P-52
環境

Effects of light and dark cycles on the growthand geosmin production in
Dolichospermum smithii
SATテクノロジー・ショーケース 2024

■ はじめにThe secondary metabolite geosmin, produced bycyanobacteria, is widely considered to be the most dominantbio-induced taste and odour in freshwater environments.
Odour events pose a serious threat to the quality of drinking
water and aquatic products. Hence, identifying the key
factors affecting geosmin synthesis is of particular importance.
Light, as the main driver of photosynthesis, is one of the mostimportant factors affecting the growth and odor synthesis ofcyanobacteria. However, to our knowledge, no previous
studies have explored the impact of light/dark cycles on
geosmin production and gene expression. In this study, weestablished three light/dark cycle conditions to investigate theinfluence of light/dark cycles on the growth of the known
geosmin producer Dolichospermum smithii (D. smithii) NIES824 and the expression of geosmin synthase gene geoA, as
well as the concentration of geosmin.
■ 活動内容１．Design of work and culture conditions
The light/dark cycle was set to 16:8 h for the long illumination
experimental group and 9:15 h for the short illumination group. A
light/dark cycle of 12:12 h was used as the control group.
2．Growth curve of D.smithii
As shown in the Figure 1, D. smithii growth entered theexponential and decay phases more rapidly with increasing
light duration. However, the peak Chl-a of D. smithii in thelong illumination group (2606 µg/L) was smaller than that of
the control (3018 µg/L) and short illumination groups (2829µg/L).
3．Expression of geoA gene
The analysis of geoA copy number is shown in Figure 2.
Combined with the growth curve analysis, the extended dailylight duration (16 h/day) caused D. smithii to enter a rapid
growth phase, while geoA expression also peaked in a shortperiod of time (day 3). The prolonged illumination contributed
to the rapid expression of the geosmin synthase gene of D.
smithii at the early stage of the experiment.
4．Release of metabolites
The optimal growth conditions (medium illumination) inthe later part of the experiment made the total geosmin muchhigher than the other two groups (Figure 3A). The analysis ofthe amount and percentage of extracellular geosmin showed

代表発表者
所属MIAO HANCHEN（ミョウ カンシン）
国立研究開発法人 産業技術総合研究所
安全科学研究部門

問合せ先〒305-0053 茨城県つくば市小野川 16-1
Email： miao93hc@gmail.com

that long light helped the cyanobacteria to release more
geosmin (Figure 3B and 3C).
The DOC changes (ΔDOC) of the three groups showed an
overall upward trend, even in the decay phase (Figure 3D). It
can be assumed that cell decay or rupture releases more
intracellular metabolites, which increase with prolongedillumination. This corresponds with the observed changes ingeosmin.
■ まとめ
Prolonged exposure to light can cause temporary surges inodor release. However, we overlooked the potential
consequences of extended odor release periods under shortillumination. Furthermore, the risk of releasing intracellulargeosmin due to cell rupture increases over time. Thus, the mosteffective method to mitigate the release of intracellularorganisms is to remove intact cyanobacterial cells.

Figure 1 The growth curve of D. smithii under thedifferent light/dark cycles

Figure 2 Effect of different light/dark cycles on relative
expression of geoA. Different letters represent significant
differences between the three treatments (p < 0.05).

AB

CD

Figure 3 Concentration of the total (A) and extracellular (B) geosmin, extracellular geosmin productivity (C)and release of ΔDOC (D) at different illumination conditions.

■キーワード： （1）Environmental microorganism（2）Cyanobacteria（3）Odor compounds■共同研究者： Ji Zhang (University of Tsukuba)
Chi Zhang (University of Tsukuba)
Kazuya shimizu (Toyo university)‒ 64 ‒