Plants that can survive in indoor light

Plants that can survive in indoor light

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Plants that can survive in indoor light environments, in contrast, are of interest in the context of providing life support for astronauts in space. Among them, the green algae *Chlorella*, a unicellular green alga, has been considered to be the most suitable because its life cycle is simpler than that of land plants, and can easily be cultured using any kind of water (liquid or solid) [[@B1]].

Sato et al. [[@B2]] reported the cultivation of the *Chlorella* green algae using an acrylic photobioreactor ([Fig. 1](#F1){ref-type="fig"}). The photobioreactor consists of a culture vessel (5.6 cm×10.2 cm×16.4 cm) made of an acrylic-polycarbonate composite with an inlet and outlet. The inlet is a tube with an inner diameter of 9 mm. The outlet is an open hole on the surface of the vessel. Two fluorescent lamps with 3,500 K light (300 to 310 nm) were used as light sources. The flow rate of the water was regulated through a flow rate controller. The oxygen content was detected by an oxygen meter (YSI, Yellow Springs, OH, USA). The culture volume of the photobioreactor is ∼2 L.

The *Chlorella* culture with no treatment for water disinfection is defined as the control. The culture under a hydrogen peroxide treatment is defined as the hydrogen peroxide treatment. The culture under ultraviolet treatment is defined as the ultraviolet treatment. The culture under both ultraviolet and hydrogen peroxide treatments is defined as the ultraviolet-hydrogen peroxide treatment.

The *Chlorella* green algae is used in the Chlorella Production Technology Research Group in Tsukuba University, Japan. A batch culture in an open reactor was carried out using the cultivation method described above. The batch cultures were performed with 10% inoculation.

The inoculation and growth of the *Chlorella* green algae were shown in [Fig. 2](#F2){ref-type="fig"}.

The inoculation rate of the inoculation is the number of organisms divided by the culture volume. The inoculation rate was calculated to be 0.002% at the beginning of the cultivation, and reached a maximum of ∼0.0009% by the 5^th^ day. There was no notable difference between the inoculation rates in the control, hydrogen peroxide treatment, and ultraviolet treatment. In contrast, the inoculation rate was reduced by 21% in the ultraviolet-hydrogen peroxide treatment, compared to that in the control.

Based on the optical density, the growth rates of the *Chlorella* green algae with and without the hydrogen peroxide, ultraviolet, and ultraviolet-hydrogen peroxide treatments were estimated to be 0.07, 0.20, and 0.04, respectively. Thus, the bacterial growth rate was higher than that of the green algae by one order of magnitude.

The inoculation rate was reduced in the ultraviolet-hydrogen peroxide treatment, compared to the control. Thus, this experiment suggests that both hydrogen peroxide and ultraviolet treatments are useful for water disinfection to be used in the cultivation of *Chlorella* green algae.

This research was supported by the Science Research Promotion Fund for the Promotion of KAO Industries (Grant Number 180124).

The authors declare that there are no conflicts of interest.


![Inoculation and growth of *Chlorella* green algae. (A) The inoculation rate. (B) The growth rate.](lar-28-79-g002){#F2}