## 5 Conclusions  
As the byproduct in the steelmaking process, there are many nutrient elements in slag that are necessary for microalgae's growth, such as Fe, P, Ca, Mg and so on. Through research and explorations on the effects of slag on microalgae's growth, it has been found that slag has enormous potential and huge advantages for promoting microalgae's growth and accumulation of metabolites. Under suitable conditions, slag can effectively promote microalgae's growth and reproduction. Culturing microalgae by slag can lower the cost and solve the problem of lacking nutrient elements during the process of microalgae's growth. It can also alleviate the phenomenon of a large amount of steel slag storage and reduce the environmental pollution caused by steel slag storage. This provides a new method for the utilization of steel slag. Meanwhile, under certain conditions, culturing microalgae by slag can also promote microalgae's accumulation of metabolites, especially lipid accumulation. Thus, slag can be used as an ideal material for microalgae to produce metabolites.  
For the research related to slag's influences on microalgae's growth, there is some key information that can hopefully be used as references in future research.  
1. Through the laboratory research on slag's influences on microalgae's growth, it was found that there are many oxide compositions in slag, which may cause the increase of a solution's pH value and conversely inhibit microalgae's growth, which in a marine  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Types of Elements in Steel Slag** & **Types of Microalgae** & **Growth Condition** & **Effects on Growth and Metabolite Accumulation** & **Reference** \\ \hline \hline \multirow{6}{*}{_Parkova viridis_} & \multirow{2}{*}{\(>\)0.5 mg/L} & Promoting Accumulation of MDA & \multirow{2}{*}{[17]} \\ \cline{3-3} \cline{5-5}  & & 3 mg/L & Promoting Accumulation of GSH, SOD, CAT, GPX & \\ \hline \multirow{6}{*}{_Ditylum brightuellii_} & \multirow{6}{*}{\(>\)0.5 mg/L} & Promoting Accumulation of SOD & \multirow{2}{*}{[86]} \\ \cline{3-3} \cline{5-5}  & & & Promoting Lipid Accumulation & \\ \cline{3-3} \cline{5-5}  & & & \multicolumn{1}{c}{} & \\ \cline{3-3} \cline{5-5}  & & & \multicolumn{1}{c}{} & \\ \cline{3-3} \cline{5-5}  & & & \multicolumn{1}{c}{} & \\ \cline{3-3} \cline{5-5}  & & & \multicolumn{1}{c}{} & \\ \cline{3-3} \cline{5-5} Cu & & & \multicolumn{1}{c}{} & \\ \cline{3-3} \clenvironment obviously shows influences of pH decrease. This effect can be verified by a pilot experiment or even a field experiment.  
2. Among the research on slag's influences on microalgae's growth, most research analyzes one certain element in slag. There are few studies on the synergistic effect of multi-elements in steel slag on the growth of microalgae. It should be further studied whether the reason why the steel slag promotes the growth of microalgae is the effect of a single element or the synergistic effect of multiple elements.  
3. Among the research on slag's influences on microalgae's growth, most research focuses on slag's influences on marine microalgae's growth. In the future, research on slag's influences on freshwater microalgae's growth should be carried out.  
Conceptualization, Y.W.; Writing--original draft preparation, T.L.; writing--review and editing, Q.Y., S.C., D.G., X.W., F.W., Y.W., J.L. and Y.Z.; Funding acquisition, Y.W. and Y.Z. All authors have read and agreed to the published version of the manuscript.  
The authors wish to acknowledge the financial support from National Natural Science Foundation of China (No: 52004095 and 51704119) and Natural Science Foundation of Hebei Province (E2017209243) and Department of Education of Hebei Province (BJ2019038).  
Not applicable.  
The authors give thanks to the anonymous reviewers and all the editors in the process of manuscript revision.  
The authors declare no conflict of interest.