木村 駿太

© 2020, Springer Nature B.V. Cyanobacteria are distributed globally in the sea, fresh water, and land. Akinetes (dormant cells) of cyanobacteria are highly resistant to dry conditions, contributing to the survival of terrestrial cyanobacteria. However, the substances that regulate akinete dormancy and germination have not been elucidated. The present study investigated the effects of the phytohormone cytokinin (CK), on the germination stage of a terrestrial filamentous cyanobacterium, Nostoc sp. HK-01. In the cell suspensions of Nostoc sp. HK-01, isopentenyladenine (iP), isopentenyladenosine (iPR), cis-zeatin (cZ), cis-zeatinriboside (cZR), trans-zeatin (tZ), and a trace amount of trans-zeatinriboside (tZR) were identified, according to their retention times and multiple reaction monitoring (MRM) transitions, using high performance liquid chromatography and electrospray ionization tandem mass spectrometry (HPLC-ESI-MS/MS). Product ion scans also identified iP, iPR, cZ, and cZR. The present study established an isolation method for the dormant Nostoc akinetes, and then treated them with CKs. iP and tZ promoted the germination of akinetes, while cZ did not. S-4893, which is a CK antagonist, inhibited germination and the promotive activity of the applied iP. The results strongly suggest that cyanobacteria have a very similar perception system to plants, and that they regulate the dormancy and germination of their own cells and/or other cells via CKs. The production of CKs by several species of cyanobacteria was previously reported. However, this is the first investigation showing the role of phytohormones in the dormant cell physiology of cyanobacteria.

A terrestrial cyanobacterium, Nostoc sp. HK-01 has several unique abilities: photosynthesis, nitrogen fixation and tolerance to space. The functions of Nostoc sp. HK-01 as a food resource in extraterrestrial environments such as Mars were investigated for future extraterrestrial agriculture, "Space Agriculture". The nutritional energy of 100 g dry weight of Nostoc sp. HK-01 was estimated to be 358.1 ± 10.9 kcal by the results of protein, sugar and lipids. Six kind of recipes cooked using cyanobacteria were demonstrated and these results indicate that Nostoc sp. HK-01 has three food functions: primary (nutritional), secondary (sensory), tertiary (physiological). We will discuss the utilization of Nostoc sp. HK-01 as a food resource in closed bio-ecosystems. Our results may contribute to the supply of food resources under severe conditions for life-support in closed bio-ecosystems.

A terrestrial cyanobacterium, Nostoc sp. HK-01, is a candidate material for introduction to extraterrestrial environments, such as Mars, given its high tolerance to drought and other extraterrestrial environments. Here, we evaluated Nostoc sp. HK-01 as food for future habitation of Mars. We found that wet colonies of Nostoc sp. HK-01 after initiation survived for over 8 years on Martian regolith simulant (MRS), although they did not survive for over 105 days on medium without MRS. Total protein per 100 g of the dried colony of Nostoc sp. HK-01 was approximately 50 g. Increases in biomass proliferation in liquid or on agar medium were determined in six strains of terrestrial cyanobacteria, including Nostoc sp. HK-01, N. commune HK-02, N. commune YK-04, N. punctiforme ATCC 29133, Brasilonema sp. HK-05 and Calothrix sp. HK-06. Nostoc sp. HK-01 grew better compared to the other five strains under both conditions. Following the incubation of Nostoc sp. HK-01, the medium exhibited radical-scavenging activity. Sugar content in the medium after a 1-day incubation of Nostoc sp. HK-01 was one-sixth of its dry weight at initiation. These results indicate that, after liquid incubation, the remaining cells may be used as recyclable materials on Mars.

The heat tolerance of dry colonies of a terrestrial cyanobacterium, Nostoc sp. HK-01 by exposure to temperatures at 100 °C, the boiling point of liquid water, was examined. The cells of Nostoc sp. HK-01 can live under these conditions for 10 hours. All of the viable cells were akinetes which are dormant cells. Akinetes have a high tolerance to high temperatures. The photosynthetic ability of cells of HK-01 was normal even after the heat exposure. The percentage of decreased water content in the dry colony was 5 percent during the heat exposure for 24 hours. The reasons for the decrease in the percentage of survival rates of colonies under the high heat condition are related to the remaining water in the dry colony, and/or, DNA damage of individual cells. Furthermore, there is a possibility that there are highly tolerant cells in the dry colony since there are cells that survived over 24 hours after the heat exposure.