*Article* **NaCl Promotes the Efficient Formation of** *Haematococcus pluvialis* **Nonmotile Cells under Phosphorus Deficiency**

**Feng Li 1,2,3,4, Ning Zhang 1, Yulei Zhang 1, Qingsheng Lian 1, Caiying Qin 1, Zuyuan Qian 1, Yanqi Wu 1, Zhiyuan Yang 1, Changling Li 1, Xianghu Huang 1 and Minggang Cai 1,2,3,4,\***


**Abstract:** Natural astaxanthin helps reduce the negative effects caused by oxidative stress and other related factors, thereby minimizing oxidative damage. Therefore, it has considerable potential and broad application prospects in human health and animal nutrition. *Haematococcus pluvialis* is considered to be the most promising cell factory for the production of natural astaxanthin. Previous studies have confirmed that nonmotile cells of *H. pluvialis* are more tolerant to high intensity of light than motile cells. Cultivating nonmotile cells as the dominant cell type in the red stage can significantly increase the overall astaxanthin productivity. However, we know very little about how to induce nonmotile cell formation. In this work, we first investigated the effect of phosphorus deficiency on the formation of nonmotile cells of *H. pluvialis*, and then investigated the effect of NaCl on the formation of nonmotile cells under the conditions of phosphorus deficiency. The results showed that, after three days of treatment with 0.1% NaCl under phosphorus deficiency, more than 80% of motile cells had been transformed into nonmotile cells. The work provides the most efficient method for the cultivation of *H. pluvialis* nonmotile cells so far, and it significantly improves the production of *H. pluvialis* astaxanthin.

**Keywords:** *Haematococcus pluvialis*; astaxanthin; nonmotile cells; phosphorus deficiency
