Epiphytic diatoms perform a variety of ecological functions. Diatoms are important primary producers and sources of oxygen which can modify the chemistry of the surrounding aquatic environment. They may live attached to plant surfaces with the help of extracellular polymeric substances and compete with plants for resources (e.g., light, nutrients). Thus, they represent an excellent model system for studies on interactions between epiphytes and their host plants under different environmental conditions. Further, the practical usage of epiphytic diatoms in biomonitoring begs questions concerning substrate specificity, diatom biodiversity, and species delimitations. This review focuses on specific aspects of freshwater epiphytic diatom ecology as adaptations for epiphytic way of life, epiphyte-host relationships, and implications for biomonitoring.

The Watanabea clade of the Trebouxiophyceae includes unicellular coccoid green microalgae that mostly thrive in subaerial microhabitats. Recently, a number of new genera and species were described on the basis of the DNA sequence data and morphological observations. The peculiar genus Phyllosiphon, which forms a monophyletic clade within the Watanabea clade, is characterized by forming siphonous parasitic stages thriving in the leaves of the Araceae. In addition, several previous studies demonstrated that members of the genus Phyllosiphon also include free-living chlorelloid individuals that occur on various subaerial substrates. A number of Phyllosiphon members sampled from European subaerial microhabitats have so far not been taxonomically described, because they were not available in cultures. In this study, we provide a taxonomic description of a new species of the genus Phyllosiphon, P. duini, isolated from a corticolous biofilm growing on Quercus pubescens in a sub-Mediterranean forest stand. The simple chlorelloid morphology of this strain did not unambiguously distinguish Phyllosiphon duini from other closely related members of the Watanabea clade. However, phylogenetic analyses based on the 18S rDNA sequences showed that this species clustered in a sister position to a Phyllosiphon species previously described from eastern Asia. Similar phylogenetic pattern was also supported by the plastid-encoded rbcL gene sequences of members of the Watanabea clade. Our data demonstrate that the genus Phyllosiphon represents a diverse phylogenetic lineage within the subaerial chlorelloid green microalgae of the Watanabea clade.

Epithemia hirudiniformis and three morphologically related taxa, described in Rhopalodia by O. Müller from material collected in East Africa at the end of the 19^th and the beginning of the 20^th century were re-evaluated and lectotypes designated. Rhopalodiella as a new subgenus is proposed which refines O. Müller's infrageneric classification and to which all these taxa belong. In addition, the type of Epithemia rhopala, described by Ehrenberg from Egypt, was studied to examine the assumed synonymy, introduced by Hustedt, with some of Müller's species. This study, using light and scanning electron microscopy, was not only based on historic material but also more recent material from Africa, including samples from the Island of Reunion from which the new species Epithemia vandevijveri is described. The distribution of Epithemia subg. Rhopalodiella, known to be restricted to tropical Africa, is discussed based on literature data and own observations. Corresponding to recent molecular-based studies, Rhopalodia is given a new status as subgenus.

To expand on the biodiversity study of the eastern Alpine region of North Tyrol, we investigated silica-scaled chrysophyte flora of unexplored high-altitude Alpine sites. We concentrated mostly on lakes that were 1000–2500 m above sea level; some of them were still partly ice covered. Overall, 27 taxa were recorded at 12 sites, despite 21 sites being sampled altogether. In general, the sites were oligotrophic and species poor. Due to a restricted data set, an extensive analysis was not possible. However, altitude and pH were found to be important variables for explaining species distribution. We described the new species Mallomonas pechlaneri from two of the sampled sites. Mallomonas pechlaneri most closely resembles Mallomonas striata var. striata and M. striata var. serrata; these three taxa are clearly distinguished by bristle morphology and scale shape. The bristles of M. pechlaneri terminate in a bifurcated tip consisting of unequal diverging branches. Scale shape was captured my means of landmark-based geometric morphometries (GM) and evaluated using multivariate statistical analyses. GM methods proved to be an efficient tool to be employed in chrysophycean taxonomy.

The cosmopolitan genus Ulva Linnaeus includes species of green macroalgae found in marine, brackish and some freshwater environments. Although there is a wide literature for the determination of Ulva taxa in Europe, they are among the most problematic algae to accurately identify, because they have few distinctive features, as well as a high intraspecific variation.

At present, the knowledge of both diversity and distribution of the genus Ulva in the Mediterranean Sea is almost entirely based on morphological studies and there is only a few published papers dealing with molecular data. Tunisia has a key position in the Mediterranean and constitutes a transition area with a rich habitat diversity between eastern and western basins. The latest inventory of marine macrophytes dates back to 1987, updated in 1995. The aim of the present paper is to provide a molecular-assisted alpha taxonomy survey of Ulva spp. along Tunisian coasts, in comparison with a few Italian sites, using the tufA marker. Nine genetic species groups were resolved, including the non indigenous species Ulva ohnoi, newly reported for Tunisia. The actual picture of the taxonomy of Ulva spp. in the Mediterranean as a whole is far to be clarified and the present data on Tunisian collections aim to be a step towards its clarification. This paper is the first DNA barcoding study on green macroalgae in Tunisia and it contributes to add records to the Barcode Of Life Data Systems which are publically available.

Calliblepharis rammediorum sp. nov. (Gigartinales, Rhodophyta) is described from the northern Levant Mediterranean shore of Israel. The new species has a prostrate creeping habit. It usually grows as an epiphyte, strongly attached by several discoidal holdfasts to the base of branches of some seaweed species, mainly on some non-native species and particularly on Laurencia chondrioides. Phylogenetic analyses of sequence data indicate that C. rammediorum is closely related to an unidentified species from the Cocos (Keeling) Islands, Australia. These two are sister to a complex of two genetic groups both assigned to the morphospecies Calliblepharis fimbriata, whose species identity clearly requires further research. Calliblepharis rammediorum is the fourth that is newly described for the genus Calliblepharis since the beginning of the last decade. Aspects of its biology and ecology are also discussed.

A constant effort in sequencing an extensive number of specimens originating from comprehensive sampling had return an unprecedented amount of information fostering our understanding of diversity, evolution and distribution of coralline algae; however, many sequences lack reliable assignation of a taxonomic name, specially at the species level. Recently, the sequencing of type material allowed to bridge this gap by providing a direct link between the DNA sequence and the type bearing name. For instance, in the genus Lithophyllum, the identity of three species, generally abundant along the European Atlantic and the Mediterranean, was demonstrated by including sequences of the type material. Nevertheless, for less conspicious species, such as Lithophyllum nitorum, data are still needed to assess distribution, anatomy, phylogenetic affinities and taxonomic status. Using DNA sequences recovered from the type material of L. nitorum, further recent collections were resolved as conspecific and used to improve the description and refine the distribution of this species. Lithophyllum nitorum consisted of subtidal, thin crusts overgrowing fragments of dead maerl, pebbles and drifted fragments of fishing lines. The presence of uniporate conceptacles markedly protruding can be considered an external morphological feature useful to identify L. nitorum from other subtidal Lithophyllum species with similar habitat. Lithophyllum nitorum is reported for the Mediterranean Sea and confirmed in the North East Atlantic, a similar distribution as for L. incrustans and L. hibernicum.