## 4 Influences of Slag on the Growth of Microalgae
### Influences of Fe in Slag on the Growth and Accumulation of Metabolites of Microalgae
[58] found that when the concentration of Fe\({}^{3+}\) was 1 \(\times\) 10\({}^{-4}\) mol/L, the growth of _Phaeodactylum tricornutum_ was the fastest and lipid content was the most, reaching up to 47.3%, which was 4.2 times higher than the _Phaeodactylum tricornutum_ cultivated without Fe\({}^{3+}\). When the concentration of Fe\({}^{3+}\) was 1 \(\times\) 10\({}^{-5}\) mol/L, the growth rate of _Ischrysis galbana_ was the fastest, and the lipid content reached up to 41.9%, which was 2.8 times higher than the _Ischrysis galbana_ cultivated without Fe\({}^{3+}\). [PERSON] et al. [59] found that under the combination conditions of nitrogen deficiency, 82.5 uE/m\({}^{2}\)s light intensity and 0.74 mM Fe\({}^{3+}\) concentration, the lipid content of _Botryococcus_ spp. was improved to 35.9%. [PERSON] et al. [60] studied the effects of Fe on the growth and lipidaccumulation of _Desmodesmus_ sp. WC08. The biomass concentration, lipid content and lipid production rate of _Desmodesmus_ sp. WC08 were tested when Fe concentration was 0 mmol/L, 0.5 \(\times\) 10\({}^{-5}\) mmol/L, 1 \(\times\) 10\({}^{-5}\) mmol/L, 2 \(\times\) 10\({}^{-5}\) mmol/L, 4 \(\times\) 10\({}^{-5}\) mmol/L, 6 \(\times\) 10\({}^{-5}\) mmol/L and 10 \(\times\) 10\({}^{-5}\) mmol/L. Research findings showed that _Desmodesmus_ sp. _WC08\({}^{\prime}\)s biomass was less affected by the concentration of Fe. When the concentration of Fe was 2 \(\times\) 10\({}^{-5}\) mmol/L, _Desmodesmus_ sp. WC08\({}^{\prime}\)s biomass reached the highest value at 2.38 \(\pm\) 0.21 g/L, which was 3 times as much as without Fe. Under other concentrations of Fe, the accumulation of _Desmodesmus_ sp. WC08\({}^{\prime}\)s biomass was relatively low. From the perspective of lipid content, Fe concentration had an obvious effect on _Desmodesmus_ sp. WC08\({}^{\prime}\)s lipid content. The lipid content showed a slowly increasing tendency when Fe concentration was 0-10 \(\times\) 10\({}^{-5}\) mmol/L. When Fe concentration was 10 \(\times\) 10\({}^{-5}\) mmol/L, the lipid content reached the highest value (31.99%). Except for the lowest lipid content group (0.5 \(\times\) 10\({}^{-5}\) mmol/L), it had no significant difference from the lipid contents of other groups. Analysis of the lipid content showed that the Fe concentration had significant influences on the lipid yield of _Desmodesmus_ sp. WC08, which meant that the role Fe played in the process of lipid accumulation could not be ignored. In the range of test concentrations, the lipid yield increased with the increase of the Fe concentration. When the concentration was 2 \(\times\) 10\({}^{-5}\) mmol/L, the lipid yield reached the highest value (0.69 g/L), which was 1.4 times as much as that of blank group. However, compared with the 1 \(\times\) 10\({}^{-5}\) mmol/L group and the 1 \(\times\) 10\({}^{-4}\) mmol/L group, there was no significant difference. Consequently, when the Fe concentration was 2 \(\times\) 10\({}^{-5}\) mmol/L, it was the most helpful for _Desmodesmus_ sp. WC08\({}^{\prime}\)s accumulation of biomass and lipids. Different iron sources had different effects on microalgae's growth. Research compared the influences of three kinds of iron compounds on _Auxencholorella protothecodes_, that is, ferrous sulfate, ferric ethylenediaminetetraacetic acid (EDTA) and ferric chloride. When the concentration of ferric chloride was 1.15 mM, the content of saturated fatty acids (SFA) of _Auxencholorella protothecodes_ reached the highest value (78.5%). When the concentration of ferrous sulfate was 1.08 mM, content of biomass of _Auxencholorella protothecodes_ reached the highest value (1520 mg/L). Ferric chloride was the most toxic for _Auxencholorella protothecodes_. When microalgae were cultivated under the condition of 0.2 mM and 14.4 mM ferrous sulfate concentration, as well as 7.19 mM ferric EDTA concentration and 0.07 mM-21.58 mM ferric chloride concentration, biodiesel with a satisfactory quality could be produced [61].