Owing to increasing global warming, the topics of greenhouse gas emission reduction and carbon sequestration have attracted worldwide attention. Our planet includes two main categories of primary producers: marine phytoplankton and terrestrial plants, each accounting for about 50% of global net primary productivity (Field et al., 1998; Schlesinger and Bernhardt, 2020). The carbon fixed by marine photoautotrophs (including macroalgae, phytoplankton, and seagrasses) is generally referred to as “Blue Carbon” (Nellemann et al. 2009; Krause-Jensen et al., 2018; Raven, 2018; Gao et al., 2022). During the growth process, macroalgae convert inorganic carbon into organic carbon through photosynthesis, and part of this organic carbon is involved in the formation of algal tissues, while the other part is released into the environment as dissolved organic carbon (DOC) and particulate organic carbon (POC). Part of the POC and DOC are eventually exported to the deep sea to be sequestered; this is the carbon sink formation mechanism of macroalgae (Krause-Jensen and Duarte, 2016; Krause-Jensen et al., 2018). Some economically important macroalgae have already been established in complete industrial chains and therefore have greater potential for application in carbon sequestration and sink enhancement (Raven, 2018; Chen et al., 2020a; Gao et al., 2021; Gao et al., 2022; Tian et al., 2023). However, the current understanding of macroalgal-mediated carbon sink processes is not sufficiently advanced to accurately assess the carbon sink potential of macroalgae (Paine et al., 2021; Hurd et al., 2022).

The mechanism of organic matter release from algae is generally considered to include two types: active release and passive leakage (Thornton, 2014; Paine et al., 2021). Overflow is a typical example of active release as it results in the leaching of DOC from cells when the production capacity of algal photosynthesis exceeds consumption (Hatcher et al., 1977; Fogg, 1983; Livanou et al., 2019). This DOC active release mechanism can avoid the excessive accumulation of photosynthetic products that leads to algal damage and thus can maximize the utility of ribulose-1,5-bisphosphate carboxylase (Rubisco). Since the released organic carbon and the release processes are closely related to photosynthesis and respiration, it is thought that those factors that could influence these two metabolic pathways may alter the release of organic carbon (Thornton, 2014; Paine et al., 2021). In this manner, stressful growth conditions due to changes in CO₂ concentration, temperature, light, salinity, and photoperiod can indirectly but strongly increase the release of organic carbon (Paine et al., 2021). However, the effects of environmental stresses on organic carbon release from macroalgae lack in-depth study.

Fujian Province ranks first in China in terms of production of macroalgae, and it has abundant wild macroalgae resources (Chen et al., 2020a; Fishery-Department-of-China, 2021). The tides in Fujian Province are semi-diurnal, and the macroalgae at the high tide zone can experience a maximum of 6 h of desiccation. Moreover, Fujian is one of the provinces of China with the highest average annual rainfall, up to 2,432.6 mm (Fujian-Meteorological-Bureau, 2021). As a result, intertidal macroalgae in Fujian coastal waters are often under pressure from hyposaline stress. In addition, many macroalgae are found in estuaries where the salinity of seawater depends on the mixing ratio of seawater and freshwater and thus is highly variable; for example, the salinity falls in the range of 0.81–50 PSU (Hurd et al., 2014; Ma, 2021). This is another cause of hyposaline stress. It has been shown that hyposaline stress due to rainfall is an important driver of DOC release from the macroalgae Enteromorpha prolifera, Chondrus crispus, and Fucus vesiculosus (Sieburth, 1969; Pregnall, 1983). However, such studies are few, and it is uncertain whether the conclusions are widely representative.

Based on the above analyzation, we hypothesized that hyposaline stress may increase DOC release from macroalgae, resulting in an underestimation of the carbon sink potential of macroalgae. Therefore, this study analyzed the effects of hyposaline stress on the growth, organic matter release rate, and tissue C, N, and P contents of five common intertidal macroalgae in the coastal areas of Fujian Province, China, and preliminarily analyzed the biogeochemical effects of hyposaline stress on the carbon sink process mediated by macroalgae.

The W28 strain of P. haitaneisis was obtained from the Laboratory of Germplasm Improvements and Applications of Pyropia in Jimei University of Fujian Province. Gracilaria lemaneiformis was obtained from the culture raft. The collection time and locations of wild Sargassum thunbergii, Enteromorpha prolifera, and Ulva lactuca are shown in Table 1 . The collected algae were quickly transported back to the laboratory, rinsed at least three times with sterilized seawater to remove impurities, and placed in five liter glass bottles for temporary incubation for 3–5 days. The photoperiod was 12 L:12 D, and the light intensity was set at 100 μmol m^-2 s^-1.

The healthy algae were selected and rinsed again with sterilized seawater three times and placed in 500 mL glass bottles. The algae were treated with three salinities (0 PSU, 5 PSU, and 30 PSU) for 6 h. During the incubation period, the cultures were aerated and irradiated with a light intensity of 100 μmol m^-2 s^-1. The incubation temperatures were the same as the temperature at the time of algal collection ( Table 1 ). Different salinities of seawater were taken from natural seawater and ultrapure water.

Macroalgae can release a large amount of DOC to the environment during growth, and this makes macroalgae have a high carbon sink potential (Krause-Jensen and Duarte, 2016; Ortega et al., 2019; Watanabe et al., 2020; Li et al., 2022). However, a variety of environmental stressors have significant effects on DOC release from macroalgae (Paine et al., 2021; Hurd et al., 2022). This makes it difficult to assess the carbon sink potential of macroalgae. For example, intertidal macroalgae often face low salinity stress caused by rainfall or estuarine flushing of freshwater, and there is a lack of research on the effects of low salinity stress on DOC release from macroalgae. To this end, this study analyzed the effects of hyposaline stress on common macroalgal species in coastal waters of Fujian Province, China, and found that hyposaline treatment significantly inhibited photosynthesis but significantly increased the DOC release rate ( Figure 2 ). Fujian is one of the province of China with the highest average annual rainfall, up to 2,432.6 mm (Fujian-Meteorological-Bureau, 2021). This suggests that the assessment of the carbon sink contribution of macroalgae in Fujian coastal waters must take into account the effects of hyposaline stress.

The tolerance level of different macroalgal species to salinity stress differs significantly (Hurd et al., 2014). This conclusion is consistent with the results of present study. For example, a low salinity of 5 PSU had no significant effect on the fv/fm of P. haitaneisis or S. thunbergii ( Figure 1 ), but significantly inhibited the fv/fm of the other three macroalgal species. In addition, 0 PSU strongly reduced the fv/fm of all five species of macroalgae. This study also found that compared to 30 PSU, hyposaline stress significantly reduced the net photosynthetic rate and total photosynthetic rate of all five macroalgal species ( Figure 3 ). A previous study also showed that the fv/fm of P. haitaneisis and Sargassum fusiforme decreased significantly with the increase of hyposaline treatment time (Chen et al., 2019; Yonemori et al., 2022). Recent studies have shown that hyposaline treatment causes osmotic damage to the cells of P. haitaneisis, resulting in down-regulated expression of relevant genes and proteins involved in photosynthesis, reduced light energy capture and transfer efficiency, and reduced photosynthetic pigment content, thus inhibiting photosynthesis (Wu et al., 2015; Feng et al., 2016; Chen et al., 2019; Wen et al., 2022). These results suggest that 6 h of hyposaline treatment inhibited the photosynthetic rate of macroalgae and negatively affected PSII function.

Under normal growth conditions, about 14% of the carbon fixed by photosynthesis is released by macroalgae to the environment in the form of DOC (Chen et al., 2020a). The present study found that hyposaline treatment significantly increased the release rate of DOC from macroalgae ( Figure 2 ) while strongly decreasing the photosynthetic rate ( Figure 3 ). A previous study by Pregnall (1983) found that osmotic stress caused by salinity fluctuations led to an increase in DOC release from E. prolifera. The DOC release rate of E. prolifera increased from 457.95 μmol C g DW^-1 h^-1 to 1290.59 μmol C g DW^-1 h^-1 when the salinity decreased from 30 PSU to 5 PSU (Pregnall, 1983). Similar results were reported by Sieburth (1969) in the macroalgae Chondrus crispus and Fucus vesiculosus. These studies and our results suggest that hyposaline stress causes a double inhibition of organic carbon accumulation in macroalgal tissue ( Figure S1 ).

By analyzing the effect of hyposaline stress on the C contents of macroalgal tissues, we found that hyposaline treatment substantially increased (> 40%) the C contents of G. lemaneiformis and E. prolifera but had a smaller effect (< 7%) on the C contents of P. haitaneisis, S. thunbergii, and U. lactuca. Thus, there were significant interspecific differences in the effects of hyposaline stress on the elemental composition of macroalgae ( Figure 4 ). A previous study found that P enrichment caused a significant increase in tissue P concentrations of the three macroalgal species Caulerpa serrulata, Laurencia intricate, and Sargassum polyphyllum, but the increases were significantly different among the three species (Reef et al., 2012). In conclusion, the physiological responses of marine algae to environmental factors such as salinity, CO₂, light, temperature, and nutrients are generally species-specific (Koch et al., 2013; Ji et al., 2016; Ji and Gao, 2021).

Macroalgae need to absorb large amounts of N and P from the environment during growth, and thus large-scale cultivation of macroalgae can alleviate the offshore eutrophication problem and thereby play an important role in the offshore N and P cycles (Xu et al., 2008; Hurd et al., 2014; Mao et al., 2018). In this study, we found significant interspecies differences in the effects of hyposaline treatment on the N and P contents of macroalgae as well as the C:N:P ratios ( Figures 4 , 5 ). This is further evidence that different species have different levels of tolerance to low salinity ( Figure 1 ). Many studies have shown that C:N and C:P ratios are inversely correlated with N and P contents (Chen et al., 2020b; Xu et al., 2021; Wang et al., 2022). The results of these studies are consistent with our findings ( Figure 6 ). Therefore, short-term hypersaline stress affects the N and P metabolism of macroalgae, changing their N and P contents and C:N:P ratios, and the resulting biogeochemical effects need to be further investigated.

It is believed that large-scale macroalgae cultivation is a low-cost, high-efficiency blue carbon strategy, which is why this method has attracted a lot of attention (Gao et al., 2021; Yang et al., 2021; Gao et al., 2022). China has abundant macroalgal resources (including wild seaweeds and artificially cultivated seaweeds) and 12.5 million square kilometers of coastal aquaculture area, laying a sufficient foundation for planning the cultivation of large seaweed (Xiao et al., 2017). Therefore, macroalgal cultivation will help China to achieve its carbon neutrality plan. However, the major uncertainty of this method is the difficulty of quantifying the role of macroalgae in the coastal carbon cycle (Paine et al., 2021; Hurd et al., 2022). The present study suggests hyposaline stress needs to be considered when quantifying DOC release from macroalgae. Besides, a recent study found that macroalgal cultivation shaped the microbial community structure by increasing dissolved oxygen, decreasing inorganic nutrients, and releasing of DOC (Xu et al., 2022). Thus, hyposaline stress may indirectly influence the microbial community structure of the tidal zone. Further studies are needed to comprehensively explore the ecological and biogeochemical influence of hyposaline stress on macroalgae.

In this study, we found that the DOC release rates of five common macroalgal species in the offshore waters of Fujian Province, namely P. haitaneisis, G. lemaneiformis, S. thunbergii, E. prolifera, and U. lactuca, all increased with decreasing salinity, while photosynthetic rates decreased with decreasing salinity. Both responses could seriously affect the accumulation of organic carbon in tissues of macroalgae. There were significant interspecific differences in the effects of low salinity stress on the carbon, nitrogen, and phosphorus contents and their ratios in the five macroalgal species, which may alter the competitive advantage and ecological functions of macroalgae, thus affecting the offshore carbon, nitrogen, and phosphorus cycles. This study showed that hyposaline stress substantially altered the carbon metabolism of macroalgae, especially increasing the DOC release rate from macroalgae, and therefore the effects of salinity stress should be considered in assessing the carbon sink potential of macroalgae.

The original contributions presented in the study are included in the article/ Supplementary Material . Further inquiries can be directed to the corresponding authors.

JC and KX conducted the data analysis and wrote the first draft of the manuscript. KX, DJ, and YX participated in data collection and sample processing. WW, CX, and CC contributed to the design and interpretation of results. All authors contributed to writing, revising, and approving the submitted version of the manuscript.