Species from the Mallomonas bronchartiana complex were examined using material from Vietnam, the U.S.A. and South Korea. The original description of Mallomonas bronchartiana is expanded and descriptions of two new taxa, M. pseudobronchartiana and M. velari, are given. All taxa possess body scales that are large, broad, with an asymmetrically-placed posterior rim, an internal honeycomb reticulation, and are covered externally with a layer containing papillae. A distinct V-rib is lacking on Mallomonas bronchartiana scales. Scales of M. pseudobronchartiana have a thin V-shaped rib situated on the scale surface, while the V-rib of M. velari is formed from an upward folding of the surface of the scale. Scales of Mallomonas velari are also easily separated from the other species by the presence of a large forward projecting wing. The positions of all three taxa within the genus are discussed, and placement in section Quadratae is proposed. The distributions and habitat conditions of each species are summarized.

A population of an unknown Psammothidium species (Bacillariophyta, Achnanthidiaceae) was found in core sediments collected from Cimera Lake, an oligotrophic, undisturbed mountain lake in Central Spain (Gredos mountain range). The morphology and ultrastructure of this taxon is hereby documented in detail by means of light (LM) and scanning electron (SEM) micrographs. Morphologically, the closest taxon is P. levanderi, and the type of this species is analyzed here to provide a differential diagnosis. A comparison with other similar small Psammothidium species with an elliptic outline show that the combination of features exhibited by this taxon is unique and it is thus described here as Psammothidium toroi sp. nov.

Several diatom species produce resting stages as part of their life cycle. These resting stages accumulate in the sediments where they can remain for a long time before being re-suspended in the water column and switching to active growth. Until now, the abundance and diversity of viable diatom resting stages have been assessed using the Serial Dilution Culture (SDC) method. In the present study, surface sediment samples from the Gulf of Naples were used to compare results obtained with the SDC method with those provided by HTS metabarcoding based on DNA extracted from the same sediment sample; the marker used was the V4 region of 18S rDNA. HTS metabarcoding showed a marked dominance of polar centric diatoms, among which Chaetoceros species were the most represented, in terms of both sequence and ribotype number. Almost all the most abundant ribotypes identified with metabarcoding matched records of species observed in SDCs. In some cases, however, this marker region could not distinguish between morphologically and phylogenetically distinct species, e.g., Skeletonema pseudocostatum and S. tropicum. As expected, molecular analysis provided a higher number of ribotypes as compared to the number of taxa recorded by SDC. Despite the well-known biases of both methodologies for quantitative assessments, our results show that DNA metabarcoding via HTS sequencing is a promising approach to explore the diversity of diatom resting stages. This study also confirms the importance of curated reference sequences to fully interpret the diversity stored in environmental sediment samples.

Complete sequence of sexual reproduction in Cylindrocystis brebissonii (Ralfs) De Bary was observed for the first time in a natural population from Sikkim, Eastern Himalayas. The observations are supported by microphotographs.

Kongsfjorden (Spitsbergen, Svalbard) is an inlet treated as a model site for studies on the impact of climate change in the Arctic due to its hydrological features. In this research, seven-days monitoring was carried out to evaluate the effects of hydrological variability on phytoplankton biomass and diversity in the late summer period. Temperature, salinity, nutrients, total suspended matter, phytoplankton abundance and biomass were determined for each sample. The thermo-haline properties of the column water seemed to affect phytoplankton communities. Their abundances and biomass were correlated with the amount of the total suspended matter. Moreover, species composition and biomass dramatically changed throughout the study period. Cold-water and Atlantic species were replaced by temperatewarm water dinoflagellates, including harmful species. An increase in phytoplankton biomass as well as the presence of dinoflagellate aggregations, mainly composed of Prorocentrum cf. gracile, were detected. This kind of algal accumulation is a new phenomenon in the Arctic and was probably related to the mobilization of sediment-rich glacial meltwaters. These findings, even if preliminary, suggest the need to study how additional biomass pulses and the increase of harmful species may alter the food web structure and the biogeochemical cycles, leading to major ecosystem changes.