Seagrasses are important foundation species in coastal ecosystems, and they provide food and habitat that supports high biodiversity. However, seagrasses are increasingly subjected to anthropogenic disturbances such as metal pollution, which has been implicated as a significant factor driving seagrass losses. There have been several reviews synthesizing the metal concentrations in seagrasses and evaluating their utility as biomonitors for metal pollution in the coastal environment at the local scale. However, the interpretation of metal data in seagrass biomonitors requires a more mechanistic understanding of the processes governing metal bioaccumulation and detoxification. In this review, the progress and trends in metal studies in seagrasses between 1973 and 2022 were analyzed to identify frontier topics in this field. In addition, we tried to (1) analyze and assess the current status of metal contamination in seagrasses on a global scale by incorporating more metal data from tropical and Indo-Pacific seagrasses, (2) summarize the geochemical and biological factors governing metal uptake and loss in seagrasses, and (3) provide an up-to-date understanding of metals' effects on seagrasses and their physiological responses to metal challenges. This review improves our understanding of the highly variable metal concentrations observed in the field.

Seagrass beds are considered to be substantial sinks of “blue carbon”. However, differentiation in the carbon sink capacities of seagrass beds in different regions with distinct nutrient conditions remains unclear. In this study, sediment carbon stocks, seagrass biomass, and microbial community structures and potential functions of seagrass beds in eutrophic seawater adjacent to Hainan Island and oligotrophic seawater around the Xisha Islands were compared. Our results showed that sediment mineralizable organic carbon and dry bulk density were substantially higher on Hainan Island than on the Xisha Islands (t-test, p < 0.05), while sediment carbon stocks and the total organic carbon were comparable between the two regions (p > 0.05). Similarly, seagrass biomass was much higher on Hainan Island (p < 0.05). Sediment carbon stocks positively correlated with sediment nitrogen and negatively correlated to sediment pH and grain size (p < 0.05). Bacterial diversities were similar in the two regions, while fungi were more diverse on Hainan Island (p < 0.05). Proteobacteria, Desulfobacterota, Ascomycota and Basidiomycota could account for degrading organic carbon on Hainan Island. Proteobacteria and Bacteroidota may contribute primarily to carbon loss in the seagrass beds of the Xisha Islands. This study strengthens our understanding of the effects of human activities on carbon sequestration in seagrass bed ecosystems.

Coral reefs are essential for marine ecology and biodiversity. Global climate change has resulted in severe coral reef degradation, partly via coral bleaching, which is caused by rising sea temperatures and solar light intensity. In this study, we examined the impact of strong light (300 µmol.m^-2.s^-1) and high temperature (33°C) on the growth, immunity, and gene expression of Galaxea fascicularis. Strong light caused coral bleaching in the absence of high sea temperatures, while no obvious bleaching was observed under high temperature alone. The effect of strong light on calcification rate of G. fascicularis is significantly weaker than that of high temperature. Both strong light and high temperatures significantly affected the immune enzyme activity of G. fascicularis symbionts, with the former having a strong effect on their photosystem. Temperature affected the digestive system, replication and repair, and cell growth and death of coral hosts, as indicated by transcriptomics analysis. These results provide a valuable for strategies to mitigate coral bleaching. TEASER: We explored the effects of strong light exposure and high temperature on coral reefs and their symbiont algae.

Introduction: Seagrasses play a key role in biodiversity maintenance, sediment stabilization, and carbon reduction. Due to the significant decline of seagrass populations worldwide, many projects to restore seagrass by transplantation have been undertaken in recent decades. However, effective restoration technologies required to rapidly restore seagrass on Hainan Island are yet to be developed.Methods: In this study, an experiment was carried out to investigate methods for alleviating continuous degradation of seagrass beds on Hainan Island. Seagrass were transplanted by “single-plant-fixed-distance transplantation” method, survival rate, shoot formation rate, coverage and shoot density of transplanting plants were monitored for 23 months.Results: Over 67% of T. hemprichii and 90% of E. acoroides were still alive, shoot density were over 11 shoots/m² and 15 shoots/m² respectively after almost two years, E. acoroides was more likely to survive than T. hemprichii both in Gaolong Bay and Xincun lagoon. Because of the damage to below-ground tissues, the survival rate, shoot formation rate, coverage and shoot density decreased during the first one year after transplantation, then increased gradually.Discussion: We considered transplantation established successfully of T. hemprichii and E. acoroides in the new site was about 12 months. In addition, T. hemprichii was suitable for transplanted in Xincun lagoon, and E. acoroides was suitable for transplanted in Gaolong bay. The results demonstrated that the growth of transplanted seagrass was greatly affected by suspend solids and DIN. The results of this study may provide data support and scientific basis for seagrass transplantation in South China.