## 4 Influences of Slag on the Growth of Microalgae
### Influences of Fe in Slag on the Growth and Accumulation of Metabolites of Microalgae  
There are a lot of iron oxides and iron complexes in slag. Fe in slag is the essential trace element that is needed in the metabolic processes of photosynthesis respiration and an important electron acceptor in microalgae's photosynthesis. Fe, which is one of the main factors limiting the growth of phytoplankton, plays a critical role in the growth of microalgae. Fe can enhance the ability of microalgae's nitrogen reduction and fixation, which plays an important role in microalgae's effective use of C, N and P, as well as in chlorophyll's biosynthesis and photosynthesis [51]. Proper concentration of iron can promote microalgae's growth and proliferation. The research findings show that when desulfurization slag was added in the reactor, total iron content of the reactor dramatically increased. Iron concentration increased with the increase of slag concentration. When the amount of slag was over 100 mg/L, pH \(>\) 11, _Chlorella_ sp. 's growth was inhibited. When the concentration of 25 mg/L desulfurization slag was added, total iron concentration of the solution was 1.1 mg/L. Compared with the control group of 0.1-0.2 mg/L total iron concentration, concentration of iron increased significantly, and total organic carbon of _Chlorella_ sp. was 56 mg/L. When the concentration of 300 mg/L desulfurization slag was added, total iron concentration of the solution was 4 mg/L and total organic carbon of _Chlorella_ sp. was 41 mg/L, because Fe released by slag and heavy metals of an extremely low concentration stimulated the increase of _Chlorella_ sp. 's biomass, while high-concentration slag would release more heavy metal ions, resulting in the increase of pH, thus inhibiting _Chlorella_ sp. 's growth. _Chlorella_ sp. in the reactor grew through absorbing total iron [52]. In the ocean, especially in the open sea, since land supply and dissolved iron are limited, photosynthesis is limited because of the lack of Fe. To deal with iron stress, slag can be used as marine algae's iron source, taking use of iron's quick oxidation and sedimentation to promote marine algae's growth [53]. Research has found that the iron source released by 20 mg slag was enough for two kinds of marine diatoms, _Thalassioira nordenskioldii_ and _Thalassioira oceanica_, to grow for 50 days. Fe or Fe\({}^{2+}\) released by slag and an inorganic iron reagent (FeCl\({}_{3}\cdot\)6H\({}_{2}\)O) could effectively promote these two kinds of marine diatoms' cell proliferation [54]. Slag is different from other inorganic iron reagents, such as Fe powder and ferrous sulfate, which can release dissolved iron ions and colloid Fe (III) and maintain the iron concentration of a solution. Compared with other iron sources, slag has a longer maintaining time. Additionally, slag can also release P and Si into a solution. Including the Fe released, the extended Redfield ratio of P, Si and Fe released by slag into the solution was 1:15:0.0075, which was the average ration of main nutrients that composed phytoplankton cells [55]. During the process of the use of slag to cultivate _Thalassioira guillardia_, it was found that, with slag as the Fe source for the proliferation of _Thalassioira guillardia_, the utilization rate of Fe of _Thalassioira guillardia_ was about 1% of Fe content in slag. With slag as the P source for the proliferation of _Thalassioira guillardia_, the utilization rate of P of _Thalassioira guillardia_ was less than 68% of the P content in slag. When the addition amount of slag was less than 50 mg/L, pH of the solution was not affected [56]. Fe is the essential component for redox reaction in cells and plays an important role in cell respiration, photosynthesis and the process of metalloprotein's catalytic reaction [57]. Research has shown that Fe plays an important role in microalgae's growth and oil accumulation. [PERSON] et al.