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Press release (In Japanese only)

Microalgae possess high potential for producing pigments, antioxidants, and lipophilic compounds for industrial applications. However, their open pond cultures are often contaminated by other undesirable organisms, including their predators. In addition, the cost of using freshwater is relatively high, which limits the location and scale of cultivation compared with using seawater. It was previously shown that Cyanidium caldarium and Galdieria sulphuraria, but not Cyanidioschyzon merolae grew in media containing NaCl at a concentration equivalent to seawater. We found that the preculture of C. merolae in the presence of a moderate NaCl concentration enabled the cells to grow in the seawater-based medium. The cultivation of cyanidialean red algae in the seawater-based medium did not require additional pH buffering chemicals. In addition, the combination of seawater and acidic conditions reduced the risk of contamination by other organisms in the nonsterile open culture of C. merolae more efficiently than the acidic condition alone.

Source: S. Hirooka, et al., Scientific Reports (2020)10: 13794 DOI:10.1038/s41598-020-70398-z

Fig: Outdoor cultivation of C. merolae in 7 L of nonsterile a freshwater-based medium and a seawater-based medium at day 0 and day 14. C. merolae cells in the seawater-based medium grew along a similar time course and to similar amounts compared with cells cultured in the freshwater-based medium. The microscopic observation of cultures 14 days after inoculation showed that the culture in the freshwater-based medium was contaminated with bacteria. In contrast, no bacteria or organisms other than C. merolae were observed in the culture in the seawater-based medium. Images were obtained by phase contrast microscopy, and the fluorescence images of chloroplasts (red) were overlaid.

Metabolism, cell cycle stages, and related transcriptomes in eukaryotic algae change with the diel cycle of light availability. In the unicellular red alga Cyanidioschyzon merolae, the S and M phases occur at night. To examine how diel transcriptomic changes in metabolic pathways are related to the cell cycle and to identify all genes, for which mRNA levels change depending on the cell cycle, we examined diel transcriptomic changes in C. merolae. In addition, we compared transcriptomic changes between the wild type and transgenic lines, in which the cell cycle was uncoupled from the diel cycle by the depletion of either cyclin-dependent kinase A (CDKA) or retinoblastoma-related (RBR) protein. Of 4,775 nucleus-encoded genes, the mRNA levels of 1,979 genes exhibited diel transcriptomic changes in the wild type. Of these, the periodic expression patterns of 454 genes were abolished in the transgenic lines, suggesting that the expression of these genes is dependent on cell cycle progression. The periodic expression patterns of most metabolic genes, except those involved in starch degradation and de novo dNTP synthesis, were not affected in the transgenic lines, indicating that the cell cycle and transcriptomic changes in most metabolic pathways are independent of the diel cycle. Approximately 40% of the cell–cycle–dependent genes were of unknown function, and approximately 19% of these genes of unknown function are shared with the green alga Chlamydomonas reinhardtii. The dataset presented in this study will facilitate further studies on the cell cycle and its relationship with metabolism in eukaryotic algae.

Figure: Cell growth and division of eukaryotic algae in accord with the diel cycle. Eukaryotic algae grow by photosynthesis in the daytime and undergo DNA replication and cell division in the night. Most metabolic genes are upregulated in the daytime whereas genes involved in DNA replication and cell division are induced dependently on cell cycle in the night.