The regression of similarity against distance unites several ecological phenomena, and thus provides a highly useful approach for illustrating the spatial turnover across sites. Our aim was to test whether the rates of decay in community similarity differ between diatom growth forms suggested to show different dispersal ability. We hypothesized that the diatom group with lower dispersal ability (i.e. periphyton) would show higher distance decay rates than a group with higher dispersal ability (i.e. plankton). Periphyton and phytoplankton samples were gathered at sites distributed over an area of approximately 800 km length in the Negro River, Amazon basin, Brazil, South America (3°08′00″S; 59°54′30″W). Distance decay relationships were then estimated using distance-based regressions, and the coefficients of these regressions were compared among the groups with different dispersal abilities to assess our predictions. We found evidence that different tributaries and reaches of the Negro River harbor different diatom communities. As expected, the rates of distance decay in community similarity were higher for periphyton than for phytoplankton indicating the lower dispersal ability of periphytic taxa. Our study demonstrates that the comparison of distance decay relationships among taxa with similar ecological requirements, but with different growth form and thus dispersal ability provides a sound approach to evaluate the effects of dispersal ability on beta diversity patterns. Our results are also in line with the growing body of evidence indicating that microorganisms exhibit biogeographic patterns. Finally, we underscore that clumbing all microbial taxa into one group may be a flawed approach to test whether microbes exhibit biogeographic patterns.

In scarcity of light and primary producers, subterranean ecosystems are generally extremely oligotrophic habitats, receiving poor supplies of degradable organic matter from the surface. Human direct impacts on cave ecosystems mainly derive from intensive tourism and recreational caving, causing important alterations to the whole subterranean environment. In particular, artificial lighting systems in show caves support the growth of autotrophic organisms (the so-called lampenflora), mainly composed of cyanobacteria, diatoms, chlorophytes, mosses and ferns producing exocellular polymeric substances (EPSs) made of polysaccharides, proteins, lipids and nucleic acids. This anionic EPSs matrix mediates to the intercellular communications and participates to the chemical exchanges with the substratum, inducing the adsorption of cations and dissolved organic molecules from the cave formations (speleothems). Coupled with the metabolic activities of heterotrophic microorganisms colonising such layer (biofilm), this phenomenon may lead to the corrosion of the mineral surfaces. In this review, we investigate the formation of biofilms, especially of diatom-dominated ones, as a consequence of artificial lighting and its impacts on speleothems. Whenever light reaches the subterranean habitat (both artificially and naturally) a relative high number of species of diatoms may indeed colonise it. Cave entrances, artificially illuminated walls and speleothems inside the cave are generally the preferred substrates. This review focuses on the diatom flora colonising subterranean habitats, summarizing the information contained in all the scientific papers published from 1900 up to date. In this review we provide a complete checklist of the diatom taxa recorded in subterranean habitats, including a total of 363 taxa, belonging to 82 genera. The most frequent and abundant species recorded in caves and other low light subterranean habitats are generally aerophilic and cosmopolitan. These are, in order of frequency: Hantzschia amphioxys, Diadesmis contenta, Orthoseira roeseana, Luticola nivalis, Pinnularia borealis, Diadesmis biceps and Luticola mutica. Due to the peculiarity of the subterranean habitats, the record of rare or new species is relatively common. The most important environmental factors driving species composition and morphological modifications observed in subterranean populations are analysed throughout the text and tables. In addition, suggestions to prevent and remove the corrosive biofilms in view of an environmentally sustainable cave management are discussed.

Two new epiphytic species of Ulnaria (Kützing) Compère, U. titicacaensis E. Morales, Ector & P.B. Hamilton and U. macilenta E. Morales, C.E. Wetzel & S.F. Rivera, are described from the Tiquina Strait, Lake Titicaca, Bolivia. The taxa have distinctive morphological features and their separation from morphologically related taxa and from each other is predominantly based on features of the valve central area and type of colonies. Whereas most morphologically similar taxa have a clear unilateral central area and form ribbon-like colonies, U. titicacaensis has a central area which is large and depressed internally, square to rectangular fascia, forms pin-cushion-like colonies mainly on Cladophora species, and has large polyphosphate bodies located toward the frustule apices. In contrast, U. macilenta has a similar central area to that in U. titicacaensis, but has narrower valves, forms short palisade groupings arranged in pin-cushion-like colonies, mainly on Oedogonium species and has polyphosphate bodies distributed throughout the frustule lumen. Morphological and ecological aspects of both new taxa are discussed in the light of available literature.

Fragilaria pinnata Ehrenberg is actually a member of the genus Denticula Kützing. However, due to a drift of its taxonomic concept, there is now a “floating” idea of F. pinnata as an araphid diatom associated to Staurosirella D.M.Williams & Round (as S. pinnata (Ehrenberg) D.M.Williams & Round). Upon investigation of the nomenclatural history of F. pinnata and examination of type material of Staurosirella mutabilis (W.Smith) E.Morales & Van de Vijver (= Odontidium mutabile (W.Smith) Kützing), a commonly reported synonym for S. pinnata, it became evident that the material studied in depth by Haworth (1974, 1975, 1976) is the most suitable to give a formal taxonomic concept to that floating idea. For this purpose, we describe a new species, Staurosirella neopinnata, from the mentioned material. This diatom is distinguished by the raised axial area and virgae, and the solid, apically flattened spines, originating from two or three points on the virgae. The largest population found in the analyzed material corresponds to a depth of 208 cm in a sediment core (early Flandrian, ca. 10226 +/- 190 YBP) from Loch Borralan, Scotland, a shallow lake exposed to wind disturbance and containing, at that depth, a diatom assemblage characterized by a high density of S. neopinnata and benthic forms that are alkaliphilous, halobion and current indifferent, non-aerophilic and non-cryophilic. Through a review of electron microscopy images in the literature, we preliminarily establish that the new taxon has a Holarctic distribution and that the great majority of F. pinnata reports from other biogeographic realms correspond to different, as yet formally undescribed species.

The taxonomy of several small-celled, ecologically significant Nitzschia species, which are frequently confused with each other or whose names are misapplied, is clarified. Following an examination of type material and modern samples by light and electron microscopy, it was concluded that N. frustulum (Kützing) Grunow, N. inconspicua Grunow, N. soratensis E. Morales & Vis and N. invisitata Hustedt are independent species. No morphological basis was found for separating N. frustulum var. subsalina Hustedt or N. boliviana E. Morales & Vis from N. inconspicua and they are therefore placed in synonymy with N. inconspicua. Nitzschia soratensis, described recently from Bolivia, has previously been misidentified in Europe, either as N. inconspicua (from which it differs most obviously in having more bluntly rounded poles, striae within the raphe canal that are composed of triplets, and fibulae that can be seen in light microscopy to widen at their bases) or as N. abbreviata Hustedt ex Simonsen (from which it differs in pore ultrastructure). Nitzschia frustulum resembles N. inconspicua in every morphological feature examined, but with wider valves and consistently higher maximum length.