## 4 Influences of Slag on the Growth of Microalgae
### Influences of Zn in Slag on the Growth and Accumulation of Metabolites of Micralgae  
Zn is a kind of heavy metal existing in slag and a necessary microelement for microalgae's growth. When slag was added to water, the Zn\({}^{2+}\) concentration in water changed since the Zn\({}^{2+}\) in slag dissolved out. The toxicity of zinc is related to the cell membrane, as it may disrupt the calcium uptake necessary for calcium ATPase activity during cell division. Research on _Arthrospin platensis_[81] showed that after adding slag, Zn\({}^{2+}\) dissolving out would influence the accumulation of carotenoids. When the Zn\({}^{2+}\) concentration was lower than 4.0 mg/L, _Arthrospin platensis_ had no obvious influences on carotenoids, but when the Zn\({}^{2+}\) concentration was higher than 6.0 mg/L, the carotenoid content produced by _Arthrospin platensis_ decreased. When the Zn\({}^{2+}\) concentration was 1.0 mg/L, the carotenoid content produced by _Arthrospin platensis_ was the highest (2.58 mg/L). When the Zn\({}^{2+}\) concentration was 4.0 mg/L, the chlorophyll-a content reached the highest value. High zinc conditions severely inhibited the growth of _Arthrospin platensis_, while under low zinc conditions, the biomass did not change much. An excessive dose of Zn\({}^{2+}\) might cause the sharp decrease in the content of Octadecenoic acid (C18:1), and even disappearance, while Palmitoleic acid (C16:1) was in contrast to Octadecenoic acid (C18:1). High zinc conditions (4.0 mg/L) might induce the production of Palmitoleic acid (C16:1). Under 8.0 mg/L, the content of Palmitoleic acid (C16:1) reached the highest value. The research found that the content of fatty acid increased with the increase of the Zn\({}^{2+}\) concentration, which meant that Zn\({}^{2+}\) was the key element for the accumulation of fatty acid. When Zn\({}^{2+}\) concentrations were 0.0 mg/L, 0.5 mg/L, 1.0 mg/L, 2.0 mg/L, 4.0 mg/L, 6.0 mg/L and 8.0 mg/L, _Coelastrella_ sp. biomass was in inverse proportion to the Zn\({}^{2+}\) concentration. When the Zn\({}^{2+}\) concentration was 0.0 mg/L, _Coelastrella_ sp. biomass reached the highest value (7.79 mg/L/d). When the Zn\({}^{2+}\) concentration was 8.0 mg/L, _Coelastrella_ sp. biomass was the lowest (3.48 mg/L/d). The protein content and glutathione content (protein/biomass g/g) were 0.207 g/g and 189.9 mg/g, respectively. When the Zn\({}^{2+}\) concentration was 6.0 mg/L, the maximum value of superoxide dismutase (SOD) was 55.5 U/mg protein. When the Zn\({}^{2+}\) concentration was 8.0 mg/L, the maximum value of adenosine triphosphate content was 1589 \(\pm\) 57 umol/g [82]. [PERSON] et al. [83] studied that the effect of Zn\({}^{2+}\) on antioxidant enzyme activity of _Chlorella vulgaris_. The experiment showed SOD activity was positively associated with Zn\({}^{2+}\) concentration. When the Zn\({}^{2+}\) concentration was 3.25 mg/L, SOD activity reached the highest amount, but a high concentration would decrease SOD activity. POD activity decreased with the increase of Zn\({}^{2+}\) concentration. [PERSON] and [PERSON] [84] found that, when _Scenedesmus_ sp. was stimulated by a small amount of heavy metal Zn\({}^{2+}\) (0.005 Mm), the peroxide TBARS content of a kind of lipid in microalgae increased dramatically. Meanwhile, activities of some antioxidant enzymes promoted greatly, such as SOD, APOX, CAT, GR and so on. [PERSON] et al. [85] studied the influences of Zn\({}^{2+}\) on _Pavlova viridis_ antioxidant enzymes and lipid peroxide. The study showed that when the Zn\({}^{2+}\) concentration was higher than 3.25 mg/L, the TBARS content in microalgae increased obviously, while when the Zn\({}^{2+}\) concentration was lower than 1.3 mg/L, the CAT content increased gradually with the increase of Zn\({}^{2+}\) concentration. When the Zn\({}^{2+}\) concentration was higher than 1.3 mg/L, the CAT content increased rapidly. When Zn\({}^{2+}\) concentrations were 3.25 mg/L and 6.5 mg/L, CAT contents reached 178.57% and 92.21% of the control samples, respectively.