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Light microscopy micrographs of the resting spore formation in Chaetoceros rotosporus sp. nov., strain DY6. All scale bars are 20 lm. Fig. 13. One immature (arrow) and several mature resting spores. Fig. 14. Two immature resting spores (arrows), each with circular wing around the girdle. The third resting spore is about to lose the wing (arrowhead). Fig. 15. Two immature resting spores, each with circular wing (arrows). Fig. 16. Four mature resting spores in their mother cells.
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With more than 400 species, the diatom genus Chaetoceros Ehrenberg is one of the most species-rich genera of marine planktonic diatoms. Although most Chaetoceros taxa are considered to be cosmopolitan, research has mainly focused on arctic and temperate North Atlantic areas. Studies on material from tropical waters suggest that a high and undescrib...
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The cosmopolitan distribution of the marine planktonic diatom Chaetoceros socialis Lauder is generally interpreted as a case of wide ecological plasticity. However, cryptic diversity has recently been observed within this taxon and this genetic diversity could extend to phenotypic differences between allopatric populations. Monoclonal strains of C....
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... In the past decades, taxonomic studies have mainly been conducted on the previous subgenus Hyalochaete and combined morphological and molecular evidence clarified the delineation of some species, and a few new species have been described (Rines et al., 2000;Li et al., 2013Li et al., , 2015Li et al., , 2017Chamnansinp et al., 2015;Gaonkar et al., 2017Gaonkar et al., , 2021Xu et al., 2019Xu et al., , 2020Hernández-Becerril, 2021;Chen et al., 2022). In contrast, very few taxonomic studies have focused on the section Chaetoceros, and those performed have mainly relied on morphological methods (Hernández-Becerril, 1992, 1993, 1996, 1998, 2000Bosak and Sarno, 2017), and only a few new species have been described, like C. octagonus and C. atlanticus var. ...
... Notably, the ultrastructural characters on setae were found to be species specific in the section Chaetoceros. The setae are often four-six sided, and covered with one to five transverse rows of roundoval poroids between costae in almost all of the species in the section Chaetoceros, whereas round and twisted setae ornamented by 4-6 single rows of poroids are seen in the other sections (Kooistra et al., 2010;Li et al., 2013Li et al., , 2015Li et al., , 2017Lee et al., 2014a, b). An exceptional seta ultrastructure in the section Chaetoceros was only observed in C. dichaeta, in which no costae but elongated poroids are arranged in a single longitudinal pattern on the octagonal setae (Lee et al., 2014b, loc. ...
Chaetoceros Ehrenberg is one of the most diverse genera of planktonic diatoms. The species in section Chaetoceros are characterized by cells and setae having numerous chloroplasts and being widely distributed. However, the delimitations of some species are problematic because of limited morphological information in the classical descriptions. Monoclonal strains of the section Chaetoceros were established, morphological features were studied using light and electron microscopy, and the hypervariable D1–D3 region of the nuclear ribosomal large subunit gene was sequenced to address phylogenetic relationships. Fifteen species belonging to the section Chaetoceros were recorded, including two new species, C. hainanensis sp. nov. and C. tridiscus sp. nov. Chaetoceros hainanensis was characterized by straight chains, narrowly lanceolate to hexagonal apertures, sibling setae diverging in nearly right angles, stipule-shaped spines on terminal setae and arrowhead-shaped spines on intercalary setae. C. tridiscus had short straight chains, narrowly lanceolate apertures, arrowhead-shaped spines and circular poroids arranged in a grid pattern on terminal and intercalary setae. The phylogenetic analyses revealed six groups formed by 19 species within the section Chaetoceros, which was found to be monophyletic. The subdivision of the section is still not well understood. The morphological characters within each group varied considerably and molecular information on more species are needed to enrich the phylogenetic profiling.
... Species recognition is normally difficult because a number of the species have been described based on light microscopy only. The delineation of several commonly recorded species has been amended-for example, C. socialis [2], C. compressus and C. contortus [3], C. debilis [4]-and new species have also been described [5][6][7][8][9][10]. ...
The globally distributed Chaetoceros elegans belongs to the C. lorenzianus complex and is characterized by having tear-shaped setae poroids. Several strains of C. elegans were established from Chinese coastal waters. The vegetative cells and the resting spores were observed using light and electron microscopy. Phylogenetic analyses of two nuclear ribosomal RNA genes (SSU and the D1–D3 region of LSU) and the internal transcribed spacer (ITS) revealed that the C. elegans strains clustered into three clades, corresponding to different morphotypes. Based on the type material, the delineation of C. elegans was amended, and two new taxa, C. macroelegans sp. nov. and C. densoelegans sp. nov., were described. The two new taxa are featured by the presence of two types of setae poroids, tear-shaped and round-oval setae poroids, whereas only tear-shaped setae poroids are seen in C. elegans. The setae base is distinct in C. elegans, but absent or short in the two new taxa. In C. macroelegans, the tear-shaped poroids on the intercalary setae are larger and less densely spaced than in the other two species. The round-oval setae poroids are more densely spaced in C. densoelegans than in C. macroelegans, although they have more or less the same size. Resting spores characterize the two new taxa, but are unknown in the amended C. elegans. When comparing the ITS2 secondary structure, two and four compensatory base changes (CBCs) distinguish C. elegans from C. macroelegans and C. densoelegans, respectively. Between the two new taxa, no CBC but five hemi-CBCs (HCBCs) are present. The shape, size and density of the setae poroids, as well as the morphology of the resting spores, are important characteristics for species identification among the presently nine known species within the C. lorenzianus complex.
... The genus is highly diverse with more than 200 accepted species (Rines & Hargraves, 1988;Hernández-Becerril, 1996;Shevchenko et al., 2006;Guiry, 2019), and new species are continuously described (e.g. Li et al., 2013;Chen et al., 2018;Kaczmarska et al., 2019;Xu et al., 2019aXu et al., , 2019b. The species appear to be widely distributed (Malviya et al., 2016;De Luca et al., 2019a) and many of the morphologically recognized species show remarkable cryptic diversity (Chamnansinp et al., 2013(Chamnansinp et al., , 2015Balzano et al., 2017;Gaonkar et al., 2017;Li et al., 2017;Xu et al., 2018Xu et al., , 2019aXu et al., , 2019bXu et al., , 2020. ...
Among the marine planktonic diatoms, Chaetoceros is among the most species-rich genera, and many Chaetoceros species are considered important primary producers. However, little is known about the ecology and distribution of the few small solitary species within this genus, including Chaetoceros tenuissimus. This article describes a minute Chaetoceros strain, identified as C. tenuissimus (named CT16ED) that was isolated at a coastal lagoon in Corsica Island, Western Mediterranean. The strain was characterized by light microscopy and scanning and transmission electron microscopy, with a specific focus on the fine structure and construction of setae and its behaviour in culture. The CT16ED strain was compared with other strains we isolated from the species type locality (Ostend Harbour, North Sea) by sequencing a fragment of the nuclear ribosomal DNA (rDNA) spanning from the 18S rDNA to the D3 region of the 28S rDNA, and the plastid rbcL gene that codes the large RuBisCO subunit. Based on the literature and the available sequence data, the analysed strains were similar to C. tenuissimus but the phylogenetic analysis indicated a C. tenuissimus species complex that contained several clades, therefore the current taxonomic status of C. tenuissimus is discussed. The comparison with the available rDNA and rbcL sequencing data of strains assigned to species considered as synonyms of C. tenuissimus, including Chaetoceros simplex var. calcitrans, Chaetoceros calcitrans and Chaetoceros calcitrans f. pumilus, suggested that these taxa are paraphyletic within the genus Chaetoceros.
... The genus is highly diverse with more than 200 accepted species (Rines & Hargraves, 1988;Hernández-Becerril, 1996;Shevchenko et al., 2006;Guiry, 2019), and new species are continuously described (e.g. Li et al., 2013;Chen et al., 2018;Kaczmarska et al., 2019;Xu et al., 2019aXu et al., , 2019b. The species appear to be widely distributed (Malviya et al., 2016;De Luca et al., 2019a) and many of the morphologically recognized species show remarkable cryptic diversity (Chamnansinp et al., 2013(Chamnansinp et al., , 2015Balzano et al., 2017;Gaonkar et al., 2017;Li et al., 2017;Xu et al., 2018Xu et al., , 2019aXu et al., , 2019bXu et al., , 2020. ...
ABSTRACT
Among the marine planktonic diatoms, Chaetoceros is among the most species-rich genera, and many Chaetoceros species are considered important primary producers. However, little is known about the ecology and distribution of few small solitary species within this genus, including Chaetoceros tenuissimus. This article describes a minute Chaetoceros strain, identified as C. tenuissimus and named CT16ED, that was isolated at a coastal lagoon in Corsica Island, Western Mediterranean. The strain was characterized by light microscopy and scanning and transmission electron microscopy, with a specific focus on the fine structure and construction of setae, and by studying its behaviour in culture. Then, the CT16ED strain was compared with other strains we isolated from the species type locality (Ostend harbour, North Sea) by sequencing a fragment of the nuclear ribosomal DNA (rDNA) spanning from the 18S rDNA to the D3 region of the 28S rDNA, and the plastid rbcL gene that codes the large RuBisCO subunit. On the basis of the literature and the available sequencing data, the analysed strains were similar to C. tenuissimus, but the phylogenetic analysis evidenced a C. tenuissimus species complex that contained several clades. The current taxonomical status of C. tenuissimus is discussed. The comparison with the available rDNA and rbcL sequencing data of strains assigned to species considered as synonyms of C. tenuissimus, including Chaetoceros simplex var. calcitrans, Chaetoceros calcitrans and Chaetoceros calcitrans f. pumilus, suggested that these taxa are paraphyletic in the genus Chaetoceros.
... New approaches (e.g. the use of combined morphological and molecular tools) have been recently used which have revealed a cryptic diversity of the genus (Gaonkar et al. 2018), with revisions of its phylogenetic relationships (Kooistra et al. 2010;De Luca et al. 2019a), its world distribution (Malviya et al. 2016;De Luca et al. 2019b), and especially with the description of many new species (Li et al. 2013(Li et al. , 2015(Li et al. , 2017Gaonkar et al. 2017;Xu et al. 2018;Kaczmarska et al. 2019). ...
During the study of various marine planktonic diatoms cultured from material of the tropical Mexican Pacifi c, a species of the genus Chaetoceros was found, which provisionally was identifi ed as Chaetoceros crinitus Schütt, given its main morphological characters. A critical review of the literature and the morphological observations of the species revealed the fi nding of a new species here named Chaetoceros crawfordii sp. nov. The morphology of Chaetoceros crawfordii was studied using LM and SEM, and the taxonomy of this species is discussed. Chaetoceros crawfordii is a chain-forming species, with straight and relatively long chains ranging from 8 to 17 cells per chain. Sibling valves are connected by both intercalary setae and four protuberances per valve located at the apices of the valves, close to where the setae arise. The valves have a pattern of costae running from slightly eccentric annuli, and only terminal valves show one rimoportula. All setae arise from the apices of the valves and may be straight or curved, and are apparently directed at random. Setae are cylindrical (circular in cross section) and have a pattern of small spines and rows of small poroids arranged in spirals along the seta. No resting spores were detected. The species looks superfi cially similar to Chaetoceros crinitus and C. pseudocrinitus Ostenfeld in LM, but it appears more closely related to Chaetoceros costatus Pavillard, because four protuberances on the valves are also found in the former species. Another species having the same type of connection by valve protuberances is Chaetoceros pseudocurvisetus Mangin, but the diff erences are stressed.
... Studies carried out using electron microscopy recognized other morphological characters supporting the distinction of the two subgenera, such as different patterns of valve and setae structures, relative thickness of setae, and the presence of rimoportulae in every valve or only in terminal valves (Evensen & Hasle 1975;Hernández-Becerril 1996). Recently, combining morphological and molecular data, the delimitation of several Chaetoceros species was re-assessed, and some novel species of subgenus Hyalochaete were described, especially from Asian warm coastal waters (Rines et al. 2000;Li et al. 2013Li et al. , 2015Li et al. , 2017Chamnansinp et al. 2015;Xu et al. 2019Xu et al. , 2020. In contrast, studies on the subgenus Chaetoceros were very few during the past decades, with only the descriptions of C. octagonus Hernández-Becerril from the coast of Baja California (Hernández-Becerril 1992) and C. atlanticus var. ...
... Chaetoceros is monophyletic whereas Chaetoceros subg. Hyalochaete is not (Kooistra et al. 2010;Li et al. 2013Li et al. , 2015. The subdivision of subgenus Chaetoceros into sections is less stable. ...
Chaetoceros Ehrenberg is one of the most diverse genera of planktonic diatoms and includes important primary producers in marine waters. A valve linking protuberance is a unique character that has only been recorded in Chaetoceros rostratus and C. rostratus var. glandazii. However the taxonomical relationship between the two taxa needs to be clarified. In order to clarify species delimitations, monoclonal strains were established from Chinese southeastern coastal waters. Vegetative cells were observed using light microscopy and electron microscopy of the frustule. The region D1–D3 of the nuclear-encoded large subunit ribosomal DNA and the small subunit ribosomal DNA were sequenced to determine phylogenetic relationships. The phylogenetic analyses inferred from both SSU and LSU rDNA showed that the strains bearing valve linking protuberance clustered into two groups, which corresponded to different morphotypes. One group included the strains isolated from Hong Kong, the type locality of C. rostratus, and matched the original description; this was used herein to delineate the characters of C. rostratus. Another group is described as a new species, Chaetoceros separatus sp. nov., characterized by a long tubular valve linking protuberance and disjoint sibling setae. Previous reports of C. rostratus and C. rostratus var. glandazii were re-examined. The phylogenetic results showed C. peruvianus grouping with species of Chaetoceros sect. Rostrata, thereby suggesting the synonymy between this section and Chaetoceros sect. Peruviana. However, the identity and morphological variability of C. peruvianus need to be re-examined.
... The genus is taxonomically well-studied with many species new to science being described frequently (e.g. Balzano et al. 2017;Gaonkar et al. 2017Gaonkar et al. , 2018Kooistra et al. 2010;Li et al. 2013Li et al. , 2015Li et al. , 2017Xu et al. 2018). ...
... nov. in that the basal parts of their terminal and intercalary setae do not bend sharply out of the apical plane, their terminal setae orient in a broad U-or V-shape and their intercalary setae orient more regularly in a narrow V-shape over their entire length, and all setae remain visible in broad valve-view. All the taxa within Clade VIIb (Gaonkar et al. 2018;Li et al. 2013) diff er from C. turingii sp. nov. in that their intercalary setae proceed for a short distance before fusing briefl y in, or just outside, the valve margin. ...
In the present study we describe and illustrate a new species in the genus Chaetoceros, C. turingii sp. nov., from the Gulf of Naples, Italy. Colonies have a brush-like appearance due to the apparent randomness in seta orientation. The defining feature of this species is the regular pattern of narrow ridges on the exterior of the terminal valve face, which is visible in electron microscopy. In addition, a petal- like collar is present at the base of each intercalary seta. Its nuclear-encoded 18S rDNA and partial 28S rDNA sequences were resolved in a clade with C. anastomosans, C. dayaensis, C. cf. vixvisibilis as well as with three other Chaetoceros species still in need of description. The new species is compared with morphologically similar species, and their differences are discussed.
... The species observed in the metabarcodes, but not in LM, could either be exceedingly rare at the LTER (C. cinctus and C. debilis) or hardif not impossibleto distinguish in LM from their close relatives (Li et al., 2013;Li et al., 2017;Gaonkar et al., 2018). Our results are in line with those of other HTS studies, which revealed many more species than determined using culture-dependent and Sanger sequencing methodologies Stoeck et al., 2009;Nolte et al., 2010;Pawlowski et al., 2011;Logares et al., 2012;Bittner et al., 2013). ...
... In the autumn, a secondary annual increase of nutrients in the water column favours a plankton community (Zingone et al., 1995) apparently including a series of unknown and only recently described species. These summer and autumnal species usually include references or environmental sequences from the tropics and from temperate regions in summer, e.g., C. elegans (Li et al., 2017) and C. rotosporus (Li et al., 2013). Many species that typically increase in spring are already found in early and midwinter in the GoN, where even in this season sunlight is still sufficient for diatom blooms to develop when stable meteorological conditions persist for some days (Zingone et al., 2010). ...
High throughput sequencing (HTS) metabarcoding is commonly applied to assess phytoplankton diversity. Usually, haplotypes are grouped into operational taxonomic units (OTUs) through clustering, whereby the resulting number of OTUs depends on chosen similarity thresholds. We applied, instead, a phylogenetic approach to infer taxa among 18S rDNA V4‐metabarcode haplotypes gathered from 48 time‐series samples using the marine planktonic diatoms Chaetoceros and Bacteriastrum as test case. The 73 recovered taxa comprised both solitary haplotypes and polytomies, the latter composed each of a highly abundant, dominant haplotype and one to several minor, peripheral haplotypes. The solitary and dominant haplotypes usually matched reference sequences, enabling species assignation of taxa. We hypothesise that the super‐abundance of reads in dominant haplotypes results from the homogenisation effect of concerted evolution. Reads of populous peripheral haplotypes and dominant haplotypes show comparable distribution patterns over the sample dates, suggesting that they are part of the same population. Many taxa revealed marked seasonality, with closely related ones generally showing distinct periodicity, whereas others occur year‐round. Phylogenies inferred from metabarcode haplotypes enable delineation of biologically meaningful taxa, whereas OTUs resulting from clustering algorithms often deviate markedly from such taxa. This article is protected by copyright. All rights reserved.
... The shape of apertures has been reported as rectangular in early descriptions (Cleve 1894, Hustedt 1930), but often hexagonal and broadly elliptical later (Cupp 1943, Rines and Hargraves 1988, Hern andez-Becerril 1996, Jensen and Moestrup 1998. Morphological variations were also reported in the resting spores, characters which are usually considered as key criteria for identification of Chaetoceros taxa (Hasle and Syvertsen 1997, Li et al. 2013, 2015. The resting spore of C. debilis was first reported by Hustedt (1930), more than thirty years after the discovery of the vegetative cells. ...
... Morphological variations have also previously been reported for the resting spores, involving characters which are usually considered as a key criterion for identification of Chaetoceros taxa (Hasle and Syvertsen 1997, Li et al. 2013, 2015. The variations mainly focus on the humps FIG. 8. Phylogenetic tree inferred from maximum likelihood based on D1-D3 region of LSU rDNA, with Chaetoceros teres (MG914628) and C. lauderi (MG914553) as outgroups. ...
Chaetoceros debilis is considered one of the most abundant and widespread diatoms in the coastal marine phytoplankton, and is often used in research studies ranging from ecophysiology and molecular biology to oceanography and aquaculture. To clarify the species delineation of C. debilis and explore the diversity among C. debilis sensu lato taxa, monoclonal strains were established from different geographical regions, including the Danish coast, close to the type locality of C. debilis, the Denmark Strait (between Iceland and Greenland) as well as the Taiwan Strait, East China Sea and Daya Bay, South China Sea. Vegetative cells and resting spores were examined using light and electron microscopy. The hypervariable D1-D3 region of the nuclear large subunit ribosomal gene and the small subunit ribosomal gene were sequenced to address phylogenetic relationships. In both SSU and LSU trees, the C. debilis sensu lato strains clustered in four distinct clades. Culture material from the type locality was, along with molecular data, used to delineate and emend the description of C. debilis. Based on molecular data and detailed morphological features, one of the clades originating from Chinese waters were described as C. galeatus sp. nov., characterized by curved and helical chains, elliptical valves, and smooth and helmet-shaped primary valves of the resting spores. The remaining two clades, which also represent two novel taxa, C. debilis-1 and C. debilis-2, were not formally described as new species due to the insufficient information on their morphology.
... Yet, their study included fewer species than that of Rines and Theriot (2003). Recent diversity studies in Chaetocerotaceae have provided detailed morphological and ultrastructural illustrations as well as sequence data of numerous taxa, many of which are new to science (Bosak et al., 2015(Bosak et al., , 2016Bosak and Sarno, 2017;Gaonkar et al., 2017Gaonkar et al., , 2018Kooistra et al., 2010;Li et al., 2013Li et al., , 2017Xu et al., 2019). However, since most of these studies generally focused on the diversity within sections, their phyletic status remains to be resolved. ...
... Instead, , following Gran's (1905) emended description, provided a broader definition, which includes also features of the vegetative cells and which accommodates the spiny resting spores of C. diadema and C. seiracanthus as well as the smooth ones of C. rotosporus . Our phylogenetic tree supports findings of Li et al. (2013) and Gaonkar et al. (2018) that the aforementioned taxa in section Diadema form a clade. Thus, all these species, including the recently described C. rotosporus, fit perfectly fine in the monophyletic section Diadema. ...
The diatom family Chaetocerotaceae (Bacillariophyta) is common in the marine plankton worldwide, especially in coastal areas and upwelling zones. Its defining character constitutes hollow processes, called setae, which emerge from the valves of the vegetative cells. The family comprises two extant genera: Bacteriastrum and Chaetoceros. Current systematics is based on morphological features of vegetative cells and resting spores and is summarised in a classification scheme subdividing Bacteriastrum in two sections, Isomorpha and Sagittata, and Chaetoceros in three subgenera: Hyalochaete, Chaetoceros (Phaeoceros) and Bacteriastroidea, and further into 22 sections. Phylogenies inferred from single molecular markers (18S and partial 28S rDNA) show only partial topological agreement and many poorly or unresolved basal ramifications. Since classification should not only satisfy practical needs but also reflect well-supported evolutionary relationships of the taxa under investigation, we inferred a multigene phylogeny of the family Chaetocerotaceae amplifying five genes of 100 strains encompassing six Bacteriastrum and 60 Chaetoceros species. We also compared the phylogenetic signal of nuclear, plastid and mitochondrial compartments to ascertain if the inferred tree topologies were congruent. Our results provided a robust multigene phylogeny of the family Chaetocerotaceae, offering a solid framework to test the validity of the traditional taxonomical classification. The genera Bacteriastrum and Chaetoceros were resolved as sister clades, whilst the subgenus Hyalochaete was found to be paraphyletic. Consequently, we rejected the subdivision in subgenera and only considered sections. Most of the already recognised sections were found to be monophyletic. We emended one section, rejected seven and erected three new ones. As a consequence of our proposed changes, all the sections investigated are supported by morphological and molecular characters alike. Thus, a natural classification is feasible for this important and very diverse marine planktonic family.
