Continental Antarctica is a polar desert containing sparse pockets of vegetation within ice-free areas. Despite the recognized association between lichens, mosses and epiphytic diatoms, the environmental factors controlling diatom community structure are poorly understood. We investigated the association between diatom communities and host vegetation characteristics by experimentally adding nutrients and/or water to two bryophyte (healthy and moribund) and two lichen (crustose and Usnea) vegetation types in the Windmill Islands. Diatom communities were morphologically characterized, diversity indices calculated and differences between treatments, vegetation type and vegetation characteristics tested. We identified 49 diatom taxa, 8 of which occurred with > 1% relative abundance. Bryophyte and lichen vegetation harboured significantly different diatom communities, both in composition and diversity indices. Specifically, Luticola muticopsis was more prevalent in moribund bryophytes and crustose lichens, and Usnea lichens showed lower species richness than other types. While nutrient and water additions did not significantly alter diatom communities, diversity indices and some species showed relationships with vegetation physiological characteristics, notably %N and δ13C, suggesting the importance of ambient gradients in water and nutrient availability. Collectively, this work suggests that future conditions favouring the dominance of a particular vegetation type may have a homogenizing effect on the terrestrial diatom communities of East Antarctica.

Diatoms are good environmental indicators and are often the main component of phytobenthos, one of the key groups of organismsrecommended by the Water Framework Directive (W.F.D.) for the assessment of ecological status of surface waters.Although diatoms are regularly used as indicators in rivers and in lakes, less is known about the benthic diatom communities inwetland systems, especially in Italy. The main aim of the project was to provide a contribution to the knowledge on distribution,composition and structure of benthic diatom communities in wetland systems in Italy. In 2005, we investigated the benthic diatomcommunities and their relationships to water variables of nine ponds and wetlands along the Tyrrhenian coast near Rome.Furthermore, a first application of seven European diatom-based indices on this type of water body was undertaken. A total of225 species and varieties belonging to 54 genera were identified. The Canonical Correspondence Analysis indicated that certainspecies were closely associated with each wetland type and their distribution mainly depended on nutrient and chloride contents:pollution tolerant species (i. e. Luticola goeppertiana, Mayamaea atomus, Navicula subminuscula) in the wetlands connected toriver mouths, halophilous species and also marine littoral species (i. e. Nitzschia elegantula, Amphora coffeaeformis, Achnanthesbrevipes var. intermedia, Epithemia adnata var. porcellus) in the coastal brackish ponds and wetlands, and less eutrophic species(i. e. Achnanthidium minutissimum, A. biasolettianum) in the lowland freshwater wetland. In general, the indices classifiedmost of the studied water bodies as bad, poor and moderate status; few sites were classified into a good class, even for at leastin one sampling occasion. Eutrophication and Pollution Index (EPI-D), Trophic Diatom Index (TDI), Taxa Pollution Tolerant(%PT), Pollution Sensitivity Index (IPS) and Generic Diatom Index (IDG) were significantly correlated to nutrient contents inthe studied wetlands. Our preliminary data set illustrated the potential of the use of these benthic algae as bioindicators in wetlands.

A new diatom (Bacillariophyceae) genus and two new species are described from the arid region of the North American southwest. The new genus, Playaensis, and new species, P. circumfimbria and P. furtiva, are distinguished by frustule morphology and autecology. Playaensis possesses unique, narrowly lunate longitudinal ribs bordering the raphe and spathulate spines, but their evolutionary relationship to known genera is unclear. In Payton Lake, New Mexico, P. circumfimbria and P. furtiva are rare in the diatom community, and their geographic and geological extent are not known, but they may be restricted to alkaline, slightly saline lakes of the region.

The development of auxospores and initial cells of the freshwater chain-forming diatom Ellerbeckia arenaria forma arenaria was studied using light and electron microscopy. Whether the auxospore mother cell is formed vegetatively (parthenogenesis) or through autogamy is still a matter for speculation. Because no signs of spermiogenesis could be detected in our material, we exclude oogamy. Further development involves retraction of the protoplast from the epivalve and the formation of a featureless siliceous cap with a central opening on its free surface. The homology and function of this cap remain uncertain. At the other end of the cell the protoplast remains tightly attached to the hypovalve throughout its further development. This hypovalve always has a concave valve face. An organic wall bearing many layers of overlapping, small silica scales begins to be formed while the cell is still enclosed within the mother cell wall, and allows the auxospore to swell, thus breaking the siliceous cap and girdle bands. External and internal surfaces of scales can be distinguished morphologically. Initial valves are formed internal to the scales at an oblique angle to the parent chain. The initial epivalve always faces the hypotheca of the auxospore mother cell, and the initial valves already show elements of the heterovalvy manifest in the vegetative cells. The significance of the observations in terms of ecology and phylogeny is discussed. The combinations Ellerbeckia arenaria (Moore ex Ralfs) R. M. Crawford, and E. arenaria forma teres (Brun ex Schmidt) R. M. Crawford are made.

Bioassays were carried out with the ‘blue diatom' Haslea ostrearia Simonsen, which is responsible for oyster greening during the fattening period of Crassostrea gigas Thunberg in oyster ponds. Samples of seawater were taken from two oyster ponds: one without oysters and the other with 20 oysters per m[sup2 ], maximal density allowed by the French AFNOR norm for ‘refinement'. The aims were to clarify the nutrient requirements of this diatom, also to elucidate the eventual influence of C. gigas at this density on the seawater fertility and to envisage the mass production of this diatom by pond fertilization. Examination of cell numeric densities at the end of bioassays allows us to conclude that silicate was the first limiting nutrient, closely followed by phosphate. Chlorophyll a concentrations led to different conclusions: phosphate was the first limiting factor, but after the seawater storage period in ponds, seawater quality evolved to a deficiency of nitrogen. Silicate addition increased cell division rate, and silicate depletion increased chl a synthesis for this species. Examination of nutrient assimilation ratios confirms that H. ostrearia requires a large amount of silicon. From these results, it was possible to prepare a N + P + Si simplified medium which has been tested in laboratory and field mesocosm conditions. In both conditions, similar results were observed: a significant increase in H. ostrearia cell concentrations and consequently an evolution up to the greening stage. Applications of this work are numerous; the principal permits us to envisage the production of this species in 25-m3 ponds, with the aim of allowing constant production of the greening phenomenon.

The dependence of the cellular (biogenic) and frustule-associated (mineralized) silica content of the diatoms Navicula salinarum and Thalassiosira weissflogii on salinity and aluminium conditions was studied in order to make it possible to manipulate silicification in vitro and maximize it to levels required for physico-chemical frustule characterization by physisorption, X-ray scattering analysis and NMR, which is our ultimate objective. Enrichments with AlCl3 increased growth and the final cell density of the pennate N. salinarum, but not of the centric species T. weissflogii. Aluminium additions did not, however, result in a significant increase in the biogenic or mineralized silica content per cell and could not be detected in the silica matrix of the frustule. In contrast, lowering the salinity from 28 practical salinity units (PSU) to 20 and 15 resulted in a significant increase in the biogenic silica content per cell of both species, which is in line with an increase in density of the chemically derived silica under low salt conditions. The silica content per cell was variable during culture growth; increase in cell densities (during exponential growth) was accompanied by a decrease in contents of biogenic silica per cell. Electron microscopy and energy dispersive X-ray analysis (EDXA) support the chemical analyses and also suggest higher biogenic silica contents at lower salinities. The results indicate that besides silicate concentration and pH, the concentration of salts is an important inorganic factor that affects the silica polymerization inside the silica deposition vesicle of diatoms.

Trois à cinq ans après un amendement de dolomie (carbonate calco-magnésien) réalisé en Belgique sur certains bassins versantsforestiers du massif ardennais, les diatomées épilithiques ont été prélevées dans deux ruisseaux situés en zone amendée etdans deux ruisseaux en zone non amendée afin de déceler les éventuels changements induits dans les peuplements algaux. Onobserve des assemblages à Eunotia typiques des eaux acides. L'espèce dominante est Eunotia exigua, souvent accompagnée parE. rhomboidea ou E. bilunaris. Dans le cadre de cette étude, aucune influence propre à l'amendement n'a pu être mise en évidencepar l'étude de ces bioindicateurs.

Achanthes inflata (Kütz) Grun. et la variété elata (Leud.-Fontm.) Hust. ont été étudiés au microscope électronique. L'espèce et la variété, outre la forme de la valve, se différencient par quelques caractères structuraux. Une attention particulière a été portée sur l'organisation du cingulum ; le nombre d'éléments qui le composent est variable en fonction du stade où se trouve la cellule entre deux divisions.

Les résultats des premières études sur les diatomées et les pigments des sédiments d'un réservoir mesotrophe sont analysés. La proportion des deux espèces de diatomées les plus fréquentes Asterionella formosa et Melosira italica est inversée vers 1981-1982, années où le lac semble avoir subi une "poussée" d'eutrophisation. Cependant, aucun changement dans les concentrations en pigments n'a été détecté. Les teneurs élevées en pigments autour de l'année 1975 ne semble pas devoir être imputées à une augmentation de la production algale ; elles semblent plutôt associées à un abaissement du niveau du lac.