Two new species in the Chaetoceros socialis complex (Bacillariophyta): C. sporotruncatus and C. dichatoensis,and characterization of its relatives,C. radicans and C. cinctus

The diatom genus Chaetoceros is one of the most abundant and diverse phytoplankton in marine and brackish waters worldwide. Within this genus, Chaetoceros socialis has been cited as one of the most common species. However, recent studies from different geographic areas have shown the presence of pseudo‐cryptic diversity within the C. socialis complex. Members of this complex are characterized by curved chains (primary colonies) aggregating into globular clusters, where one of the four setae of each cell curves toward the center of the cluster and the other three orient outwards. New light and electron microscopy observations as well as molecular data on marine planktonic diatoms from the coastal waters off Chile revealed the presence of two new species, Chaetoceros sporotruncatus sp. nov. and C. dichatoensis. sp. nov. belonging to the C. socialis complex. The two new species are similar to other members of the complex (i.e., C. socialis and C. gelidus) in the primary and secondary structure of the colony, the orientation pattern of the setae, and the valve ultrastructure. The only morphological characters that can be used to differentiate the species of this complex are aspects related to resting spore morphology. The two newly described species are closely related to each other and form a sister clade to C. gelidus in molecular phylogenies. We also provide a phylogenetic status along with the morphological characterization of C. radicans and C. cintus, which are genetically related to the C. socialis complex.     

Multiple losses of photosynthesis in Nitzschia (Bacillariophyceae)

In order to obtain insights into the evolution of colorless (apochlorotic) diatoms, we investigated newly established apochlorotic strains of Nitzschia spp. using light and electron microscopy and molecular phylogenetic analyses. Fluorescence microscopic observations demonstrated that the apochlorotic diatoms lack chlorophylls. Transmission electron microscopy of two apochlorotic strains also demonstrated that their plastids lacked thylakoids; instead, having four‐membrane‐bound organelles without thylakoids, similar to nonphotosynthetic plastid remnants. From the apochlorotic strains, we also found plastid small subunit rRNA genes that were unusually long branched in phylogenetic analyses, as observed in other nonphotosynthetic plastids. Molecular phylogenetic analysis of the nucleus‐encoded large subunit rRNA genes showed eight distinct lineages for apochlorotic diatoms. The eight apochlorotic lineages were not monophyletic, suggesting that the loss of photosynthesis took place multiple times independently within Nitzschia. Several diatoms, including Nitzschia spp., are mixotrophic, which is an expected mode of nutrition that would help explain the evolutionary switch from a photosynthetic lifestyle to a heterotrophic lifestyle.     

Description of a new diatom genus Dorofeyukea gen. nov. with remarks on phylogeny of the family Stauroneidaceae

Type material of Navicula kotschyi was studied, and this species was transferred to Dorofeyukea gen. nov. as D. kotschyi comb. nov. Dorofeyukea was described on the basis of DNA sequence and morphological data. Additional species assigned to this genus that were previously included in Navicula include: D. ancisa comb. nov., D. grimmei comb. nov., D. ivatoensis comb. nov., D. orangiana comb. nov., D. rostellata comb. nov. & stat. nov., D. savannahiana comb. nov., D. tenuipunctata comb. nov., and D. texana comb. nov. All Dorofeyukea species share the same morphological features, including having a narrow stauroid fascia surrounded by 1–3 irregularly shortened striae, uniseriate, and weakly radiate striae, circular, or rectangular puncta in the striae that are covered internally by dome‐shaped hymenes, presence of a pseudoseptum at each apex and absence of septa. Partial DNA sequences of SSU and rbcL loci show Dorofeuykae belongs to the clade of stauroneioid diatoms together with Stauroneis, Prestauroneis, Craticula, Karayevia, Madinithidium, Fistulifera, Parlibellus, and, possibly, Schizostauron. A new species from the monoraphid genus Madinithidium, M. vietnamica sp. nov., was described based on valve and chloroplast morphology as well as DNA sequence data.     

MORPHOLOGY AND MOLECULAR PHYLOGENY OF ANKISTRODINIUM GEN. NOV. (DINOPHYCEAE), A NEW GENUS OF MARINE SAND‐DWELLING DINOFLAGELLATES FORMERLY CLASSIFIED WITHIN AMPHIDINIUM1

The classical athecate dinoflagellate genera (Amphidinium, Gymnodinium, Gyrodinium) have long been recognized to be polyphyletic. Amphidinium sensu lato is the most diverse of all marine benthic dinoflagellate genera; however, following the redefinition of this genus ～100 species remain now of uncertain or unknown generic affiliation. In an effort to improve our taxonomic and phylogenetic understanding of one of these species, namely Amphidinium semilunatum, we re‐investigated organisms from several distant sites around the world using light and scanning electron microscopy and molecular phylogenetic methods. Our results enabled us to describe this species within a new heterotrophic genus, Ankistrodinium. Cells of A. semilunatum were strongly laterally flattened, rounded‐quadrangular to oval in lateral view, and possessed a small asymmetrical epicone. The sulcus was wide and characteristically deeply incised on the hypocone running around the antapex and reaching the dorsal side. The straight acrobase with hook‐shaped end started at the sulcal extension and continued onto the epicone. The molecular phylogenetic results clearly showed that A. semilunatum is a distinct taxon and is only distantly related to species within the genus Amphidinium sensu stricto. The nearest sister group to Ankistrodinium could not be reliably determined.     

广东鉴江水系底栖硅藻多样性与时空分布特征

为了解广东省鉴江水系底栖硅藻多样性和时空分布特征,对全流域进行了底栖硅藻采样调查。结果表明,从19个采样点4次采样中共检出底栖硅藻10科52属242种,其中舟形藻属(Navicula)、菱形藻属(Nitzschia)和异极藻属(Gomphonema)是优势类群,出现频次和相对丰度较高。硅藻多样性指数(丰富度、真香农多样性指数和真辛普森多样性指数)随河流等级呈现一定的空间分布特征,但它们季节变化不明显。底栖硅藻群落相异性在上游和下游河段较高,从一级到三级河流递减,四级河流又增加。底栖硅藻群落结构空间变化明显,季节变化显著。群落丰富度的稀疏曲线表明,热带河流底栖硅藻群落以400个体计数,不能完整反映底栖硅藻多样性。这些为鉴江水系河流健康监测和水生态保护奠定了基础。     

Depth distribution of photosynthetic pigments and diatoms in the sediments of a microtidal fjord

The depth distribution of photosynthetic pigments and benthic marine diatoms was investigated in late spring at three different sites on the Swedish west coast. At each site, sediment cores were taken at six depths (7–35 m) by scuba divers. It was hypothesized that (1) living benthic diatoms constitute a substantial part of the benthic microflora even at depths where the light levels are <1% of the surface irradiance, and (2) the changing light environment along the depth gradient will be reflected in (a) the composition of diatom assemblages, and (b) different pigment ratios. Sediment microalgal communities were analysed using epifluorescence microscopy (to study live cells), light microscopy and scanning electron microscopy (diatom preparations), and HPLC (photosynthetic pigments). Pigments were calculated as concentrations (mg m^–2) and as ratios relative to chlorophyll a. Hypothesis (1) was accepted. At 20 m, the irradiance was 0.2% of surface irradiance and at 7 m, 1%. Living (epifluorescent) benthic diatoms were found down to 20 m at all sites. The cell counts corroborated the diatom pigment concentrations, decreasing with depth from 7 to 25 m, levelling out between 25 and 35 m. There were significant positive correlations between chlorophyll a and living (epifluorescent) benthic diatoms and between the diatom pigment fucoxanthin and chlorophyll a. Hypothesis (2) was only partly accepted because it could not be shown that light was the main environmental factor. A principal component analysis on diatom species showed that pelagic forms characterized the deeper locations (25–35 m), and epipelic–epipsammic taxa the shallower sites (7–20 m). Redundancy analyses showed a significant relationship between diatom taxa and environmental factors – temperature, salinity, and light intensities explained 57% of diatom taxa variations.     

Microalgal–meiofaunal interactions in a sublittoral site of the Gulf of Trieste (northern Adriatic Sea, Italy): A three-year study

Monthly sediment sampling was carried out from February 1999 to December 2001 at a 21-m deep station. Benthic diatom biomass and meiobenthic abundance were estimated together with abiotic parameters (PAR irradiance, temperature, salinity and dissolved oxygen). During the three-year study average microphytobenthic abundance (ABU) and biomass (BIOM) were 4.7 ± 2.6 × 10⁴ cell cm^− 3 and 14.3 ± 8.1 µg C cm^− 3, respectively, while the mean meiofaunal abundance was 923 ± 210 ind 10 cm^− 2. The microphytobenthic community was mainly composed of Bacillariophyceae (99.3%) with a smaller percentage of phytoflagellates (0.7%). Among diatoms 39 genera were observed with a total of 110 taxa and 100 species. The main benthic diatom genera were Navicula, Paralia, Nitzschia and Diploneis. A total of 18 meiobenthic taxa were detected: 7 taxa belonged to permanent meiofauna (Nematoda, Copepoda Harpacticoida and their nauplius stages, Kinorhyncha, Turbellaria, Gastrotricha, Ostracoda and Acarina). The remaining 11 taxa belonged to temporary meiofauna (juvenile macrofauna: Polychaeta, Cumacea, Amphipoda, Isopoda, Nemertea, Decapoda, Sipunculida, Ophiurida, Gastropoda, Bivalvia and Cnidaria). Positive correlations between PAR and the biomass of the major diatom genera were revealed. Temperature did not seem to influence the microphytobenthic community as a whole, although we found statistically significant correlation between temperature and the genera Nitzschia and Amphora. Salinity showed significant positive correlation only with the genus Pleurosigma, in contrast, dissolved oxygen did not seem to affect the microalgal community. The major meiofaunal taxa were not correlated with abiotic variables, but were positively correlated with most diatom genera. Paralia was negatively correlated with the three main meiofaunal taxa. The three-point moving average applied to our biotic data allowed us to better the signal without the background noise which hid the actual variations of the studied communities. Applying the three-point moving average to the main taxa, it resulted that microphyto- and meiobenthos were in phase. The principal component analysis (PCA), constructed considering both biotic and abiotic variables, accounted for 58% of the total variance. PC1 axis explained 39.74% of the total variance and was correlated with Navicula, Diploneis and Nitzschia. PC2 axis explained 18.40% of the remaining variance and was correlated with Copepoda, Paralia and PAR. In contrast to 1999 and 2001, a mucilage event occurred in 2000, which resulted in the formation of a false bottom and a decrease in the abundance of microphyto- and meiobenthos.     

The epizoic diatom community on four bryozoan species from Helgoland (German Bight, North Sea)

The composition of the diatom community on the bryozoans Electra pilosa, Membranipora membranacea, Flustra foliacea, and Alcyonidium gelatinosum from the German Bight was studied by light and scanning electron microscopy. In total, members of 26 diatom genera were found, with Cocconeis, Tabularia, Licmophora, Amphora, and Navicula being the most abundant. The amount and the composition of the diatom covering seem to be typical for single bryozoan species. Electra pilosa and Membranipora membranacea showed a rather dense covering with 71–547 cells/mm² and 77–110 cells/mm², respectively. The most prominent genus on Electra pilosa was Cocconeis, reaching up to 58% of all diatoms in one sample, followed by Navicula, Tabularia and Amphora. The most abundant genera on Membranipora membranacea were Tabularia and Licmophora, making up almost 70% of all diatoms in one sample, followed by Navicula, Cocconeis and Amphora. The diatom composition was very stable on all Electra samples, but varied on Membranipora samples. With <1–27 cells/mm², diatoms were much less abundant on Alcyonidium gelatinosum. Members of the genera Tabularia and Navicula were the most frequently found benthic diatoms, whereas the planktonic forms Coscinodiscus, Cyclotella, and Thalassiosira made up 35% of the diatoms. On Flustra foliacea, diatoms were virtually absent, with fewer than 5 cells/mm². The low diatom numbers are probably due to toxic metabolites produced by the host. The same may be true for Alcyonidium gelatinosum, but here they might also be a consequence of the surface properties of the bryozoan. Electronic Publication     

MATING SYSTEM,SEXUAL REPRODUCTION,AND AUXOSPORULATION IN THE ANOMALOUS RAPHID DIATOM EUNOTIA (BACILLARIOPHYTA)1

The diatom genus Eunotia is unusual among raphid diatoms in having a raphe system consisting of two short slits that are not integrated into the primary pattern center. This and other characteristics, particularly the presence of rimoportulae, are consistent with the hypothesis that Eunotia is a basal lineage within the raphid group. We studied auxosporulation in E. bilunaris (Ehrenberg) Mills and E. tropica Hustedt for comparison with other raphid pennate diatoms and with araphid pennates; E. bilunaris was studied in parental and F1 generations. Like araphid pennates, E. bilunaris and E. tropica are heterothallic. Clones of the same mating type did not interact sexually, and intraclonal sexual reproduction was absent or very rare. Clones retained the same sex throughout the life cycle, as shown by experiments using abrupt size reduction to produce clones of similar age but different size and using subclones derived from a single initial cell within six mitotic generations. Unlike in araphid pennate diatoms, in the Eunotia species the gametes are not visibly or behaviorally differentiated. Gametogenesis is merogenous, because the gametangium formed a supernumerary cell as well as a single gametic cell, both undergoing meiosis II to form a surviving functional nucleus and a nucleus that quickly degenerated. Plasmogamy is via papillae that grew out toward each other from the ends of the gametangia to create a copulation canal. After plasmogamy, the gametes moves bodily into the copulation canal, producing an elongate zygote, which expands to form a curved sausage‐like auxospore.     

Morphology and molecular phylogeny of epizoic araphid diatoms on marine zooplankton,including Pseudofalcula hyalina gen. & comb. nov. (Fragilariophyceae,Bacillariophyta)

Some diatoms are able to colonize as epibionts on their potential zooplankton predators. Here, we report Pseudohimantidium pacificum living on the copepod Corycaeus giesbrechti and as a new finding on Oithona nana, Protoraphis atlantica living on the copepod Pontellopsis brevis, Protoraphis hustedtiana on the cypris larvae of barnacles, and Falcula hyalina on the copepod Acartia lilljeborgii. The epizoic diatoms were able to grow as free‐living forms under culture conditions. Pseudohimantidium pacificum and P. atlantica appeared as the most derived species from their benthic diatom ancestors. The mucilage pad or stalk of the strains of these species showed important morphological distinction when compared with their epizoic forms. Barnacle larvae explore benthic habitats before settlement, and epibiosis on them is an example where P. hustedtiana profits from the host behavior for dispersal of its benthic populations. Molecular phylogenies based on the SSU rRNA and RuBisCO large subunit (rbcL) gene sequences revealed F. hyalina as an independent lineage within the Fragilariales (Tabularia, Catacombas, and others), consistent with its morphological distinction in the low number of rows (≤6) in the ocellulimbus, among other features. We propose the transfer of F. hyalina to the genus Pseudofalcula gen. nov. Molecular phylogeny suggests a single order for the members of the Cyclophorales and the Protoraphidales, and that the epibioses of araphid diatoms on marine zooplankton have been independently acquired several times. These clades are constituted of both epizoic and epiphytic/epilithic forms that evidence a recent acquisition of the epizoic modus vivendi.     

Photoprotection and xanthophyll‐cycle activity in three marine diatoms1

Light is one of the most important factors affecting marine phytoplankton growth, and its variability in time and space strongly influences algal performance and success. The hypothesis tested in this work is that the activity of the xanthophyll cycle and the development of nonphotochemical quenching could be considered a functional trait of algal diversity. If this hypothesis is true, a relationship must exist between fast‐activated pigment variations linked to photoprotective behavior and the ecology of the species. This assumption was tested on three diatoms: Skeletonema marinoi Sarno et Zingone, Thalassiosira rotula Meunier, and Chaetoceros socialis Lauder. These three diatoms occupy different ecological niches. Strains of these diatoms were subjected to five changes in irradiance. Xanthophyll‐cycle activity, quantum yield of fluorescence, and electron transport rate were the main parameters determined. There were marked interspecific differences in xanthophyll‐cycle activity, and these differences were dependent on the light history of the cells. Chaetoceros socialis responded efficiently to changing irradiance, which might relate to its dominance during the spring bloom in some coastal areas. In contrast, T. rotula responded with a slower photoprotection activation, which seems to reflect its more offshore ecological distribution. The photoresponse of S. marinoi (a late‐winter coastal species blooming in the Adriatic Sea) was light‐history dependent, becoming photoinhibited under high light when acclimated to low light, but capable of reaching a high photoprotection level when acclimated to moderate light. Our hypothesis on the photoprotection capacity as a functional trait in microalgae seems to be validated given the results of this study.     

Morphological and molecular characterization within 26 strains of the genus Cylindrospermum (Nostocaceae,Cyanobacteria), with descriptions of three new species

Twenty‐six strains morphologically identified as Cylindrospermum as well as the closely related taxon Cronbergia siamensis were examined microscopically as well as phylogenetically using sequence data for the 16S rRNA gene and the 16S‐23S internal transcribed spacer (ITS) region. Phylogenetic analysis of the 16S rRNA revealed three distinct clades. The clade we designate as Cylindrospermum sensu stricto contained all five of the foundational species, C. maius, C. stagnale, C. licheniforme, C. muscicola, and C. catenatum. In addition to these taxa, three species new to science in this clade were described: C. badium, C. moravicum, and C. pellucidum. Our evidence indicated that Cronbergia is a later synonym of Cylindrospermum. The phylogenetic position of Cylindrospermum within the Nostocaceae was not clearly resolved in our analyses. Cylindrospermum is unusual among cyanobacterial genera in that the morphological diversity appears to be more evident than sequence divergence. Taxa were clearly separable using morphology, but had very high percent similarity among ribosomal sequences. Given the high diversity we noted in this study, we conclude that there is likely much more diversity remaining to be described in this genus.     

Fine structure and 18S rDNA phylogeny of a marine araphid pennate diatom Plagiostriata goreensis gen. et sp. nov. (Bacillariophyta)

A marine araphid pennate diatom Plagiostriata goreensis is described from the sand grains of Goree Island, Dakar, Republic of Senegal, based on observations of fine structure of its frustule. The most striking feature of the species is its striation, which is angled at approximately 60° across the robust sternum. The other defining features of the species are its one highly reduced rimoportula and apical pores located at both ends of the valve margin. In the 18S rDNA phylogeny, the species appears as a member of a ‘small‐celled clade’ of araphid pennate diatoms that consist of Nanofrustulum, Opephora and Staurosira. The results of the phylogenetic analyses suggest that the distinct characters of the diatom; namely, oblique striae and apical pores, may have been acquired independently. However, it remains unclear whether the rimoportula of P. goreensis is a reduced state or P. goreensis acquired its morphologically curious rimoportula independently after the loss of an ancient rimoportula at the root of the small‐celled clade.     

Observations of the morphology of some known and new fragilarioid diatoms (Bacillariophyceae) from rivers in the USA

Morphological studies at the light microscopy and scanning electron microscopy levels of selected fragilarioid diatoms occurring in North American streams and lakes are presented herein. The majority of the samples studied were collected by the US Geological Survey's National Water Quality Assessment Program, which concentrates on stream water quality monitoring throughout the continental USA and Hawaii. Two new species (Staurosirella confusa Morales and Punctastriata mimetica Morales) and a new forma (Pseudostaurosiropsis geocollegarum f. triradiatum Morales) are described and two new combinations (Pseudostaurosira subsalina (Hustedt) Morales and Staurosirella olden‐burgiana (Hustedt) Morales) are provided. Morphological details of an additional taxon, Staurosira construens var. binodis (Ehrenberg) Hamilton in Hamilton et al. are also presented. The taxonomic affinities of all these taxa, as well as some evolutionary aspects and ecologic characteristics, are discussed in the light of published material.     

Phylogeny of North American Tolypella (Charophyceae,Charophyta) based on plastid DNA sequences with a description of Tolypella ramosissima sp. nov.

Characeae (Charophyceae, Charophyta) contains two tribes with six genera: tribe Chareae with four genera and tribe Nitelleae, which includes Tolypella and Nitella. This paper uses molecular and morphological data to elucidate the phylogeny of Tolypella species in North America. In the most comprehensive taxonomic treatment of Characeae, 16 Tolypella species worldwide were subsumed into two species, T. intricata and T. nidifica, in two sections, Rothia and Tolypella respectively. It was further suggested that Tolypella might be a derived group within Nitella. In this investigation into species diversity and relationships in North American Tolypella, sequence data from the plastid genes atpB, psbC, and rbcL were assembled for a broad range of charophycean and land plant taxa. Molecular data were used in conjunction with morphology to test monophyly of the genus and species within it. Phylogenetic analyses of the sequence data showed that Characeae is monophyletic but that Nitelleae is paraphyletic with Tolypella sister to a monophyletic Nitella + Chareae. The results also supported the monophyly of Tolypella and the sections Rothia and Tolypella. Morphologically defined species were supported as clades with little or no DNA sequence differences. In addition, molecular data revealed several lineages and a new species (T. ramosissima sp. nov.), which suggests greater species diversity in Tolypella than previously recognized.     

Morphological and genetic diversity of Beaufort Sea diatoms with high contributions from the Chaetoceros neogracilis species complex

Seventy‐five diatom strains isolated from the Beaufort Sea (Canadian Arctic) in the summer of 2009 were characterized by light and electron microscopy (SEM and TEM), as well as 18S and 28S rRNA gene sequencing. These strains group into 20 genotypes and 17 morphotypes and are affiliated with the genera Arcocellulus, Attheya, Chaetoceros, Cylindrotheca, Eucampia, Nitzschia, Porosira, Pseudo‐nitzschia, Shionodiscus, Thalassiosira, and Synedropsis. Most of the species have a distribution confined to the northern/polar area. Chaetoceros neogracilis and Chaetoceros gelidus were the most represented taxa. Strains of C. neogracilis were morphologically similar and shared identical 18S rRNA gene sequences, but belonged to four distinct genetic clades based on 28S rRNA, ITS‐1 and ITS‐2 phylogenies. Secondary structure prediction revealed that these four clades differ in hemi‐compensatory base changes (HCBCs) in paired positions of the ITS‐2, suggesting their inability to interbreed. Reproductively isolated C. neogracilis genotypes can thus co‐occur in summer phytoplankton communities in the Beaufort Sea. C. neogracilis generally occurred as single cells but also formed short colonies. It is phylogenetically distinct from an Antarctic species, erroneously identified in some previous studies as C. neogracilis, but named here as Chaetoceros sp. This work provides taxonomically validated sequences for 20 Arctic diatom taxa, which will facilitate future metabarcoding studies on phytoplankton in this region.