Microbial Influence on Carbon Storage and Trace Element Speciation in Restored and Natural Mangrove Sediments: A Synthesis of the Current Understanding

Mangrove microorganisms play a fundamental role in regulating sediment biogeochemical processes, particularly carbon storage and trace element cycling. Although numerous studies have examined microbial roles in individual processes, an integrated understanding of how microbial communities simultaneously regulate carbon storage and trace element dynamics in natural and restored mangrove ecosystems remains limited. This review synthesizes the global status of mangrove microbial research and the influence of microbes on carbon storage, trace element accumulation, and speciation in natural and restored mangrove ecosystems, while identifying emerging research trends and knowledge gaps. Our investigation revealed that research on the influence of microbes on carbon storage in mangrove sediments is increasing globally, with considerably increasing trends after 2017. However, less research has been reported on microbial trace metal interrelations than on carbon storage relationships, suggesting that there is limited focus from researchers on this topic. The available evidence indicates that sulfate reduction, microbial extracellular polymeric substances (EPS), microbial necromass formation, redox-driven biogeochemical coupling, and microbially mediated mineral transformations are the principal mechanisms promoting long-term carbon stabilization in mangrove sediments. Therefore, several studies have also suggested that microbial diversity regulates trace element accumulation and speciation through different pathways, such as redox transformation, bioadsorption, EPS-mediated binding or aggregation, biomineralization, and sulfide precipitation, in mangrove sediment. The principal conceptual contribution of this review is the development of an integrated framework demonstrating that microbial processes act as a central biogeochemical bridge connecting carbon storage and trace element cycling, rather than regulating these functions independently. Finally, we identify critical challenges and research priorities, including functional gene characterization, integrated metal–microbe–plant interactions, multi-omics approaches, long-term monitoring, and global meta-analyses, to improve mechanistic understanding and support evidence-based mangrove restoration and blue carbon management.