**1. Introduction**

In recent years, multiple large-scale outbreaks of golden tide caused by *Sargassum* have occurred along the coastal waters of the Pacific and Atlantic Ocean [1–4]. This phenomenon may be closely correlated with marine ecological deterioration, including seawater eutrophication and warming [5,6]. Along the northwest Pacific coast, *Sargassum horneri* is a predominant golden-tide-forming seaweed that originally forms extensive underwater forests [7,8]. Sessile populations of *S. horneri* have continuously decreased in this region [9], whereas a prominent drifting biomass has been frequently detected in the coastal regions of China in the last decade [3,10]. Golden tides have caused considerable disruption to coastal ecosystems, including the death of aquatic organisms by hypoxia, resource competition with native species, and even shifts in biological community structures [11,12]. Furthermore, it has brought considerable damage to local economic industries such as tourism, traditional fisheries, and mariculture [13–15].

Positive and negative interactions among plants are widespread in both terrestrial and aquatic ecosystems, and have complicated and changeable effects on population dynamics, community structure, and biodiversity patterns [16–18]. In coastal regions, a common interspecific interaction pattern is for marine plants with overlapping niches to compete for diverse limited environmental resources [19–21]. Epiphytic *Ulva lactuca* has been proven to severely interfere with mariculture of *Neoporphyra haitanensis*, as they

**Citation:** Song, H.; Liu, Y.; Li, J.; Gong, Q.; Gao, X. Interactions between Cultivated *Gracilariopsis lemaneiformis* and Floating *Sargassum horneri* under Controlled Laboratory Conditions. *Water* **2022**, *14*, 2664. https://doi.org/10.3390/w14172664

Academic Editors: Xiangli Tian and Li Li

Received: 20 July 2022 Accepted: 26 August 2022 Published: 28 August 2022

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compete for carbon and nitrogen resources [22]. Additionally, the growth of *Neopyropia yezoensis* was apparently restrained by *Ulva prolifera* proliferation because of competition for space, nutrients, and light for spore attachments [23,24]. Allelopathy is another pattern of interspecific interaction among marine algae, which can stimulate or inhibit each other through the release of biochemical metabolites [25–27]. Yuan et al. [28] showed that aqueous extract from *Sargassum fusiforme* promoted the growth of *Karenia mikimotoi* at 0.2 g L−1, while its growth was inhibited at 1.6 g L−1. Similarly, Patil et al. [29] documented that the growth of *Skeletonema costatum* was not significantly decreased by culture filtrates of *Pyropia haitanensis* at 0.625–10 g FW L−1, while it was greatly inhibited at 15 and 20 g FW L−1. These results imply that interaction through allelopathy is closely associated with the biomass density ratio among species. However, thus far there has been little information produced regarding the influence of the biomass density ratio on the interaction between marine macroalgae.

*Gracilariopsis lemaneiformis* is a common intertidal red seaweed that is naturally distributed across temperate regions worldwide [30–32]. In China, *G. lemaneiformis* is extensively cultivated due to its widespread applications in agar production, the food industry, and abalone aquaculture [33–35]. It is the seaweed species with the third highest production rate in terms of mass, with 368,967 tons of dry weight produced in 2020 [36]. Additionally, the cultivation of this species is beneficial to coastal ecosystems in many ways, including alleviation of harmful microalgal blooms, increasing the dissolved oxygen in the seawater, and maintaining coastal ecological balance [37]. In recent years, the deterioration of the marine environment caused by climate change and human activities has posed a severe threat to the mariculture production of seaweeds [38–40]. Due to the economic and ecological importance of *G. lemaneiformis*, a considerable number of studies about the impacts of diverse abiotic factors on its physiological and biochemical performance have been conducted [41–44]. Nevertheless, the effects of biotic stresses such as macroalgal blooms on *G. lemaneiformis* have rarely been reported.

In the current study, we performed a short-term culture experiment to examine the interactive influences between cultivated *G. lemaneiformis* and floating *S. horneri*. Changes in growth, photosynthesis, NO3-N uptake, and biochemical compositions were estimated. Our results provide significant information to improve the mariculture management of this valuable species and to address marine ecological hazards.

#### **2. Materials and Methods**

#### *2.1. Sampling and Maintenance*

*G. lemaneiformis* thalli were sampled from farmed populations on Lidao Island (36◦26 N, 122◦56 E), China, in June 2020. Meanwhile, *S. horneri* thalli were sampled from floating populations in the cultivation area of *G. lemaneiformis*. Healthy samples were chosen and completely rinsed using sterilized seawater to remove epiphytes and detritus. Algal fragments of *G. lemaneiformis* (6 cm in length) and *S. horneri* (3 cm in length) were respectively excised from the apical position of branches for the experiments. Then, they were incubated in tanks containing 6 L of 25% PESI medium [45]. These fragments were kept for three days at 20 ◦C, 90 μmol photons m−<sup>2</sup> s<sup>−</sup>1, and a 12:12 L:D cycle in order to minimize the impacts of excision.

#### *2.2. Culture Experiment*

A twelve-day culture experiment was performed under mono-culture conditions of *G. lemaneiformis* and *S. horneri* and co-culture at different biomass density ratios (BDRs). For the mono-culture, the initial biomass density was set at 6 g for both species. For the co-culture, the initial BDR was set at 2:1 (4 g *G. lemaneiformis* cultured with 2 g *S. horneri*), 1:1 (3 g *G. lemaneiformis* cultured with 3 g *S. horneri*), and 1:2 (2 g *G. lemaneiformis* cultured with 4 g *S. horneri*). A total of five treatments were set up, and each treatment was conducted in three replicates. In this experiment, 20 ◦C, 90 μmol photons m−<sup>2</sup> s−1, and a 12:12 L:D cycle were maintained. This experiment used fifteen tanks, with each tank containing 6 L

of sterilized seawater enriched with 25% PESI medium. Gentle aeration was conducted during this experiment. The medium was changed every three days. The fresh weights (FWs) of all fragments before and after incubation were determined after each fragment was blotted dry. The relative growth rate (RGR; % day<sup>−</sup>1) was calculated by Equation (1):

$$\text{RGR} = 100 \times (\text{lnW}\_{\text{t}} - \text{lnW}\_{0}) \; / \; \text{t} \tag{1}$$

where W0 is the initial FW, Wt is the final FW, and t is the culture time in days.

#### *2.3. Measurement of Photosynthesis*

Following the culture experiment, we determined the net photosynthetic rates of *G. lemaneiformis* and *S. horneri* using a manual FireStingO2II oxygen meter (Pyro Science GmbH, Aachen, Germany). For each species, 0.33 g (FW) of samples from each treatment were moved to the oxygen electrode cuvette containing 330 mL of 25% PESI medium. Next, the medium was continuously stirred to ensure homogenous oxygen diffusion. The culture conditions were the same as for the experiment above. The oxygen increase in the seawater was regarded as the net photosynthetic rate after an increase in the light density. Before measurements, these samples were placed into the cuvette for 5 min acclimation. The oxygen concentration in the medium was documented every 1 min for 15 min.
