Peridinium. volzii

Holzer, Victoria J. C., Kretschmann, Juliane, Knechtel, Johanna, Owsianny, Paweł M. & Gottschling, Marc, 2022, Morphological and molecular variability of Peridinium volzii Lemmerm. (Peridiniaceae, Dinophyceae) and its relevance for infraspecific taxonomy, Organisms Diversity & Evolution 22 (1), pp. 1-15 : 3-5

Morphology of Peridinium. volzii

All investigated strains exhibited cells that were either flagellated ( Fig. 1a, i ) or coccoid ( Fig. 1k–m ), with the motile cells being predominant. Motile cells swam homogenously within the medium or occasionally, they accumulated towards the light source. Each cell, except for necrotic cells, contained numerous gold- through olive-brownish chloroplasts and an orange to red accumulation body ( Fig. 1i ). An eyespot was absent. Necrotic cells included plastids that appeared grey through silver.

All strains were indistinguishable from each other in gross morphology, and motile cells were continuously covered by a theca built of cellulosic plates (as inferred from astra blue staining). The thecate cells had an ovoid shape and were slightly compressed dorso-ventrally. The hypotheca was generally smaller than the epitheca. Average cell sizes of motile cells within the ten strains under detailed study ranged from 41 to 51 µm in length and from 36 to 48 µm in width ( Table 1 ). However, cell sizes of particular strains were significantly different from each other ( Fig. 2a, b ). All strains presented the thecal formula 4′, 3a, 7′′, 5c, 5s, 5′′′, 2′′′′, with few exceptions (see below). The surface of the plates was strongly reticulate, and each major plate of the epi- and hypothecae contained one or two pores.

The epithecal plate pattern was largely symmetrical, although the apical plate 2′ was always smaller than the apical plate 4′ ( Fig. 1a , c-d, f). The apical plate 1′ appeared small, was rhomboid and ca 12–14 μm tall. Two significantly different suture lengths between the plates 1′ and 4′ were distinguishable across the strains ( Fig. 2c ), being shorter ( Fig. 1a ) or longer ( Fig. 1 c-d). Both apical plates 2′ and 4′ appeared hexagonal in shape and plate 3′ pentagonal. The apical plate 3′ is separated from plate 1′ by plates 2′ and 4′ and was either as wide as tall ( Fig. 1f ) or slightly wider than tall ( Fig. 1c ). The intercalary plates 1a and 3a were pentagonal, had the same size and were smaller than the elongated plate 2a ( Fig. 1c, f ).

Deviations from the archetypical plate pattern could be stated ( Tables 1 , 2 and 3 ). Usually tetragonal, precingular plate 4′′ was occasionally pentagonal ( Fig. 3a, b ) and abutted plate 3a, from which it was otherwise separated. This conformation affected the position and size of the intercalary plate 2a, which appeared smaller and less elongated. In few individual cells of all strain, the intercalary plate 2a ( Fig. 3c ), the apical plate 3′ ( Fig. 3m ) or the postcingular plate 3′′′ ( Fig. 3i ) were split, and also unusual fusions of plates could be observed ( Table 3 ; Fig. 3d, f ).

The cingulum was median or sub-median and descending. The displacement was significantly different between the strains ( Table 1 ), with either ca 1 cingulum width ( Fig. 1a ) or ca 1.5–2 cingulum widths ( Fig. 1d ). Notably, this distinction correlated with the two different suture lengths between apical plates 1′ and 4′ ( Table 1 ). The sulcus appeared narrow and extended into the epitheca for approximately a cingulum width ( Fig. 1a, d ). Two flagella originated from the junction between sulcus and cingulum and were visible in LM.

Cell division was by eleutheroschisis. The resulting empty thecae were collected in high quantity at the bottom of the cultivation plate. The theca opened along the edge of the cingulum that epitheca and hypotheca were separated. The predominant opening of the epitheca was exhibited at the apical side between the three intercalary plates and the plates 2′ and 4′ ( Fig. 1f ). The cell opening continued at the sutures between the precingular plates 5′′ and 6′′ as well as 2′′ and 3′′, resulting in the release of the dorsal part of the epitheca. Subsequently, this thecal split led to the release of the ventral epitheca, as well as the separation of epitheca and hypotheca ( Fig. 1m ). This specific dehiscence line was present in all strains with a frequency of 66–82% ( Table 1 ). Deviations from the regular opening line predominantly occurred at the precingular plates in all strains investigated. Sutures between thecal plates varied from thin lines through wide bands, the latter exhibiting cross striations ( Figs. 1f , 3b, e, g ).

Coccoid cells varied in their average size from 39 to 50 µm in length and from 32 to 45 µm in width ( Table 1 ) and were either thecate or athecate. Their shapes were variable ranging from spherical through obovoid. Some coccoid cells exhibited a slight dorso-ventrally flattening ( Fig. 1l ). Within these cells, brown granules of varying sizes were observed in addition to the plate-like chloroplasts. A single coccoid cell was observed to develop intrathecately ( Fig. 1m ), and the ecdysing cell was released by thecal opening along the edge of the cingulum.

Figures

◂Fig.1 Morphology of thecate and coccoid cells, with labelled thecal plates. a–c, i, m Light microscopy, d–h, k–l scanning electron microscopy. a Ventral view of strain GeoM*788; b dorsal view of strain GeoM*793; c apical view of strain GeoK*044; d ventral view of strain GeoK*037; e dorsal view of strain GeoM*788; f apical view of strain GeoK*024, with the dehiscence of epithecal opening indicated by a blue line; g antapical view of strain GeoK*044; h leftlateral view of strain GeoM*866; i motile cell of strain GeoK*037; k–m coccoid cells showing variability in shape and size of strains k GeoM*866, l GeoM*793 and m GeoK*024. Abbreviations: n′: apical plate, n′′: precingular plate, n′′′: postcingular plate, n′′′′: antapical plate, na: anterior intercalary plate, nC: cingular plate, Sa: anterior sulcal plate, Sd: right sulcal plate, Sp: posterior sulcal plate. Ss: left sulcal plate. Scale bar: 10 µm. UA: 15 kV

Fig. 2 Box plots displaying correlations between cell size (of motile cells; A, B) or side length (of empty thecate cells; C) in selected strains. Colours correspond to the ribotype of each strain (blue: ribotype I; magenta: ribotype II; orange: ribotype III). Statistically significant clusters are indicated with letters a, b and c and were calculated with Tukey’s Honest Significant Difference (HSD) test (p-values<0.05). Box plots depict percentile values from 25–75% (box), median (bar inside the box), standard deviation (whiskers) and outliers (dots)

◂Fig.3 Historically described phenotypical variations and yet undiscovered deviations in the plate pattern of P. volzii. b–c, f, i, l–m Light microscopy, a, d–e, g–h, k scanning electron microscopy. a–f Newly identified deviations a–b plate 4′′ pentagonal in strains a GeoM*793; b GeoM*788; c plate 2a split (strain GeoK*024); d plates 2′′ and 3′′ fused (strain GeoM*866); e plates 1′′′ and 1′′′′ fused (strain GeoM*788); f plates 1a and 3′ fused (strain GeoM*788). g–m Historic infraspecific taxa; g P. guestrowiense forma lineatum (strain GeoM*866); h P. guestrowiense forma compressum (strain GeoM*866); i P. guestrowiense subvar. originale (strain GeoK*024); k P. volzii var. cinctiforme (strain GeoM*793); l P. volzii var. simplex (strain GeoM*789); m P. volzii forma complexum (strain GeoM*793). Abbreviations: n′: apical plate, n′′: precingular plate, n′′′: postcingular plate, n′′′′: antapical plate, na: anterior intercalary plate, nC: cingular plate, split or fused plates are indicated by asterisks. Scale bar=10 µm. U A=15 kV