**4. Conclusions**

In this study, we screened mutant strains with photosensitivity using the negative phototaxis of the *H. pluvialis* in microfluidic device. It was confirmed that the more sensitive the mutant, the higher the astaxanthin productivity. In addition, large-scale cultivation in 100 L photo-bioreactor shows that the strains selected through phototaxis technology can be used in a more scaled-up culture for commercialization. We are planning follow-up studies to screen more cells faster and more efficiently. This screening strategy using the phototaxis of microalgae is advantageous in securement of *H. pluvialis* with improved growth rate and astaxanthin productivity.

**Author Contributions:** Conceptualization, K.H.L. and Y.C.; data curation, K.H.L., Y.C. and J.H.L.; funding acquisition, H.S.K.; methodology, K.H.L., Y.C. and J.H.L.; project administration, C.P., H.Y.Y. and H.S.K.; software, K.H.L., Y.C. and J.H.L.; supervision, C.P., H.Y.Y. and H.S.K.; writing—original draft, K.H.L. and Y.C.; writing—review and editing, C.P., H.Y.Y. and H.S.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Research Foundation of Korea (NRF) gran<sup>t</sup> funded by the Korean Government (NRF-2019R1F1A1058873).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.
