At high latitudes, the polar night poses a great challenge to photosynthetic organisms that must survive up to six months without light. Numerous studies have already shed light on the physiological changes involved in the acclimation of microalgae to prolonged darkness and subsequent re‐illumination. However, these studies have never considered inter‐individual variability because they have mainly been conducted with bulk measurements. On the other hand, such long periods are likely to impact within‐population selection processes. In this study, we hypothesized that distinct subpopulations with specific traits may emerge during acclimation of a population of diatoms to darkness. We addressed this hypothesis using flow cytometry (FCM), which allow to individually characterize large numbers of cells. The ecologically dominant polar pennate diatom Fragilariopsis cylindrus was subjected to three dark acclimation (DA) experiments of one, three, and five months duration, during which all cultures showed signs of recovery once light became available again. Our results suggest that darkness survival of F. cylindrus relies on reduction of metabolic activity and consumption of carbon reserves. In addition, FCM allowed us to record three different causes of death, each shared by significant numbers of individuals. The first rendered cells were unable to survive the stress caused by the return to light, probably due to a lack of sufficient photoprotective defenses. The other two were observed in two subpopulations of cells whose physiological state deviated from the original population. The data suggest that starvation and failure to maintain dormancy were the cause of cell mortality in these two subpopulations.
Diatoms, the major eukaryotic phytoplankton in polar regions, are essential to sustain Arctic and Antarctic ecosystems. As such, it is fundamental to understand the physiological mechanisms and associated molecular basis of their resilience to the long polar night. Here, we report an integrative approach revealing that in prolonged darkness, diatom cells enter a state of quiescence associated with reduced metabolic and transcriptional activity during which no cell division occurs. We propose that minimal energy is provided by respiration and degradation of protein, carbohydrate, and lipid stores and that homeostasis is maintained by autophagy in prolonged darkness. We also report internal structural changes that manifest the morphological acclimation of cells to darkness. Our results further indicate that immediately following a return to light, diatom cells are able to use photoprotective mechanisms and rapidly resume photosynthesis. Cell division resumed rates similar to those before darkness. Our study demonstrates the remarkable robustness of polar diatoms to prolonged darkness at low temperatures.
A new crystallization process for sodium bicarbonate (NaHCO3) was studied, proposing the use of osmotic membrane distillation crystallization. Crystallization takes place due to the saturation of the feed solution after water evaporation on the feed side, permeating through the membrane pores to the osmotic side. The process operational parameters, i.e., feed and osmotic velocities, feed concentration, and temperature were studied to determine the optimal operating conditions. Regarding the feed and osmotic velocities, values of 0.038 and 0.0101 m/s, respectively, showed the highest transmembrane flux, i.e., 4.4 × 10−8 m3/m2·s. Moreover, study of the temperature variation illustrated that higher temperatures have a positive effect on the size and purity of the obtained crystals. The purity of the crystals obtained varied from 96.4 to 100% In addition, the flux changed from 2 × 10−8 to 7 × 10−8 m3/m2·s with an increase in temperature from 15 to 40 °C. However, due to heat exchange between the feed and the osmotic solutions, the energy loss in osmotic membrane distillation crystallization is higher at higher temperatures.
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