## 4 Influences of Slag on the Growth of Microalgae
### Influences of Cu in Slag on the Growth and Accumulation of Metabolites of Micralgae  
Similar to Zn, Cu is a kind of heavy metal in slag. Many research studies have shown that slag could help microalgae synthetize glutathione, mercaptan, proline, superoxide dismutase and so on. The reason is that when microalgae was stimulated by Cu\({}^{2+}\) dissolving from slag, microalgae showed a kind of self-protection mechanism to defend theoxidative toxicity caused by Cu\({}^{2+}\)[85; 86]. [PERSON] et al. [17] found that, when stimulated by Cu\({}^{2+}\), _Paolova viridis_'s MDA yield increased obviously. When the Cu\({}^{2+}\) concentration was 3 mg/L, antioxidants (GSH) and antioxidant enzymes (SOD, CAT and GPX) increased dramatically. Additionally, [PERSON] et al. [86] also found that a relatively high concentration of Cu\({}^{2+}\) might cause an obvious increase of SOD in _Ditylun brighttwelli_. [PERSON] et al. [87] studied influences of Cu\({}^{2+}\) on _Chorella protothecoides_'s lipid synthesis. It was found that under the stimulation of Cu\({}^{2+}\), the highest lipid yield was 5.78 g/L. [PERSON] et al. [88] found that the optimum concentration of Cu\({}^{2+}\) for the growth of _Microystis aeruginosa_ was \(10^{-6}\) mol/L. When the Cu\({}^{2+}\) concentration was between \(10^{-5}\) mol/L and \(10^{-2}\) mol/L, with the increase of Cu\({}^{2+}\) concentration, the inhibition of Cu\({}^{2+}\) on the growth of _Microystis aeruginosa_ increased. [PERSON] et al. [89] studied effects of Cu\({}^{2+}\) of different concentrations on three kinds of algae. The research findings showed that different algae showed various sensitivities to the toxicity of the heavy metal, Cu\({}^{2+}\). The inhibition rate of 200 ug/L Cu\({}^{2+}\) to _Chlorelella ellipsoidea_, _Monoraphidium contourum_ and _Scendesmus acuminatus_ was 1%, 54% and 53%, respectively. The EC\({}_{50}\) of _C. ellipsoidea_, _M. contortum_ and _S. acuminatus_ was 489 ug/L, 164 ug/L and 170 ug/L, respectively. [PERSON] et al. [90] studied the effects of Cu\({}^{2+}\) on the growth and physiological characteristics of _Skeletonema costatum_ and _Phaeodactylum tricornutum_ and found that a relatively high concentration of Cu\({}^{2+}\) had obvious inhibitory effects on the growth of _Skeletonema costatum_ and _Phaeodactylum tricornutum_, the EC\({}_{50}\) of 72 h of which was (0.546 \(\pm\) 0.068) and (0.531 \(\pm\) 0.037) mg/L, respectively. The effects of Cu\({}^{2+}\) on the activities of SOD and POD of _Skeletonema costatum_ and _Phaeodactylum tricornutum_ displayed as low concentration induction and high concentration inhibition. The content of MDA gradually increased with the increase of Cu\({}^{2+}\) concentration. [PERSON] et al. [91] found that different concentrations of heavy metal ions (Cu\({}^{2+}\), Mn\({}^{2+}\) and Zn\({}^{2+}\)) under different time treatments inhibited the growth of _Scenedesmus obliquus_, and the ascending sequence of toxicity was Cu\({}^{2+}\)\(>\)Zn\({}^{2+}\)\(>\)Mn\({}^{2+}\). With the march of the stress time, the chlorophyll fluorescence parameters, PS II maximum light energy conversion efficiency (Fv/Fm), PS II actual light energy conversion efficiency (Y), maximum relative electron transfer efficiency (Re, t, max), half-saturation light intensity (IK) and efficiency of light energy utilization (\(\alpha\)) all decreased and the decreasing range increased gradually. Different types of elements in steel slag have different influence on growth and the metabolite accumulation of microalgae. Table 1 shows the influence of different types of elements in steel slag on the growth and metabolite accumulation of microalgae.