indicating changes in their surface properties. The size distribution and SEM did not revealed major changes regarding the average diameter and shape of the CM. Internal crosslinking was confirmed by submitting the CM to dissociation conditions, followed by ultracentrifugation and analyses of the mass of pellet obtained. Regarding the viscosity, all the samples behaved as Newtonian fluids, nevertheless the values decreased progressively according to cross-linking intensity. Contrarily, the final values of surface tension as well as the adsorption rates at air/water interfaces increased gradually. Thanks to this new reagent, it was possible to create originally modified CM. Further researches are necessary to elucidate the modifications in their internal structure and the consequences in their functional properties, like gels, emulsions and foams