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)

Variabilité spatio-temporelle des groupements végétaux d'un étang angevin (France)

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Crystal structure of cytochrome c3 from Desulfovibrio desulfuricans Norway at 1.7 A resolution.

International audienceThe crystal structure of cytochrome c3 (M(r) 13,000) from Desulfovibrio desulfuricans (118 residues, four heme groups) has been crystallographically refined to 1.7 A resolution using a simulated annealing method, based on the structure-model at 2.5 A resolution, already published. The final R-factor for 10,549 reflections was 0.198 covering the range from 5.5 to 1.7 A resolution. The individual temperature factors were refined for a total of 1059 protein atoms, together with 126 bound solvent molecules. The structure has been analyzed with respect to its detailed conformational properties, secondary structure features, temperature factor behaviour, bound solvent sites and heme geometry and ligation. The characteristic secondary structures of the polypeptide chain of this molecule are one extended alpha-helix, a short beta-strand and 13 reverse turns. The four heme groups are located in different structural environments, all highly exposed to solvent. The particular structural features of the heme environments are compared to the four hemes of the cytochrome c3 from Desulfovibrio vulgaris Miyazaki.The crystal structure of cytochrome c3 (M(r) 13,000) from Desulfovibrio desulfuricans (118 residues, four heme groups) has been crystallographically refined to 1.7 A resolution using a simulated annealing method, based on the structure-model at 2.5 A resolution, already published. The final R-factor for 10,549 reflections was 0.198 covering the range from 5.5 to 1.7 A resolution. The individual temperature factors were refined for a total of 1059 protein atoms, together with 126 bound solvent molecules. The structure has been analyzed with respect to its detailed conformational properties, secondary structure features, temperature factor behaviour, bound solvent sites and heme geometry and ligation. The characteristic secondary structures of the polypeptide chain of this molecule are one extended alpha-helix, a short beta-strand and 13 reverse turns. The four heme groups are located in different structural environments, all highly exposed to solvent. The particular structural features of the heme environments are compared to the four hemes of the cytochrome c3 from Desulfovibrio vulgaris Miyazaki