Systématique et biogéographie du genre Lamprothamnium (Characées) Caractéristique des biotopes aquatiques saumâtres

La publication fait le point des connaissances sur le genre Lamprothamnium dans les domaines de la morphologie et du cycle de développement, de l'écologie et des végétations associées, de la répartition des espèces.Toutes les espèces monoïques de Lamprothamnium, parfois difficiles à distinguer les unes des autres, forment un groupe naturel dans la tribu des Chareae. Elles sont caractérisées par un thalle sans cortication et une seule rangée de stipulodes plus ou moins développés et disposés un sous chaque phylloïde. WOOD et IMAHORI (1965) distinguent trais espèces (avec six formes) auxquelles DONTERBERG (1984) ajoute un nouveau taxon endémique d'Argentine. Une clé systématique remaniée est proposée.C'est le seul genre de Characées dont toutes les formes, peuplant les eaux saumàtres et salées, peuvent supporter de larges et soudaines fluctuations du taux de salinité (eau hypersaline à eau douce et vice versa).La germination s'opère en eau de faible salinité, à température moyenne et sur un substrat meuble reposant sur de la vase compacte et salée dans des milieux souvent peu profonds, ensoleillés, donc à échauffement diurne notable.Les biotopes aquatiques, temporaires ou permanents, sont généralement peuplés de végétations mono- ou polyspécifiques de Characées, souvent denses, où peuvent s'introduire des Algues et des Phanérogames halophiles.Toutes les espèces sont en voie de raréfaction notable en raison de la profonde modification des milieux très spéciaux qui leur sont favorables.L. papulosumse trouve être, à l'échelle mondiale, la première espèce de Cryptogames à recevoir le statut de plante protégée en Grande-Bretagne (MOORE, 1991).All monoecious species of Lamprothamnium J. Gr., which often are difficult to distinguish from each another, from a natural group within the Chareae Tribe. The plants, from 5 to 80 cm high, are characterized by a thallus without codex round the axes and the branchlets, and one tier of stipulodes these ones are uniform or of irregular size, as numerous as the branchlets, and inserted one below each branchlet. The bract-cells are well developed and verticillate (fig. 1). The common form tends to develop compact foxtail-like upper portions ; but a plant with the diffuse habit and extremely elongated branchlets can grow tell in some deeper brackishwater biotopes.The male and female gametangia are conjoined or sejoined on the branchlet modes (one of each) and sometimes aggregated at the base of the branchlets. The oogonium below the antheridium is the predominating position. The table 1 is bringing out the measurements of the male and female gametangia published in a few taxa of Lamprothamnium. It is obvious that the differences measured between them are so small that they are not a bit help in the species determination.The dried plants kept in the herbarium have become often breakable and cannot be investigated. The fresh plants must be preserved in ethanol at 70 % or in formalin at 5 %. The measurements of all parts of the fresh plants have been taken in water under the stereo- or the lightmicroscope with the help of a micrometer eyepiece. The mature plants with sex organs were fixed in acetic ethanol in the ratio of 1/3 for karyological studies, then the standard staining and squashing methods were followed. The chromosomes have been counted in the cells of the antheridial filaments (GUERLESQUIN,1967).A few chromosome numbers of four taxa have been published in Lamprothamniumpapulosum (n = ca 25, 50, 56 in Europe; n = 70, 72 in Ouzbekistan; n =14, 28, 30 in Australia on some doubtful samples); Lamprothamnium macropogon (n = 28, 55 in Australia); Lamprothamnium succinctum (n =14, 42 in India; n =42 in New Caledonia); Lamprothamnium succinctum var. australiensis (n = 42 in Australia).WOOD and IMAHORI (1965) have distinguished three similar species and six forms to which Donterberg (1984) added a new taxon endemic in Argentina. A revised key to the species and varieties of Lamprothamnium is propounded (table 2):1) Stipulodes well developed, of uniform size, male and female gametangia growing together:a) gametangtaa atbranchiet nodes and at base of branchlet : L. papulosum;b) gametangia restricted to branchlet nodes : L. hansenii;2) Stipulodes irregular in size or absent :a) male and female gametangia growing on different branchlet nodes and at base of branchlets : L. succinctum ;b) male and female gametangia growing on the same branchlet nodes only: L. haesseliae.A few taxa of an uncertain value have been described such as Lamprothamnium mediterraneum (Lovric, 1979, 1980).Lamprothamnium is the only genus of Characeae of which all brackish and saline water forms can tolerate wide and sudden fluctuations of the salinity rate (fresh- to hypersaline water and vice versa, NaCl, MgCl2, calcium sulphate). The physicochemical composition of a few brackishwater biotopes of Western Europe is mentioned in the table 3. The ratio of Cl- can vary from 9 g. 1-1 to 59 g. 1-1 (more than six times); this one of the total salts from 32 g. l-1 to 170 g. l-1 (about five times). This tact is making clear the adaptability of these plants to the fluctuations. All the taxa of Lamprothamnium are sun-loving plants and thereby they can grow in the biotopes with an appreciable warming action of the sun during the day. In the temporary ponds, the plants are enough quick to develop their life cycle completely before the whole drying up.The germination occurs in a slightly brackishwater with a mild temperature, after a dormancy time more or less long (from a few months to several years : six in Morocco, GUERLESQUIN et al., 1987). From the germination of the oospore to the fertile plant with mature gametangia, the developmental process needs two or three months.The aquatic biotopes can be covered with dense mono- or polyspecific vegetations of Characeae into which halophilous Algae (Cyanophyceae, Chlorophyceae) and Angiosperme (Ruppia sp., Zannichellia sp., Althenia sp., Zostera sp. pl., Potamogeton pecfinatus, Groenlendia densa, etc.), are growing together. The brackish biotopes must be permanent (salt- or brackish lakes, stagnant or lightly running water, etc.), temporary (coastal lagoons, salt-marshes, etc.), continental or by the sea.The taxa of Lamprothamnium are widespread in scattered sites between 20° N and 59° N, from Sahara to Southern Norway, also in South Africa, Mauritius, Australia and Tasmania, New Zeatand, New Caledonia, Japan, Eastern Asia (Pakistan, India, China), South America (Bolivia, Argentina). Now they are unknown in North and Central America, and the Caribbean Islands. L. papulosum is the mort widely distributed species (maps 1 and 2).All the forms are becoming rarefied considerably owing to the great modifications of the very special favourable environments such as the draining or the filling in of the salt-water lakes and the coastal salt-pans. The localities where L. papulosum, the only species present in France, was seen again recently, are very few (map 1).In its quinquennial review (1987), the Nature Conservancy Council has decided to protect L. papulosum, a rare Charophyte in Great Britain, and its favourable habitats. These are often regarded as unproductive « wasteland » and frequently threatened by « the development at recreational amenities, holiday accomodations, shipping and salt industries » which induce pollution, disturbance and complete drying. Thus L. papulosum will be the first species of Cryptogams to receive the status of protected plant in the whole world (Moore, 1991)

Model of a complex between the tetrahemic cytochrome c3 and the ferredoxin I fromDesulfovibrio desulfuricans (Norway strain)

International audienceA three-dimensional model of an electron-transfer complex between the tetrahemic cytochrome c3 and the ferredoxin I from the sulfatereducing bacterium Desulfovibrio desulfuricans (Norway strain) has been generated through computer graphics methods. The model is based on the known X-ray structure of the cytochrome and on a model of the ferredoxin that has been derived through computer graphics modeling and energy minimization methods, from the X-ray structure of the homologous ferredoxin from Peptococcus aerogenes. Four possible models of interaction between the two molecules were examined by bringing in close proximity each of the four hemes and the redox center (4Fe-4S) of the ferredoxin and by optimizing the ion pairs interactions. One of these models shows by far the “best” structure in terms of charges, interactions, and complementary f the topology of the contact surfaces. In this complex, the distance between the iron atoms of the ferredoxin redox center and the hemic iron atom is 11.8 Å, which compares well with those found between redox centers in other complexes. The contact surface area between the two molecules is 170 Å2