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Updated: May 18, 2026

Visualizing Methane-Cycling Microbial Dynamics in Coastal Wetlands
Published on: January 31, 2025
Microbially driven iron redox cycling dominates Fe-bound organic carbon formation in coastal wetlands under sea level
Zhuang Huang1, Shaoying Lin2, Chun Wang3
1Key Laboratory of Humid Subtropical Eco-geographical Process, Ministry of Education, Fujian Normal University, Fuzhou, 350117, China; Institute of Geography, Fujian Normal University, Fuzhou 350117, China; Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, No.4888, Shengbei Street, Changchun 130102, China.
Abstract:
Global sea level is projected to rise 0.28-1.02 m by 2100, exacerbating flooding and saltwater intrusion in estuarine wetlands. This threatens to alter soil iron (Fe) and organic carbon (C) pools, potentially affecting Fe-bound organic C (Fe-OC) formation. Given that Fe-OC represents a critically stable carbon reservoir-often referred to as the "rusty carbon sink"-its preservation is essential for the long-term carbon sequestration function of estuarine wetlands. Therefore, understanding how sea level rise influences Fe-OC dynamics is crucial for predicting future changes in coastal wetland carbon sink capacity. However, how enhanced flooding and salinity regulate Fe-OC via Fe/C pools and microbial processes remains unclear. We conducted a one-year in-situ experiment in a Phragmites australis wetland (Min River Estuary) with control (CK, natural tidal flooding) and two treatments receiving additional tidal water (TW) and saline water (SW) besides natural tidal flooding. Results showed that: (1) TW and SW increased soil free Fe oxide (Fed) by 70.7 ± 5.6% and 94.8 ± 8.3% (p < 0.05), while soil organic C remained unchanged (p > 0.05), indicating that the soil Fe pool is more responsive to sea level rise (enhanced flooding and salinity) than the soil organic C pool. (2) Soil Fe-OC content increased by 10% (TW) and 20% (SW), with its contribution to organic C rising by 5.3% and 9.2%. Concurrently, soil CO2 emission fluxes decreased significantly under both treatments and correlated negatively with Fe-OC content. Aggregated boosted tree (ABT) analysis confirmed Fed as the key driver of Fe-OC formation. (3) The diversity and abundance of the soil C-fixing microbial community (cbbL) remained unchanged (p > 0.05), but Fe-oxidizing bacteria (FeOB) and Fe-reducing bacteria (FeRB) increased with flooding-salinity (p < 0.05), especially FeRB. This microbial shift did not immediately alter soil Fe(II) or Fe(III), implying compensatory processes (e.g., root oxygen release). Mantel analysis revealed stronger FeOB/FeRB-Fe pool correlations than cbbL-C pool links. (4) Path analysis revealed that sea-level rise regulates FeOB/FeRB community structure to drive Fe redox cycling, transforming Fe minerals and dominating Fe-OC formation. This study reveals a "mineral-microorganism" synergistic mechanism underlying Fe-OC sequestration under sea-level rise: the coupled activities of FeRB (reductive dissolution) and FeOB (oxidative precipitation) drive Fe redox cycling, endowing estuarine wetland C pools with resilience to environmental stress. These findings offer new insights for coastal blue carbon management, suggesting that regulating Fe cycles could optimize Fe-OC stability and improve coastal C sink adaptability.
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