The Adaptability of Salicornia bigelovii to Salinity and Nutrient Levels in Aquaculture Effluent

The rapid development of mariculture has resulted in the direct discharge of untreated nutrient-rich effluents containing organic matter, inorganic nitrogen, phosphorus, and other nutrients into the ocean, which can lead to eutrophication, causing excessive algae growth, disrupting marine ecological balance, and presenting serious threats to coastal ecosystems. Therefore, the effective treatment and resource utilization of aquaculture effluents have become urgent considerations. The current methods for treating aquaculture effluents include physical, chemical, and biological approaches. Biological treatment, particularly using plants, is widely used owing to its environmental friend nature. Plants can absorb nutrients such as nitrogen and phosphorus from water, thereby facilitating the degradation and treatment of pollutants. Therefore, using nutrient-rich aquaculture effluents for plant irrigation reduces environmental pollution and cultivates economically viable halophytes, maximizing resource utilization. However, the high salinity of mariculture effluents limits the use of traditional terrestrial plants, making the selection and cultivation of halophytes that can thrive in high-salinity environments a notable research topic. Although various halophytes have demonstrated efficacy in the treatment of mariculture effluents, their salinity tolerance range is often below that of natural seawater, with some plants presenting ecological risks or having poor regional adaptability. Therefore, Salicornia bigelovii, known for its unique salt tolerance, broad ecological adaptability, and economic value, has attracted considerable attention. This study aimed to explore the adaptability and potential of S. bigelovii in treating aquaculture effluents by analyzing its growth and physiological-biochemical responses under different nutrient concentrations and salinity conditions. Through a cross-experiment with four salinity levels (0, 20, 30 and 40) and three eutrophication levels (LNC: low nutrient concentration, MNC: moderate nutrient concentration, and HNC: high nutrient concentration) over 60 days, we monitored the growth indices (aboveground height, number of nodes, number of axillary buds and branches, and biomass) and physiological-biochemical indices (chlorophyll content, and MDA content) of S. bigelovii to analyze its adaptability to different mariculture effluents. The results showed that, under medium-low nutrient levels, S. bigelovii exhibited strong growth adaptability within the 0–30 salinity range. Its aboveground growth height, number of axillary buds and branches, node number, and fresh and dry weight accumulation were significantly higher than those at 40 salinity level (P < 0.05). Additionally, 20 and 30 salinity levels were more conducive to node differentiation, indicating that S. bigelovii can effectively cope with moderate salt stress environments through self-regulation mechanisms and maintain stable growth patterns. When the salinity reached 40, the MDA content was significantly higher than those of other salinity levels (P < 0.05), indicating substantial significant stress, suggesting that its salt endurance is limited despite having good salt tolerance. Moreover, increasing nutrient concentrations effectively reduced the impact of various salinity levels on the differential growth of S. bigelovii and mitigated the stress effects at 40 salinity while promoting chlorophyll synthesis. This indicates that S. bigelovii is adaptable to high-nutrient environments and can effectively absorb and utilize nutrients from aquaculture effluents. Additionally, under combined high-nutrient and high-salinity (40) treatments, S. bigelovii's growth indices remained relatively stable, and MDA levels did not significantly increase, although some effect was observed. This further confirmed the important role of increased nutrient concentrations in enhancing plant resistance and survival ability under high-salinity conditions. The reasons may be that under high-salinity stress, S. bigelovii adopted multiple physiological and biochemical responses: Increasing leaf succulence to accumulate salt ions in succulent leaves and green vacuoles, thereby reducing salt ion toxicity; synthesizing osmotic regulatory substances to enhance osmotic regulation capacity, ensuring normal water supply to cells, synthesizing and accumulating osmotic protectants to enhance cell osmotic regulation and maintain water balance; and increasing antioxidant enzyme activity to scavenge reactive oxygen species, thus reducing cellular damage. The mitigation of stress effects with increased nutrient concentrations may be due to the presence of abundant nitrogen, phosphorus, and trace elements such as iron, copper, zinc, and silicon in the effluents, which are beneficial for plant growth. Consequently, S. bigelovii can be used as a phytoremediation plant for treating high-salinity eutrophic mariculture effluents and has the potential for large-scale promotion as a salt-tolerant economic plant.