# Effects of Nano-TiO\({}_{2}\) Mediated Photocatalysis on _Microystis aeruginosa_ Cells
## 3 Results and Discussion  
### Metabolic Heat of M. aeruginosa Cells  
The growth of microorganisms includes the lag stage, the log stage, the stationary stage, and the decline stage [16; 17]. Figure 3 shows the metabolic heat of _M. aeruginosa_ cells after treatment with nano-TiO\({}_{2}\) mediated photocatalysis and without nutrients from the culture. On the whole, the metabolic heat declined gradually and ceased finally.  
_M. aeruginosa_ makes use of nitrogen, phosphorus, and other nutrients to maintain its metabolism. Without nutrition from external sources, the cells can still live on endogenous nutrients for some time. Glycogen is the energy and carbon source stored in the cells. During photosynthesis, a large amount of optical energy is converted into chemical energy and stored in glycogen.  
Compared with the control _M. aeruginosa_ cells, the nano-TiO\({}_{2}\) photocatalyzed cells experienced inhibited metabolism. The inhibition was more severe when the nano-TiO\({}_{2}\) mediated photocatalysis lasted longer.  
Table 1 lists the metabolic parameters for the _M. aeruginosa_ cells that were treated with nano-TiO\({}_{2}\). In the table, \(P_{max}\) is the measured maximum output power during metabolism, and \(Q_{T}\) is the total heat produced for the entire metabolic process. Clearly, both \(P_{max}\) and \(Q_{T}\) decreased upon prolonged nano-TiO\({}_{2}\) mediated photocatalysis. In particular, the cells that underwent nano-TiO\({}_{2}\) mediated photocatalysis for 6 h generated negligible metabolic heat.  
\begin{table}
\begin{tabular}{c c c} \hline
**Sample** & \(P_{max}\) **(mW)** & \(Q_{T}\) **(0)** \\ \hline Control & 0.22 \(\pm\) 0.012 & 36.21 \(\pm\) 1.87 \\
1 h treated Cells & 0.12 \(\pm\) 0.051 & 19.32 \(\pm\) 1.05 \\
2 h treated Cells & 0.08 \(\pm\) 0.0054 & 13.88 \(\pm\) 0.95 \\
4 h treated Cells & 0.05 \(\pm\) 0.0023 & 10.65 \(\pm\) 0.56 \\
6 h treated Cells & 0.02 \(\pm\) 0.0010 & 0.89 \(\pm\) 0.067 \\ \hline \end{tabular}
\end{table}
Table 1: Metabolic parameters for the _M. aeruginosa_ cells by nano-TiO\({}_{2}\) mediated photocatalysis.  
Figure 3: The metabolic heat of _M. aeruginosa_ after nano-TiO\({}_{2}\) mediated photocatalysis.  
A simple photolysis test suggested that the illumination without nano-TiO\({}_{2}\) showed little action on _M. aeruginosa_. Nano-TiO\({}_{2}\), particularly in the anatase form, is a photocatalyst. Upon irradiation with UV light, TiO\({}_{2}\) can produce electrons (e\({}^{-}\)) and holes (h\({}^{+}\)) on its surface. The holes are strongly oxidative and can oxidize water to create hydroxyl radicals, and can also oxidize organic molecules directly [18; 19].  
\[h\
u+\text{TiO}_{2}\rightarrow\text{hole}^{+}+\text{e}^{-} \tag{1}\]  
\[\text{H}_{2}\text{O}+\text{e}^{-}\rightarrow\text{OH}^{-}+\text{HO}\cdot \tag{2}\]  
\[\text{H}_{2}\text{O}+\text{h}^{+}\rightarrow\text{H}^{+}+\text{HO}_{2}\cdot \tag{3}\]  
\[\text{O}_{2}+\text{e}^{-}\rightarrow\text{O}_{2}\cdot \tag{4}\]  
The HO\(\cdot\), O\({}_{2}\cdot\), and HO\({}_{2}\cdot\) radicals produced in the photocatalytic process are reactive enough to attack many organic bonds, such as C-C, C-H, C-N, C-O, and H-O [20; 21]. Thus, the free radicals produced by nano-TiO\({}_{2}\) can efficiently modify the molecules in microorganisms. As a result, nano-TiO\({}_{2}\) mediated photocatalysis can inhibit the metabolic activity and even damage the cells. The band gap of nano-TiO\({}_{2}\) is 3.2 eV, so the wavelength of light should be less than or equal to 387.5 nm. Therefore, in our study, a monochromatic lamp was used for the photocatalysis with high efficiency.