## 4 Influences of Slag on the Growth of Microalgae
### Influences of P in Slag on the Growth and Accumulation of Metabolites of Microalgae  
There was a certain amount of P2O5 in the slag. P is an essential factor for the primary productivity of aquatic ecosystems, such as oceans and lakes, and is one of the basic nutrients that marine life relies on to grow, as well as being a necessary nutrient source for algae proliferation. P, the substrate and regulator, directly participated in all aspects of photosynthesis in our research, including light energy absorption, formation of assimilatory power, the Calvin cycle, transport of assimilates and playing a regulatory role in the activity of some key enzymes [63]. P, the main component of nucleic acid, proteins and phospholipids, was necessary for chlorophyll synthesis. Compared with nitrogen compounds, the mobility of phosphorus-containing compounds in the natural world was relatively low. Most algae were able to fix nitrogen. Thus, the content of P was the symbol of eutrophication.  
The lack of P limited the growth rate of most microalgae; when P from the external environment was exhausted, microalgae would consume P reserved by itself until the lowest cell content of P that was suitable for growth was achieved [64]. To further research the reliability that whether slag could be used as nutrient source for marine microalgae and the promoting effects of decarbonization slag and dephosphorylation on the growth of _Thalassioira guillardii_, an experiment of _Thalassioira guillardii_ batch culture using decarbonization slag and dephosphorylation slag, without controlling the pH value at the start of the study, was carried out. The research found that, when the slag addition was 33 mg/L, _Thalassioira guillardii_'s chlorophyll-a fluorescence value increased dramatically. The use of slag boosted the _Thalassioira guillardii_'s growth obviously. When the slag concentration increased by 100 times (3300 mg/L), _Thalassioira guillardii_'s chlorophyll-a fluorescence value decreased. _Thalassioira guillardii_'s growth was inhibited. This was because calcium precipitation in slag decreased the content of phosphate in the culture. Meanwhile, an excessive dose of slag might result in the increase of pH value of the seawater medium, which might greatly reduce the solubility of Fe in slag [65].  
P was of great importance for microalgae's growth. Without enough P, the Calvin cycle efficiency declined and the recycled circulation of NADP+ and NADPH was obstructed. While NADP+ was the terminal electron acceptor of the PSI end of the photosynthetic electron transport chain, when the supply was inefficient, PSII function was sure to decline, which displayed as the decrease of photochemical efficiency of PSII and quantum yield, that is, the \(\mathrm{Fv/Fm}\) ratio decreased [66; 67; 68].  
P was the primary limiting factor for _Phaeocystis globosa_'s growth [69]. P was the main factor influencing _Chlorella_ sp.'s cell growth [70]. Various algae had different needs for P. _Chrysophyta_'s optimum P moore concentration was 5 \(\upmu\)mol/L [71]. _Prorocentrum micans_' optimum P moore concentration was 0.85 \(\upmu\)mol/L [72]. [PERSON]'s research found that _Heterosigma akashiwo_ nearly could not grow under phosphate-free conditions. Under low phosphorus concentration conditions from 0 mg/L to 0.05 mg/L, _Heterosigma akashiwo_'s growth was obviously inhibited. Under the medium phosphorus concentration conditions from 0 mg/L to 1.0 mg/L, its growth rate was directly proportional to concentration of phosphorus. Under the higher phosphorus condition where the concentration reached 5.0 mg/L, the increase of _Heterosigma akashiwo_'s growth rate decelerated [73]. Similar to the lack of N, the lack of P might also influence microalgae's photosynthesis sometimes. According to [PERSON]'s research, after P limitation for 4 days, _Chlamydomonas reinhardtii_'s oxygen evolution rate decreased by 75% [74].