Chrysosphaerella septentrionalis sp. nov. (Chrysophyceae, Chromulinales), a New Species from the Arctic Including the Description of Chrysosphaerellaceae, fam. nov.

Abstract

A new species, Chrysosphaerella septentrionalis, is described from a peat bog located on the bank of the Paz River (Pasvik Nature Reserve, Murmansk Region, Russia). Scale ultrastructure was studied using a scanning electron microscope. Morphologically, C. septentrionalis seems to closely resemble C. longispina. However, C. septentrionalis possesses subcircular scales in addition to the oval scales, and they are much smaller than in C. longispina. We suppose that C. septentrionalis is the first pseudocryptic species within the C. longispina complex. Additionally, we proposed an infrageneric classification of Chrysosphaerella based on the scale structure and divided the genus into three sections: Chrysosphaerella, Brevispinae sect. nov. and Septispinae sect. nov. The formal description of a new family Chrysosphaerellaceae fam. nov. is also provided.

1. Introduction

The genus Chrysosphaerella—with a single species, C. longispina—was described by Lauterborn [1]. Its spherical colonies have a Synura-like appearance but possess long spines. Several years later, he provided additional observations and a figure of this species [2].

Korshikov [3] described a second species within the genus, Chrysosphaerella brevispina. He carefully illustrated the scales and spines of both species and showed the difference in scale ornamentation and spine length. Moreover, he discovered a second short flagellum in both C. longispina and C. brevispina and proposed to place Chrysosphaerella within the family Synuraceae.

The first electron microscopical observations on the ultrastructure of Chrysosphaerella scales were made by Fott and Ludvík [4] who studied Chrysosphaerella brevispina. Later, Harris and Bradley [5] amended the description of this species and synonymized C. rodhei Skuja [6] with it. In 1964, Bradley—based on light and electron microscopical observations–described a new species, C. multispina [7]. This species resembled C. longispina but differed mainly in having a greater number of spines of different length. At the time of description of C. multispina, the scale ultrastructure of C. longispina was unknown, so Bradley [7] made a comparison based on Korshikov’s light microscopical observations. According to Bradley, the scales of C. multispina have a smooth margin and reticulate pattern at the center, whereas in C. longispina, scales have a “thickened inner ring with spokes radiating to margin” ([7], p. 331). Subsequently, many authors pointed out that C. longispina and C. multispina might be conspecific [8,9,10,11]. Finally, Nicholls [12] amended C. longispina and regarded C. multispina as its synonym.

Almost simultaneously, Wujek et al. [13] and Preisig and Takahashi [14] described the same species under the names Chrysosphaerella coronacircumspina and C. solitaria, respectively. Unlike C. longispina—the generitype—which is a colonial organism, this species is unicellular. Preisig and Takahashi [14] even established a new subgenus, Chrysosphaerella subgen. Pseudochrysosphaerella, to accommodate a solitary species of Chrysosphaerella. However, this taxon no longer exists because its type species, Chrysosphaerella salina Birch-Andersen is not a chrysophyte, and it has been transferred to Thaumatomastix [15].

After the description of a noncolonial species, the differences between Chrysosphaerella and Spiniferomonas, which has similar scale ultrastructure but is solitary rather than colonial [16], became unclear. Nicholls [12] proposed that both genera could be distinguished by the structure of spine-scales: in Spiniferomonas the base of the spine is cone-shaped, cup-shaped, or the spine is fixed to a simple flat disc, whereas in Chrysosphaerella, the base of the spine is bobbin- or pulley-like. However, after the discovery of two new species which had scale morphological features common to both Spiniferomonas and Chrysosphaerella, he changed his mind and decided to separate these genera based on cell habit—colonial in Chrysosphaerella or solitary in Spiniferomonas [17]. Kristiansen and Tong [18] disagreed with him and argued that morphology of siliceous structures has more taxonomic value than cell habit. The latter point of view is currently widely accepted; however, no additional data are available to support any of these hypotheses.

Taxonomic placement of Chrysosphaerella remained unclear. It was included together with other photosynthetic (Spiniferomonas and Polylepidomonas) and heterotrophic (Paraphysomonas sensu lato) genera in the family Paraphysomonadaceae [19]. Molecular analysis performed by Škaloud et al. [20] clearly showed that Chrysosphaerella is unrelated to Paraphysomonas, so, Chrysosphaerella cannot belong to the Paraphysomonadaceae, which currently include only the heterotrophic genus Paraphysomonas sensu stricto [21]. Recently, Kapustin et al. [22] have proposed a provisional family name Chrysosphaerellaceae to classify Chrysosphaerella within the Chromulinales. Therefore, we aimed to describe a new species of Chrysosphaerella—C. septentrionalis, belonging to the C. longispina complex—and formally describe a new family, Chrysosphaerellaceae.

2. Results

Chrysosphaerella septentrionalis Kapustin sp. nov. (Figure 1).

Colonies multicellular, dimensions unknown. Cells covered with plate-like and spine-like siliceous scales. Plate-like scales oval to subcircular, 2.4–3.0 × 1.8–3.0 µm, consist of a raised and smooth central area (cupola), a plain smooth marginal rim and radial ribs between them; small oval unpatterned scales (2.1 × 1.1 µm) also occur. Spine-scales (4.3–8.3 µm and 13.0–31.3 µm in length) consist of two baseplates connected by a wineglass-shaped shaft and a tubular spine with a flattened and bifurcate tip; a large circular hole presents at the spine base. Stomatocysts unknown.

Holotype (here designated): Portion of a single gathering of cells on SEM stub #P32 deposited at the Herbarium of the Papanin Institute for Biology of Inland Waters RAS, Borok (IBIW). Sample was collected on 19 June 2019. Figure 1A illustrates the holotype.

Type Locality: Peat bog on the bank of the Paz River (69°23.489′ N, 29°45.388′ E), Pasvik Nature Reserve, Murmansk Region (Russia).

Etymology: The species epithet, which means “northern” in Latin, refers to the distribution of this species in a high-latitude region.

Distribution: So far, this species is known from its type locality only.

3. Discussion

3.1. Species Diversity of Chrysosphaerella

Like in the case of other silica-scaled chrysophytes, the taxonomy of the genus Chryso-sphaerella is based almost exclusively on the ultrastructure of spines and scales. With the addition of our new species, there are 12 currently accepted taxa within the genus (

Table 1. Checklist of all previously described species of the genus Chrysosphaerella.

Taxon	Taxonomic Status	Reference(s)
Chrysosphaerella annulata Kristiansen & D. Tong	Accepted	[18]
Chrysosphaerella astrea Dürrschmidt	Accepted	[23]
Chrysosphaerella baikalensis Popovskaya	Accepted (Probably, it is conspecific with C. brevispina)	[24]
Chrysosphaerella brevispina Korshikov	Accepted	[5]
Chrysosphaerella conradii Bourrelly	Synonym of C. brevispina	[10]
Chrysosphaerella coronacircumspina Wujek & Kristiansen var. coronacircumspina	Accepted	[13]
Chrysosphaerella coronacircumspina var. grandibasa Balonov	Accepted	[25]
Chrysosphaerella enigmata (K.H. Nicholls) Kristiansen & D. Tong	Accepted	[18]
Chrysosphaerella longispina Lauterborn	Accepted	[12]
Chrysosphaerella multispina Bradley	Invalid Synonym of C. longispina	[12]
Chrysosphaerella nichollsii D. Kapustin & E.S. Gusev	Accepted	[26]
Chrysosphaerella parva Asmund	Synonym of two Spiniferomonas taxa, S. abei E. Takahashi and S. bilacunosa E. Takahashi	[27]
Chrysosphaerella patelliformis E. Takahashi & Hara	Synonym of Thaumatomastix patelliformis (E. Takahashi & Hara) Beech & Moestrup	[15]
Chrysosphaerella rodhei Skuja	Synonym of C. brevispina	[10]
Chrysosphaerella rotundata Škaloudová & Škaloud	Accepted	[28]
Chrysosphaerella salina Birch-Andersen	Synonym of Thaumatomastix salina (Birch-Andersen) P.L. Beech & Moestrup	[15]
Chrysosphaerella septispina (K.H. Nicholls) Kristiansen & D. Tong	Accepted	[18]
Chrysosphaerella setifera Schiller	Insufficiently described	[10]
Chrysosphaerella solitaria Preisig & E. Takahashi	Synonym of C. coronacircumspina	[29]
Chrysosphaerella triangulata Balonov	Synonym of Thaumatomastix triangulata (Balonov) P.L. Beech & Moestrup emend. K.H. Nicholls	[15,30]
Chrysosphaerella tripus E. Takahashi & Hara	Synonym of Thaumatomastix tripus (E. Takahashi & Hara) P.L. Beech & Moestrup	[15]

). Molecular data are available for three species, namely C. brevispina, C. longispina, and C. rotundata.

Our new species, Chrysosphaerella septentrionalis, is extremely similar to C. longispina (Figure 2), the generitype, and C. multispina. The latter taxon is considered to be conspecific to C. longispina [12]. Additionally, in terms of botanical nomenclature, C. multispina is invalid because the type has not been indicated (Art. 40.1).

In contrast to C. longispina which has elliptical or oval plate-scales up to 6 µm long, the majority of plate-scales in C. septentrionalis are subcircular and do not exceed 3 µm in diameter. The scale structure in both species is similar (see Figure 1 and Figure 2). It consists of a raised and smooth central area (cupola) connected by radial ribs to a smooth marginal rim [12,31]. Interestingly, those subcircular or broadly elliptic plate-like scales were depicted in micrographs published by Bradley (Pl. 6, Figure 44, [7]) and Asmund (Figure 5, [10]) for C. multispina or Siver et al. (Figure 7H, [32]) for C. longispina. However, unlike C. septentrionalis in the abovementioned cases, the circular scales are not a dominant scale type and, their size does not exceed the size of elliptic scales.

Additionally, C. septentrionalis has spine-scales of two size classes, similarly to C. longispina. However, in C. longispina, they have a wider range of length [12,31]. Morphometrical data of both species are summarized in

Table 2. Morphometrical comparison between Chrysosphaerella septentrionalis and C. longispina (incl. C. multispina).

Species	Plate-like Scales (Length × Width, µm)	Spine-like Scales (Length, µm)	Reference
Chrysosphaerella septentrionalis	2.4–3.0 × 1.8–3.0 unpatterned scales: 2.1 × 1.1	4.3–8.3 13.0–31.3	This study
Chrysosphaerella longispina (=C. multispina)	0.6–6.0	5–10 20–25 35–40	[7]
	2.0–2.5 × 1.25–1.6 4.6–6.0 × 2.1–3.3 unpatterned scales: 1.3–1.7 × 0.9–1.0	3–4 up to 50	[8]
	3.5–6.0 × 2.2–3.0 unpatterned scales: 1.8–2.5 × 1.0–1.6	3–85	[12]
	4.2–2.1 (mean size)	3.7–6.8 13–53 up to 71	[31]

.

Relatively recently, Škaloudová and Škaloud [28] clearly showed the existence of hidden diversity within the genus Chrysosphaerella and described the first pseudocryptic species, C. rotundata which can be attributed to the C. brevispina-species complex. Chrysosphaerella septentrionalis belongs to another species complex, namely C. longispina. Probably, the specimen depicted by Bessudova et al. [33] and identified as C. longispina represents a currently undescribed species from this species complex. Therefore, we totally agree with Němcová et al. [34], who stated that the diversity of Chrysosphaerella is largely undescribed, and our discovery of a new species C. septentrionalis supports this view.

3.2. Infrageneric Classification of the Genus Chrysosphaerella

There has only been one attempt to develop an infrageneric classification of Chrysosphaerella based on colonial vs. solitary habitat [14]. Unfortunately, the type of the subgenus Pseudochrysosphaerella, which comprises solitary species, belongs to the thaumatomonads (Cercozoa) rather than the chrysophytes. Thus, this name cannot be used anymore.

In our opinion, based on the scale ultrastructure the genus can be divided into the following three sections: Chrysosphaerella, Brevispinae, and Septispinae.

• Chrysosphaerella sect. Chrysosphaerella

Exclusively colonial chrysophytes. Plate-scales consist of a raised and smooth central area (cupola) connected by radial ribs to a smooth marginal rim. Spine-scales consist of two baseplates and a spine with a large hole at its base.

Type species: Chrysosphaerella longispina Lauterborn. 1896. Zool. Anz. 19: 16.

At present, this section consists of two species: C. longispina and C. septentrionalis.

2.

Chrysosphaerella sect. Brevispinae Kapustin, sect. nov.

Colonial and solitary chrysophytes. Plate-scales with a thickened oval ring in the central part on the exterior surface and ornamented with a scalloped oval shaped pattern on the undersurface. Spine-scales consist of two baseplates and a spine with a large hole at its base.

Type species (here designated): Chrysosphaerella brevispina Korshikov. 1941. Arch. Protistenk. 95: 31, 32, Figure 7.

At present, this section includes eight taxa: C. astrea, C. baikalensis, C. brevispina, C. coronacircumspina var. coronacircumspina, C. coronacircumspina var. grandibasa, C. enigmata, C. nichollsii, and C. rotundata.

3.

Chrysosphaerella sect. Septispinae Kapustin, sect. nov.

Exclusively solitary chrysophytes. Plate-scales with more or less oval rings with crenulated margins, or with 10–15 min crenulated annular structures. Spine-scales consist of a single baseplate separated from the spine by a septum. A circular hole in the spine wall is located at various distances above the septum.

Type species (here designated): Chrysosphaerella septispina (K.H. Nicholls) Kristiansen and D. Tong. 1989. Nord. J. Bot. 9: 331. (≡Spiniferomonas septispina K.H. Nicholls. 1984. Pl. Syst. Evol. 148: 104, 105, Figures 1–5).

At present, this section consists of two species: C. septispina and C. annulata.

3.3. Taxonomic Placement of the Genus Chrysosphaerella

The views on the taxonomic placement of Chrysosphaerella have changed drastically for over the last 120 years. Lemmermann [35] placed Chrysosphaerella in the Mallomonadaceae, within the order Phaeozoosporinae. Interestingly, Actinoglena klebsiana Zacharias, which is now considered to be conspecific with C. longispina, was placed by him in the Synuraceae under the name Synura klebsiana (Zacharias) Lemmermann. Later, Pascher [36] classified Chrysosphaerella within the family Mallomonadaceae, in the order Chromulinales.

It should be noted that the number of visible flagella and their length was considered as an important taxonomic character at the ordinal level [37,38]. Pascher recognized three orders: Chromulinales (one flagellum), Isochrysidales (two equal flagella), and Ochromonadales (two unequal flagella). Therefore, when Korshikov [3] discovered a second short flagellum in both C. brevispina and C. longispina, he proposed to transfer Chrysosphaerella to the family Synuraceae within the order Ochromonadales. This point of view was accepted in famous treatments on chrysophytes by Bourrelly [39] and Starmach [40].

Preisig and Hibberd [19] showed that cell ultrastructure of the members from the genera Chrysosphaerella, Paraphysomonas, Spiniferomonas, and Polylepidomonas is much more similar to that of Ochromonas and Chromulina than to that of Mallomonas and Synura. Therefore, they decided to establish a new family, Paraphysomonadaceae, to accommodate Chrysosphaerella, Paraphysomonas, Spiniferomonas, and Polylepidomonas.

Cavalier-Smith et al. [41] erected the order Paraphysomonadales for exclusively colorless chrysophytes. In several studies, it was shown that Paraphysomonas sensu lato formed a distinct lineage which took a basal position to all other chrysophytes [20,42,43].

The phylogenetic position of Chrysosphaerella was reported by Andersen [44] based on a single unidentified colony. This isolate was closely related to the nonscaled genera Chromulina, Chrysamoeba, and Oikomonas. Subsequently, Škaloud et al. [20] corroborated this phylogenetic position of Chrysosphaerella by adding SSU rDNA and rbcL sequences from cultured Chrysosphaerella taxa. Therefore, they clearly showed that Chrysosphaerella is unrelated to Paraphysomonas. Currently, the family Paraphysomonadaceae is restricted to a single genus, Paraphysomonas sensu stricto, and together with another monotypic family, Lepidochromonadaceae (=Clathromonadidae), they form the order Paraphysomonadales [21,45].

Although Kapustin et al. [22] used the provisional family name Chrysosphaerellaceae, they did not provide its description. It should be noted that the name Chrysosphaerellaceae in Pascher [46] is a misprint of Chrysosphaeraceae. Therefore, a new family is formally described below:

Chrysosphaerellaceae Kapustin, fam. nov.

Colonial or solitary photosynthetic chrysophytes. Flagella two unequal. Chloroplasts one or two yellow-brown. Cells covered with siliceous scales of two main types, plate-like and spine-like. Plate-like scales elliptical, oval, or subcircular. Spine-like scales consist of a single or two base-plates and flat or tubular spine. Two genera: Chrysosphaerella and Spiniferomonas (=Chromophysomonas Preisig & Hibberd).

Type genus (here designated): Chrysosphaerella Lauterborn.

Although the members of Spiniferomonas remain unsequenced, we tentatively place this genus in the Chrysosphaerellaceae based on the similarities in scale structure. The genus Polylepidomonas most likely requires its own separate family.

4. Materials and Methods

A sample containing a putatively new species of Chrysosphaerella was collected from a peat bog located on the bank of the Paz River (69°23.489′ N, 29°45.388′ E), Pasvik Nature Reserve, Murmansk Region (Russia) by squeezing water from Sphagnum on 19 June 2019. A sample containing C. longispina was collected from the surface water layer of the Marfino bog (56°04′10.2″ N 37°32′31.8″ E), Moscow Region (Russia) using a 20 µm mesh plankton net on 14 May 2022. Environmental variables were not measured.

For scanning electron microscope (SEM) studies, a few drops from the unfixed samples were placed on aluminum stubs, air-dried, and sputter-coated with gold for 10 min. Observations were carried out with JEOL 6510 LV (IBIW RAS) or TESCAN Vega III (PIN RAS) scanning electron microscopes.