## 5 Conclusions  
This study conducted semi-continuous cultures of _A. platensis_ with medium recycling and CO\({}_{2}\) supply. Medium recycling enabled 68% of total medium cost reduction and did not affect the algal growth although DIC and DOC accumulated in the medium. Carbon dioxide recovery rates of 63-100% and steady algal growth of 0.31 gDW L\({}^{-1}\) d\({}^{-1}\) were achieved even with the excessive carbon supply compared with the _A. platensis_ carbon assimilation. Carbon mass balance analysis revealed that up to 24% of the assimilated carbon escapes into the medium in a form of DOC, and thus care needs to be taken in evaluating the process. This study demonstrated the effectiveness and stability of the medium-recycling process even at high CO\({}_{2}\) supply rates. Further study is needed to elucidate the mechanisms of DOC excretion and its effects on the growth and biomass characteristics.  
The following are available online at [[http://www.mdpi.com/2076-3417/10/1/228/s1](http://www.mdpi.com/2076-3417/10/1/228/s1)]([http://www.mdpi.com/2076-3417/10/1/228/s1](http://www.mdpi.com/2076-3417/10/1/228/s1)): explanations on Figure S1. Modified SOT medium titration with KOH (2) CO\({}_{2}\) partial pressure in the absorption columns, Figure S2. Calculated and experimental amount of the absorbed CO\({}_{2}\) over 24 h (CO2_abs_) at different initial dissolved inorganic carbon (DIC). Plots: experimental values; solid lines: calculated values; and dashed lines: theoretical 100% absorption of supplied CO\({}_{2}\) Figure S3. Dissolved inorganic carbon concentrations in CO\({}_{2}\) absorption columns; (**a**,**c**,**e**) after CO\({}_{2}\) supply, and (**b**,**d**,**f**) after medium recycle from the photobioreactors. Low pH after CO\({}_{2}\) supply induced decrease of CO\({}_{3}\)\({}^{-2}\) fraction and increase of HCO\({}_{3}\)\({}^{-2}\) and CO\({}_{2}\) fractions, Figure S4. Dissolved inorganic carbon concentration in photobioreactors; (**a**,**c**,**e**) after 1-day incubation, and (**b**,**d**,**f**) after medium input from CO\({}_{2}\) absorption columns. Slight reduction in total DIC after incubation was observed. Figure S5. Carbon mass flux during 18-day operation. CO\({}_{2}\) supply rate of (**a**) Run I, (**b**) Run I, (**b**) Run I, and (**c**) Run 3. Each box represents total cumulative mass (in milligram) of carbon fraction introduced or discharged over 18 days in the forms of CO\({}_{2}\), dissolved inorganic carbon (DIC), dissolved organic carbon (DOC) or algal biomass (particulate organic carbon; POC). The mass inside the CO\({}_{2}\) absorption column and photobioreactor represent differences from the initial state.  
Conceptualization, M.K. and T.T.; methodology, M.K., Y.Y., and T.K.; software, T.M.; formal analysis, M.K. and T.M.; investigation, M.K. and Y.Y.; writing--Original draft preparation, M.K.; writing--Review and editing, T.K., T.M., and T.T; supervision, T.T.; funding acquisition, T.T. All authors have read and agreed to the published version of the manuscript.  
This study was partially supported by Japan Science and Technology Agency (JST)/Japan International Cooperation Agency (JICA), Science and Technology Research Partnership for Sustainable Development (SATREPS) and PLANE3T Project funded by the Private University Research Branding Project supported by Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT).  
The authors declare no conflict of interest.