Inheritance of spheroid body and plastid in the raphid diatom Epithemia (Bacillariophyta) during sexual reproduction". Phycologia 60 (3). 265-273. Taylor and Francis.
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.
In aquatic ecosystems, marine and freshwater environments are separated by steep salinity gradients. The osmotic stress induced by this ‘invisible wall’ forms an insurmountable barrier for many aquatic lifeforms, including bacteria, algae and animals. Because the osmotic differences when transiting a salinity divide are so hard to overcome, most species have adapted exclusively to a marine or a freshwater lifestyle. A major consequence of this physiological specialization into marine and freshwater organisms is that transitions are relatively rare, impeding regular contact and colonization. While some animals use specialized organs or behaviour to cope with unfavourable salinity levels, unicellular algae such as diatoms are completely dependent on cellular mechanisms to mitigate salinity stress. In this issue of Molecular Ecology, Downey and colleagues investigate the transcriptomic response of a salinity‐tolerant diatom to a shock treatment with freshwater (Molecular Ecology, 2023). Through frequent sampling and integration of existing RNA sequencing data, a fine‐grained model of the acclimation to hypo‐osmotic stress emerges. Deciphering the pathways that drive the acute and long‐term acclimation to freshwater has major implications for diatom ecology, diversification and resilience to global change.
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