## 5 Conclusions  
In this study, we conducted field and laboratory experiments under the same conditions. In the laboratory, to reduce contamination during _H. pluvialis_ growth, we minimised contact with the outside environment and used temperature-controlled tanks. In an outdoor \begin{table}
\begin{tabular}{c c c} \hline  & **Laboratory Culture** & **Outdoor Culture** \\ \hline Biomass (g L\({}^{-1}\) DW) & 0.71 \(\pm\) 0.020 \({}^{\star}\) & 0.51 \(\pm\) 0.130 \\ Chl \(a\) (ug mL\({}^{-1}\)) & 0.110 \(\pm\) 0.010 & 0.090 \(\pm\) 0.012 \\ Chl \(b\) (ug mL\({}^{-1}\)) & 0.112 \(\pm\) 0.024 & 0.090 \(\pm\) 0.014 \\ Total carotenoid content (ug mL\({}^{-1}\)) & 1.125 \(\pm\) 0.011 \({}^{\star}\) & 0.841 \(\pm\) 0.240 \\ Astaxanthin content (ug mL\({}^{-1}\)) & 0.752 \(\pm\) 0.014 \({}^{\star}\) & 0.521 \(\pm\) 0.171 \\ \hline \end{tabular} \({}^{\star}\)\(p\) < 0.001 (Student’s _t_-test).  
\end{table} 
Table 1: Biomass, Chl \(a\), Chl \(b\), total carotenoid, and astaxanthin contents produced in the laboratory and hybrid outdoor culture systems (_t_ = 5). experiment to promote astaxanthin accumulation, _H. pluvialis_ was also grown in a 120 m\({}^{3}\) water tank under a light intensity of 160 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\) as in the laboratory experiment. However, both the biomass and astaxanthin contents were approximately 30% lower in the outdoor culture system than in the indoor culture system. Various factors such as light periodicity, light source, water temperature, and external pollution likely affected the outdoor culture system, in comparison to the indoor culture system. Our open culture system produced approximately 50 kg of biomass per culture at an average rate of 0.51 g L\({}^{-1}\) DW in a 120 m\({}^{3}\) water tank. Although a higher temperature would be preferred to reduce production costs, we performed the mass cultivation of _H. pluvialis_ at 18 \({}^{\circ}\)C \(\pm\) 2 \({}^{\circ}\)C, as the National Institute for Environmental Studies (NIES) in Japan reported that the optimum temperature for _H. pluvialis_ growth is 20 \({}^{\circ}\)C [57]. Using sunlight, a natural source of light, is beneficial in terms of reducing capital expenditure. However, its intensity is not constant and varies with changes in weather [58]. As a solution to this drawback, we applied a multilayered polyolefin vinyl screen in our open-closed pond system to maintain a consistent light intensity of 25 \(\pm\) 5 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\). The industrial application of algae to produce astaxanthin has several limitations, which can directly affect the final price of the product. As large amounts of energy are required, inexpensive open outdoor culture systems should be adopted as an alternative to costly closed photobioreactors. Moreover, astaxanthin production rates must be increased through additional research on the effects of light periodicity, changes in nutrient levels, and water temperature. Under increasing consumer demand for natural products derived from microalgae, the proposed system may also be used for the mass cultivation of various other types of microalgae. Conceptualization, H.S. and S.J. ; methodology, J.L. ; software, V.R. ; validation, H.B., Y.A. and T.K. ; formal analysis, J.L. ; investigation, T.K. ; resources, H.S. ; data curation, S.J. ; writing--original draft preparation, S.J. ; writing--review and editing, S.J. and V.R. ; visualization, V.R. ; supervision, H.S. All authors have read and agreed to the published version of the manuscript. This research was supported by the'regional innovation mega project' program through the Korea Innovation Foundation funded by the Ministry of Science and ICT (Project Number: 2023-DD-UP-0007) and by the Basic Science Research Program (NRF-2021R1A6A1A03039503).