Planoplatessa gen. nov.—A New, Neglected Monoraphid Diatom Genus with a Cavum

A new monoraphid diatom genus, Planoplatessa gen. nov., is described on the basis of a detailed morphological investigation using light and scanning electron microscopy. We transferred Achnanthes joursacensis Héribaud, a species described from fossil deposits in France, to our new genus. We investigated material from Mongolia from Recent populations. This taxon is known from fossils to Recent across the Holarctic. Achnanthes joursacensis was previously transferred to the genera Planothidium and Platessa, but the morphology of A. joursacensis does not share all morphological features with these two genera. We discuss important morphological features for the delimitation of monoraphid genera based on careful morphological and molecular investigations we presented previously. Planoplatessa gen. nov. as a genus is characterized by having uniseriate striae on both the raphe and the rapheless valves, a cavum in rapheless valves only, and straight distal raphe ends on the valve face.

Of the above genera, only Gliwiczia, Planothidium, and Skabitschewskia have a very interesting morphological feature that is visible under a light microscope known as a horseshoe-shaped area [10]. The morphology of this horseshoe-like structure as seen with scanning electron microscopy includes a rimmed depression or sinus and hood or cavum (see description in [16]). Interestingly, species that possess a sinus and cavum, and those species where these features are weakly expressed have been shown to be closely related to one another and included in the genus Planothidium [16]. According to molecular data, these species can be recognized as three distinct clades: (a) species with a sinus, (b) species with a cavum, or (c) species without these features [16]. The genus Skabitschewskia is characterized by having only a cavum on rapheless valves in all known species, but this genus differs from Planothidium by striae morphology [10]. Striae in Planothidium are multiseriate on both valves, whereas, in Skabitschewskia, striae are uniseriate on raphe valves and biseriate on rapheless valves [10]. In the genus Gliwiczia, a cavum is present on both valves, and the striae are also uniseriate on both valves [9]. It is evident from the data discussed above that presence and number of horseshoe-like structures and morphology of striae are important features for the delimitation of genera between freshwater monoraphid taxa. The morphology of striae between monoraphid genera was comprehensively discussed by us during the description of the new genus Platebaikalia [14]. Freshwater monoraphid genera have different combinations of striae on raphe and rapheless valves that can include the presence of uniseriate, biseriate, or multiseriate striae. On the basis of this morphological feature, we prepared a revision of the genus Platessa, a genus that was described on the basis of species with biseriate striae in both valves and the absence of horseshoe-shaped structure [17]. After this description, many species were transferred to this genus, and Platessa soon became a catch-all taxon [14]. We described eight groups within the genus Platessa, and some of these groups have subsequently been described as new genera [14]. One of the eight groups of Platessa species includes the very interesting species Platessa joursacense (Héribaud) Chudaev which has uniseriate striae on both valves, but this species possesses a cavum on the rapheless valves [18].
P. joursacense was firstly described as Achnanthes joursacensis Héribaud 1903 [19] and for a long time was a neglected taxon. The generic position of this species has been interpreted differently by several authors. This species was included in the genus Planothidum as Planothidium joursacense (Héribaud) Lange-Bertalot 1999 [20] and later in Platessa as Platessa joursacensis (Héribaud) Chudaev [18]. The reason for this taxonomical instability is, on the one hand, the presence of cavum being a feature considered typical for Planothidum and, on the other hand, uniseriate striae in both valves not being typical for Planothidum. The presence of distal raphe ends that terminate on the valve face and straight is typical for Platessa. For a long time, this species was neglected due to its rarity and limited (only Holarctic) distribution [11]. Morphology of the raphe and rapheless valves was first investigated by Chudaev et al. [18].
The aim of this publication is to provide additional morphological evidence for the monoraphid diatom, Achnanthes joursacensis Héribaud 1903 and, on the basis of the results of this study, to describe the genus Planoplatessa gen. nov. LM, rapheless valves ( Figure 1J-R). Axial area narrowly lanceolate. Striae spaced slightly wider than in raphe valves. Visible horseshoe-shaped structure is present.

Results
Scanning electron microscopy (SEM), raphe valves (Figure 2A-F). Striae uniseriate (Figure 2A,D, white arrows). Raphe filiform and straight (Figure 2A, black arrows). Distal raphe ends straight and present on valve face outside ( Figure 2C, white arrow). Central raphe ends straight and tear-shaped outside ( Figure 2B, black arrows). Mantle is short, with two small areolae present in prolongation of every striae (Figure 2A, black arrowhead). Interstriae are wider than striae inside and outside (Figure 2A,D, white arrowhead). Inside, distal raphe ends in smaller helictoglossae and curved in opposite direction ( Figure 2F, black arrow). Central raphe ends are slim and curved to different sides ( Figure 2E, black arrows). Distal and central raphe ends are curved opposite to another. Areolae ≈ 60 in 10 µm.  SEM, rapheless valves ( Figure 3A-F). Striae uniseriate ( Figure 3D, black arrow). Interstriae are wider than striae. Areolae covered by silica membrane internally [18].  Figure 3D, black arrow). Interstriae are wider than striae. Areolae covered by silica membrane internally [18]. Externally, central area large and flat on one side of valve ( Figure 3A,B, white arrows); in another part, all striae are normally present or one central stria is slightly shorter ( Figure 3A, B, black arrows). Internally, a large cavum is present ( Figure 3E, black arrow). Striae are situated in deep alveoli ( Figure 3D, F, black arrows).

Etymology
Combining epithet refers to the similarity with two monoraphid diatom genera, Platessa and Planothidium.  Externally, central area large and flat on one side of valve ( Figure 3A,B, white arrows); in another part, all striae are normally present or one central stria is slightly shorter ( Figure 3A,B, black arrows). Internally, a large cavum is present ( Figure 3E, black arrow). Striae are situated in deep alveoli ( Figure 3D,F, black arrows).

Etymology
Combining epithet refers to the similarity with two monoraphid diatom genera, Platessa and Planothidium.
All findings of this species, with both LM and SEM, correspond to observations of the lectotype investigated previously. This taxon is characterized by uniseriate striae on both valves and the presence of a cavum on rapheless valves; external distal raphe ends are straight and placed on the valve face and do not extend onto the mantle [18,29]. This combination of morphological features makes this species easily recognizable. Populations of this species were found in the Holarctic, both fossil to Recent [18].
The presence of a cavum was a good reason to transfer this species to the genus Planothidium [20]. However, Planothidium as a genus is also characterized by having multiseriate striae on both valves. Planoplatessa joursacensis comb. nov., however, only has uniseriate striae [10]. External distal raphe ends extend onto the valve mantle in Planothidium; however, in Planoplatessa joursacensis comb. nov., they are straight and only occur on the valve face. As discussed above, Planothidium as a genus can be divided into three groups, one with cavum, a second with sinus, and a third without a cavum and sinus. However, all the taxa in these groups of Planothidium have multiseriate striae. Skabitschewskia is another genus with the presence of a cavum in rapheless valves. However, Skabitschewskia is characterized by uniseriate striae on the raphe valves and biseriate striae on the rapheless valves [10]. The genus Gliwiczia is characterized by the presence of a cavum and uniseriate striae on both valves similar to the situation in Planoplatessa gen. nov. However, Gliwiczia has a cavum on both raphe and rapheless valves, a situation that is unique among monoraphid genera.
Genera such as Achnanthidium, Eucocconeis, Psammothidium, Trifonovia, Gololobovia, and Gogorevia are characterized by having uniseriate striae on both valves [8,10,12,13]. However, these genera do not have a cavum, a feature that is typical for Planoplatessa gen. nov. All these genera are very easy to distinguish from Planoplatessa gen. nov. by a combination of morphological features such as valve shape, presence or absence of sternum and stauros internally and externally, and morphology of distal and central raphe ends (see Table 1). Combinations of stria morphology with the presence or absence of a cavum and stauros are important taxonomical features that were shown with molecular data to help diagnose freshwater monoraphid genera such as Achnanthidium, Psammothidium, Gogorevia, Karayevia, Planothidium, and Lemnicola [13,16,[30][31][32][33][34]. Planoplatessa gen. nov. as a genus is distinguished from other known monoraphid genera. Previous transfer of A. joursacensis to the genus Platessa was based on some morphological features shared between them, especially external distal raphe ends being straight and extending onto the valve face only [18]. Transfer of taxa that were divided on some morphological features between similar genera to different genera is normal practice. We discussed the same situation with the previous transfer of Achnanthes exigua Grunow 1880 to the genera Achnanthidium and Lemnicola [13]. However, our molecular investigation of species from this complex showed that a new genus was required, which we described as Gogorevia. Future research combining morphological and molecular datasets will be necessary to further resolve relationship of the other freshwater monoraphid genera and help to identify features that allow us to not only recognize and distinguish the genera, but also diagnose monophyletic taxa and create a natural classification for them [35].
The samples were boiled in concentrated hydrogen peroxide (≈37%) to dissolve organic matter. The samples were then washed with deionized water four times at 12 h intervals. After decanting and rinsing with up to 100 mL of deionized water, the suspension was spread onto coverslips and left to dry at room temperature. Permanent diatom slides were mounted in Naphrax ® . The slide was designated no. 02605. Light microscopic (LM) observations were performed using a Zeiss Scope A1 microscope equipped with an oil immersion objective (100×, n.a. 1.4, differential interference contrast (DIC)) and Zeiss AxioCam ERc 5 s camera. For scanning electron microscopy (SEM), parts of the suspensions were fixed on aluminum stubs after air-drying. The stubs were sputter-coated with 50 nm of gold. The valve ultrastructure was examined by means of a JSM-6510LV scanning electron microscope (Institute for Biology of Inland Waters RAS, Borok, Russia).

Conclusions
New insights into character combinations found in freshwater monoraphid diatoms are demonstrating key features used to diagnose genera. An easily recognizable feature in both light and scanning electron microscopy is a cavum, a hollow covering found in the valve interior of certain monoraphid genera. We can now recognize four monoraphid genera that possess a cavum: Planothidium, Skabitschewskia, Gliwiczia, and Planoplatessa gen. nov. These genera are recognizable from one another on the basis of whether there is a single cavum per frustule or two (found in Gliwiczia only), multiseriate versus uniseriate striae, and whether the external distal raphe ends extend onto the valve mantle or are restricted to the valve face. While we expect these groups to constitute a monophyletic lineage, formal analyses based on morphology and molecules will be needed to verify taxon relationships and monophyly.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the result.