*Article* **Interactions between Cultivated** *Gracilariopsis lemaneiformis* **and Floating** *Sargassum horneri* **under Controlled Laboratory Conditions**

**Hanmo Song, Yan Liu \*, Jingyu Li, Qingli Gong and Xu Gao \***

Key Laboratory of Mariculture (Ministry of Education), Fisheries College, Ocean University of China, Qingdao 266003, China

**\*** Correspondence: qd\_liuyan@ouc.edu.cn (Y.L.); gaoxu@ouc.edu.cn (X.G.); Tel.: +86-532-8203-2377 (Y.L. & X.G.)

**Abstract:** The golden tide dominated by *Sargassum* has become a frequently-occurring marine ecological event that may constitute major biotic threats to seaweed aquaculture. In this study, the interaction between cultivated *Gracilariopsis lemaneiformis* (GL) and floating *Sargassum horneri* (SH) was investigated by physiological and biochemical measurements under mono-culture and co-culture with different biomass density ratios of 2:1 (2GL:1SH), 1:1 (1GL:1SH), and 1:2 (1GL:2SH). The relative growth rate, net photosynthetic rate, and NO3-N uptake rate of *G. lemaneiformis* were significantly greater at the biomass density ratio of 2:1 than at mono-culture. However, these physiological parameters and biochemical composition contents (chlorophyll *a* and soluble protein) of *G. lemaneiformis* decreased significantly with increasing biomass of *S. horneri.* Meanwhile, these physiological and biochemical parameters of *S. horneri* were greater in all co-culture models than at mono-culture. They decreased significantly with decreasing biomass of *G. lemaneiformis*. These results indicate that the occurrence of floating *S. horneri* with low biomass can stimulate the growth of *G. lemaneiformis*, whereas its outbreak may significantly reduce the production and quality of *G. lemaneiformis*. *G. lemaneiformis* cultivation may be beneficial to the increased biomass of floating *S. horneri*.

**Keywords:** biochemical composition; golden tide; growth; NO3-N uptake; photosynthesis; seaweed cultivation
