*3.1. Growth*

The RGRs of *G. lemaneiformis* were significantly affected by culture model (Figure 1A and Table 1). The RGR at BDR of 2:1 was significantly greater than that at mono-culture. However, the RGR decreased significantly with increasing biomass of *S. horneri*, from BDR of 2:1 to 1:2. The RGRs of *S. horneri* differed significantly among culture models (Figure 1B and Table 1). The RGRs of all co-culture treatments were greater than that at mono-culture, and significantly higher values were found at BDRs of 2:1 and 1:1. Additionally, the RGR at BDR of 2:1 was significantly greater than that at BDR of 1:1.

**Table 1.** One-way ANOVA testing of the effects of biomass density ratio on RGR, net photosynthetic rate, NO3-N uptake rate, Chl *a*, soluble protein, and carbohydrate of *Gracilariopsis lemaneiformis* and *Sargassum horneri*.


**Figure 1.** The RGRs of *Gracilariopsis lemaneiformis* (**A**) and *Sargassum horneri* (**B**) cultured for twelve days at mono-culture and co-culture with biomass density ratios of 2:1, 1:1, and 1:2. Different lowercase letters represent significant differences (*p* < 0.05) among culture models.

## *3.2. Photosynthesis*

The net photosynthetic rates of *G. lemaneiformis* differed significantly among culture models (Figure 2A and Table 1). The net photosynthetic rate at BDR of 2:1 was significantly higher than those of the other culture models. Additionally, the net photosynthetic rate at BDR of 1:2 was significantly lower than that at mono-culture. The net photosynthetic rates of *S. horneri* were significantly affected by culture model (Figure 2B and Table 1). The net photosynthetic rate at BDR of 2:1 was significantly higher than that at mono-culture. However, the net photosynthetic rates decreased significantly with increasing biomass of *S. horneri*, from BDR of 2:1 to 1:2.

**Figure 2.** The net photosynthetic rates of *Gracilariopsis lemaneiformis* (**A**) and *Sargassum horneri* (**B**) cultured for twelve days at mono-culture and co-culture with biomass density ratios of 2:1, 1:1, and 1:2. Different lowercase letters represent significant differences (*p* < 0.05) among culture models.
