Effects of Silicon-Limitation on Growth and Morphology of <i>Triparma laevis</i> NIES-2565 (Parmales, Heterokontophyta)

<div><p>The order Parmales (Heterokontophyta) is a group of small-sized unicellular marine phytoplankton, which is distributed widely from tropical to polar waters. The cells of Parmales are surrounded by a distinctive cell wall, which consists of several siliceous plates fitting edge to edge. Phylogenetic and morphological analyses suggest that Parmales is one of the key organisms for elucidating the evolutionary origin of Bacillariophyceae (diatoms), the most successful heterokontophyta. The effects of silicon-limitation on growth and morphogenesis of plates were studied using a strain of <i>Triparma laevis</i> NIES-2565, which was cultured for the first time in artificial sea water. The cells of <i>T. laevis</i> were surrounded by eight plates when grown with sufficient silicon. However, plate formation became incomplete when cells were cultured in a medium containing low silicate (ca. <10 µM). Cells finally lost almost all plates in a medium containing silicate concentrations lower than ca. 1 µM. However, silicon-limitation did not affect growth rate; cells continued to divide without changing their growth rate, even after all plates were lost. Loss of plates was reversible; when cells without plates were transferred to a medium containing sufficient silicate, regeneration of shield and ventral plates was followed by the formation of girdle and triradiate plates. The results indicate that the response to silicon-limitation of <i>T. laevis</i> is different from that of diatoms, where cell division becomes inhibited under such conditions.</p></div

Various morphologies of siliceous plates observed during the cell wall regeneration process of <i>Triparma laevis</i> NIES-2565.

<p>Cells lost all plates, which cultured in 1 µM-silicate medium were transferred to 100 µM-silicate medium, and SEM observations were performed with cells collected within 24 h incubation. (a) Cell with immature shield (S) or ventral (V) plate that has fimbriate edge. (b) Cell with shield and ventral plates. (c) Cell with almost mature shield plates but without a triradiate plate. (d) Cell with almost mature ventral and shield plates but without a girdle plate. (e), (f) Cells with all plates regenerated. G: girdle plate; T: triradiate plate. Bars indicate 1 µm.</p

Effects of silicate concentration on morphology of siliceous cell wall of <i>Triparma laevis</i> NIES-2565.

<p>Cells cultured in 100 µM-silicate medium (a)–(d) or in 10 µM-silicate medium (e)–(i). (a)–(c) Cells viewed from different directions; cell wall is composed with three shield plates (S), one ventral plate (V), three girdle plates (G) and one triradiate plate (T). (d) Few cells have a plate, presumably triradiate plate, with abnormal shape (asterisk). (e) Cell with incomplete girdle plate (arrow) and without triradiate plate. Two shield plates seem to contact directly (arrow head) (f) Cell with ventral and shield plates but without girdle plate. Ventral plate and shield plates seem to contact directly each other (arrow head). (g) Cell with two shield and/or ventral plates. Two plates seem to contact directly (arrow head). (h) Cell with only one shield or ventral plate. (i) Cell without any plates. Bars indicate 1 µm.</p

Quantitative analysis of silicate plate morphogenesis of <i>Triparma laevis</i> NIES-2565 under silicon-limitation.

<p>Cells cultured in 100, 10 or 1µM -silicate medium for 4 d (a, c and e respectively) and 12 d (b, d and f respectively). The measured silicate concentrations at the time when cells were harvested were 96.9, 89.5, 6.8, 1.1, 1.3 and lower than 0.7 µM (undetectable by silicomolybdate methods) for (a), (b), (c), (d), (e) and (f), respectively. Categorizations of each cells are as follows; Normal type: the cells with eight complete plates contact each other without any gap (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0103289#pone-0103289-g004" target="_blank">Figure 4a–c</a>), Incomplete girdle-triradiate type (In-GT): the cells with shield and ventral plates and incomplete girdle and/or triradiate plates (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0103289#pone-0103289-g004" target="_blank">Figure 4e</a>), Round type (R): the cells without girdle and triradiate plates (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0103289#pone-0103289-g004" target="_blank">Figure 4f–h</a>) and Naked type: the cells without any plate (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0103289#pone-0103289-g004" target="_blank">Figure 4i</a>). Values are the mean of randomly selected 100 cells from triplicate experiments.</p

Effects of silicate concentration on cellular silicon contents of <i>Triparma laevis</i> NIES-2565.

<p>Results are the mean ± SD of triplicate cultures.</p

Growth of <i>Triparma laevis</i> NIES-2565 in batch culture.

<p>Open squares and closed circles are for enriched sea water medium and artificial sea water medium, respectively. Results are the mean ± SD of triplicate cultures. Error bars are omitted when ± SD was too small to be shown.</p

Incorporation of silicon detected by PDMPO fluorescence during the cell wall regeneration process of <i>Triparma laevis</i> NIES-2565.

<p>Cells that had lost all plates when cultured in 1 µM-silicate medium were transferred to 100 µM-silicate medium containing PDMPO at 0 d. Results are the mean ± SD of triplicate cultures.</p

Incorporation and deposition of silicon by means of PDMPO fluorescence in <i>Triparma laevis</i> NIES-2565.

<p>(a), (b) Cells cultured in 1 µM-silicate medium. (c), (d) Cells that had lost all plates when cultured in 1 µM-silicate medium were transferred into 100 µM-silicate medium containing PDMPO and incubated for 24 h. Cells observed under light or epifluorescent microscope for (a), (c) and (b), (d), respectively. Bars indicate 1 µm.</p

Bolidophyceae, a sister picoplanktonic group of diatoms - a review - Supplementary material

Supplementary material to :<div><br></div><div>Kuwata, A., Yamada, K., Ichinomiya, M., Yoshikawa, S., Tragin, M., Vaulot, D. & Lopes dos Santos, A. 2018. Bolidophyceae, a Sister Picoplanktonic Group of Diatoms – A Review. Front. Mar. Sci. 5:370.<br></div