Morphological plasticity in response to salinity change in the euryhaline diatom Pleurosira laevis (Bacillariophyta)

Abstract

Pleurosira laevis is a salt-tolerant diatom distributed around the world. The valve of P. laevis has distinct structures called ocelli, which are sharply defined areas with fine, densely packed pores. Two formae of this diatom, P. laevis f. laevis and P. laevis f. polymorpha, are distinguished from each other by their flat or dome-shaped valve faces and degree of elevation of the ocelli, respectively. In this study, we established 4 strains of P. laevis isolated from freshwaters or coastal areas in Japan and the United States, and tracked the formation of newly formed valves with the fluorescent SDVspecific dye PDMPO in culture under several salinity conditions. The result clearly demonstrated the morphological plasticity of the valves, controlled by environmental salinity. The laevis form and polymorpha form valves were produced at salinities of 2 and 7, respectively. The salinity thresholds dictating the morphological plasticity of the valve were consistent in all 4 strains. A similar morphology to the polymorpha form was reproduced in a freshwater medium with the addition of sorbitol, suggesting that osmotic pressure plays a key role in this morphological plasticity. The highly reproducible and easily manipulated change in morphology makes this diatom an ideal model for lab experiments focusing on the molecular and genetic factors involved with valve morphogenesis.

Transcriptional responses to salinity‐induced changes in cell wall morphology of the euryhaline diatom Pleurosira laevis

“…More genes encoding chaperones, heat shock proteins, and heat shock factors were downregulated than up-regulated (Figure S7b). Although we did not see significant differences in growth rates and apparent cellular stress in Pleurosira laevis between salinity 2 and 7 in culture experiments (Kamakura et al, 2022), the cells appear to be sensitive to the variations in stress levels between the conditions.…”

“…The association between osmotic pressure and valve morphology has been supported by studies in Skeletonema species (Balzano et al, 2011;Paasche et al, 1975) and Pleurosira laevis (Kamakura et al, 2022). Kamakura et al (2022) showed that P. laevis changed its valve morphology in response to the environmental osmotic pressure. Pleurosira laevis formed a flat valve face at salinity 2, whereas it formed a dome-shaped valve face at salinity 7.…”

“…The strains HA-01 and HA-02 of Pleurosira laevis, obtained from freshwater (Kamakura et al, 2022), were cultivated using WC medium (Guillard & Lorenzen, 1972) modified to salinity 2 and 7 by adding a salt stock described by Nakov et al (2020). The media were adjusted to pH 8 by dropwise addition of 1 M HCl and then sterilized through a membrane filter (pore size 0.2 μm, mixed cellulose ester, Advantec, Tokyo, Japan).…”

“…The natural growth environments of the selected diatoms were determined based on information from collection sites provided by the Roscoff Culture Collection (http:// rosco ff-cultu re-colle ction. org/ ), as well as from Schmidt (1875), Ashworth et al (2013), , Kamakura et al (2022), and appendix 1 in Mann (1999).…”

“…Conversely, the opposite occurs in some marine diatoms: When the force on the girdle region is greater than that on the valve face, the daughter membrane tension is low, and the cell sometimes plasmolyzes, resulting in a valve face with ridges and grooves (see also Mann, 1984). The association between osmotic pressure and valve morphology has been supported by studies in Skeletonema species (Balzano et al, 2011;Paasche et al, 1975) and Pleurosira laevis (Kamakura et al, 2022). Kamakura et al (2022) showed that P. laevis changed its valve morphology in response to the environmental osmotic pressure.…”