Hiroyuki Mizuta’s research while affiliated with Hokkaido University and other places

Conservation units are still unclear for the commercially important kelp Saccharina japonica. Previous analyses of population genetic structure have yielded inconsistent results regarding the number of clusters, especially in Japan. Thus, the genetic structure of S. japonica in Japan was studied using the mitochondrial nad3-16S rDNA region. We detected 88 haplotypes in 483 individuals collected from 46 localities. Unique haplotypes and one or a few shared haplotypes at a local scale were found in most localities. The observed genetic structure revealed cryptic invasions of S. japonica within Japan and the value of the nad3-16S rDNA region for identifying the geographic origin of kelp. Bayesian Analysis of Population Structure analysis and FST revealed the genetic divergence of populations in southwestern Hokkaido. The haplotype network showed a more detailed starburst pattern compared with the results of previous studies based on mitochondrial COI and trnW-trnI. Accordingly, S. japonica in Japan may represent one genetic group that experienced a recent expansion. Unique or locally shared haplotypes and similarity in haplotype diversity on different coasts of Hokkaido could be explained by refugia during the Last Glacial Maximum on various coasts. Furthermore, the present study demonstrated inconsistencies between the genetic structure and distribution of S. japonica varieties. Therefore, further investigations focused on the taxonomic validation of varieties are needed.

Sargassum fusiforme is a macroalga of commercial importance in Eastern Asia, and understanding the reproductive strategy of this macroalga is necessary for conservation and recovery. Here, we used a population genetics approach to identify the reproductive strategy of S. fusiforme . To do this, we performed two sampling procedures: random sampling and quadrat sampling. For random sampling, we collected 80 samples at > 1 m intervals in Site A, B, C and D apart 700 m from each other. For quadrat sampling, we collected 207 samples using 50 cm × 50 cm quadrats comprising 10 cm grid, in two sites Site B and D. Clonemates within these samples were identified by genotyping using 14 (random sampling) or 13 microsatellites (quadrat sampling). For samples obtained by random sampling, only three clonal pairs were detected. For samples obtained by quadrat sampling, each quadrat contained 4–7 genets, and the size was 23.2 ± 14.3 cm (standard deviation) on average and 70.7 cm at maximum. Levels of asexuality was higher in Site B than in Site D, perhaps due to longer exposure time. Site B located rear edge of the intertidal zonation of this species. Over 65% of gene flow via sexual reproduction was limited to within the quadrats, whereas at least 10% extended for meters to kilometers. Taken together, these results suggest that S. fusiforme disperses its offspring sexually and asexually on a small scale, and sexually on a larger scale, with levels of asexuality varying depending on the stress originate from exposure.

Saccharina japonica, a significant brown macroalga in the Pacific Ocean, serves as a food source and industrial material. In aquaculture, collecting mature sporophytes for seedling production is essential but challenging due to environmental changes. In this study, transcriptomic analysis of vegetative and sorus tissues was done to identify differentially expressed genes (DEGs) and enhance our understanding of sorus formation regulation in S. japonica. KEGG pathway and Gene Otology (GO) analysis revealed that upregulated DEGs were involved in folate biosynthesis, riboflavin metabolism, and amino acid biosynthesis. In addition, the upregulation of genes associated with cell wall remodeling, such as mannuronan C-5-epimerases, vanadium-dependent haloperoxidases, and NADPH oxidase, was observed in sorus parts. Meanwhile, downregulated DEGs in sorus portions included genes related to chloroplast function. These findings will help us understand the regulatory mechanisms behind sorus formation in S. japonica and extracellular matrix remodeling in brown algae.

Essential information for the conservation unit is still unclear in commercially important kelp Saccharina japonica . Previous analyses of population genetic structure have yielded inconsistent results regarding the number of clusters, especially in Japan. Thus, the genetic structure of S. japonica in Japan was studied using the mitochondrial nad 3-16S rDNA region. We detected 88 haplotypes in the 483 individuals collected from 46 localities. Unique haplotypes and one or a few shared haplotypes at a local scale were found in most localities. The observed genetic structure revealed cryptic invasions of S. japonica within Japan and the potential for the nad 3-16S rDNA region to identify the geographic origin. Bayesian Analysis of Population Structure analysis and F ST suggested genetic distinctiveness in southwestern Hokkaido. The haplotype network showed a more detailed starburst pattern compared with the results of previous studies based on mitochondrial COI and trn W- trn I. Accordingly, S. japonica in Japan may represent one genetic group that experienced a recent expansion. Unique or locally shared haplotypes and similarity in haplotype diversity on various coasts of Hokkaido could be explained by the refugia of S. japonica during the Last Glacial Maximum on various coasts. Furthermore, the present study also recognized inconsistencies in the genetic structure and distribution of S. japonica varieties . Therefore, further investigations focused on the taxonomic validation of varieties are needed.

The processes involved in the life cycling of the brown alga Saccharina japonica were investigated through laboratory culturing experiments. We observed that meiospores were released relatively early from the ripe sorus on the sporophyte. Following the release of free meiospores, there was a partial detachment of the sorus, leading to the release of other reproductive cells. When a small piece of exfoliated sorus tissue was cultured separately, the appearance of gametophytes and then sporophytes occurred later for the free meiospores. These observations suggest that at least two processes are involved in the production of new sporophytes. This phenomenon appears to be an active and regulated reproductive strategy, as these processes were stimulated by the abscisic acid hormone. Additionally, we examined the effects of grazing by sea urchins on the dissemination of reproductive cells. Undigested sorus tissue segments containing sporangia and paraphyses were observed in the digestive tracts, as well as in the faeces released by sea urchins. From these undigested sorus tissue segments, were observed the development of gametophytes and then sporophytes when the faeces were cultured independently. These findings show that herbivores play a role in the dissemination of brown algae reproductive cells and provide evidence for the sustained protective function of sorus tissues, even after detachment from the mother plants and exposure to sea urchin feeding. We discuss the potential importance of these reproductive processes in the persistence and diversity of kelp populations.

Seaweeds or macroalgae are important primary producers that serve as a habitat for functioning ecosystems. A sustainable production of macroalgae has been maintained by a diverse range of life cycles. Reproduction is the most dynamic change to occur during its life cycle, and it is a key developmental event to ensure the species’ survival. There is gradually accumulating evidence that plant hormones, such as abscisic acid and auxin, have a role on the sporogenesis of brown alga (Saccharina japonica). Recent studies reported that 1-aminocylopropane-1-carboxylic acid, an ethylene precursor, regulates sexual reproduction in red alga (Neopyropia yezoensis) independently from ethylene. In addition, these macroalgae have an enhanced tolerance against abiotic and biotic stresses during reproduction to protect their gametes and spores. Herein, we reviewed the current understanding on the regulatory mechanisms of red and brown algae on their transition from vegetative to reproductive phase.

Epigenetic regulation by histone modification can activate or repress transcription through changes in chromatin dynamics and regulates development and the response to environmental signals in both animals and plants. Chromatin immunoprecipitation (ChIP) is an indispensable tool to identify histones with specific post-translational modifications. The lack of a ChIP technique for macroalgae has hindered understanding of the role of histone modification in the expression of genes in this organism. In this study, a ChIP method with several modifications, based on existing protocols for plant cells, has been developed for the red macroalga, Neopyropia yezoensis, that consists of a heterogeneous alternation of macroscopic leaf-like gametophytes and microscopic filamentous sporophytes. ChIP method coupled with qPCR enables the identification of a histone mark in generation-specific genes from N. yezoensis. The results indicate that acetylation of histone H3 at lysine 9 in the 5′ flanking and coding regions from generation-specific genes was maintained at relatively high levels, even in generation-repressed gene expression. The use of this ChIP method will contribute significantly to identify the epigenetic regulatory mechanisms through histone modifications that control a variety of biological processes in red macroalgae.

Heat stress disrupts algal growth, development, and physiological processes, such as photosynthesis, and eventually decreases seaweed productivity. Previous studies of crop plants have revealed that exogenous application of phytohormones prior or parallel to stress can alleviate the negative effects of abiotic stressors, including heat stress. However, there is limited information on phytohormone-induced tolerance to abiotic stressors in seaweed. In the present study, the application of the major plant hormones abscisic acid and salicylic acid failed to mitigate the negative effects of heat stress on the marine red alga Neopyropia yezoensis, whereas 1-aminocyclopropane-1-carboxylic acid (ACC), the direct precursor of the plant hormone ethylene, regulates thermotolerance. In addition, the ACC analogs 1-aminocyclobutane-1-carboxylic acid and α-aminoisobutyric acid enhanced tolerance to heat stress. ACC increased the expression of genes involved in antioxidant defense systems to protect photosynthesis and respiration. These results suggest ACC acts as a phytohormone to mitigate the impact on heat stress independent of ethylene in N. yezoensis.

Background 1-aminocyclopropane 1-carboxylic acid (ACC) is the immediate precursor of the plant hormone ethylene. However, recent studies have suggested that ACC also acts as a signaling molecule to regulate development and growth independently from ethylene biosynthesis. In red algae, ACC stimulates the switch from a vegetative to a sexual reproductive phase. However, despite evidence that ACC signaling in plants and algae is widespread, the mechanistic basis of the ACC signaling pathway remains unknown. Results We demonstrate that exogenous ACC increased the activity of phospholipase D (PLD) and induced the accumulation of PLD transcripts in the marine red alga Neopyropia yezoensis . The product of PLD, the lipid second messenger phosphatidic acid (PA), also increased in response to ACC. Furthermore, the pharmacological inhibition of PLD by 1-butanol blocked ACC-induced spermatangia and carpospore production, but the inactive isomer t-butanol did not. In addition, 1-butanol prevented ACC-induced growth inhibition and inhibited transcript accumulation of genes upregulated by ACC, including extracellular matrix (ECM)-related genes, and alleviated the transcriptional decrease of genes downregulated by ACC, including photosynthesis-related genes. Conclusions These results indicate that PLD is a positive regulator of sexual cell differentiation and a negative regulator of growth. This study demonstrates that PLD and its product, PA, are components of ACC signaling during sexual reproduction in N. yezoensis .

Many organisms are subjected to a daily cycle of light and darkness, which significantly influences metabolic and physiological processes. In the present study, Neopyropia yezoensis, one of the major cultivated seaweeds used in “nori,” was harvested in the morning and evening during light/dark treatments to investigate daily changes in gene expression using RNA-sequencing. A high abundance of transcripts in the morning includes the genes associated with carbon–nitrogen assimilations, polyunsaturated fatty acid, and starch synthesis. In contrast, the upregulation of a subset of the genes associated with the pentose phosphate pathway, cell cycle, and DNA replication at evening is necessary for the tight control of light-sensitive processes, such as DNA replication. Additionally, a high abundance of transcripts at dusk encoding asparaginase and glutamate dehydrogenase imply that regulation of asparagine catabolism and tricarboxylic acid cycle possibly contributes to supply nitrogen and carbon, respectively, for growth during the dark. In addition, genes encoding cryptochrome/photolyase family and histone modification proteins were identified as potential key players for regulating diurnal rhythmic genes.