Elevated suspended particulate matter reorganises carbon fraction in mangrove ecosystems

Mangrove wetlands are a core component of blue carbon ecosystems worldwide. Elevated suspended particulate matter (SPM) in natural waters is a ubiquitous environmental stressor, yet its effects on the carbon cycling of mangroves remain poorly unelucidated. Here, we conducted a 90-day microcosm experiment using Kandelia obovata and integrated high-resolution chemical and biological profiling to systematically elucidate how SPM loading affects mangrove carbon-cycling processes. As SPM increased from 0 to 30 mg/L, the photosynthetic activity of phytoplankton declined, accompanied by suppressed Calvin-Benson-Bassham-cycle signatures and a relative enrichment of glycolysis- and TCA-related signatures, resulting in a 0.71 mg/L increase in dissolved inorganic carbon (DIC) and a 1.12 mg/L decrease in dissolved organic carbon (DOC) on day 90. Concurrently, suppressed sediment β-glucosidase activity contributed to a 1.76 mg/g increase in sediment organic carbon (SOC). As SPM further increased from 30 to 70 mg/L, strengthened autotrophic carbon fixation and attenuated heterotrophic carbon oxidation in the water column contributed to a 2.45 mg/L decrease in DIC and a 0.30 mg/L increase in DOC. Excessive SPM load compromised the antioxidant defence system of mangroves, leading to oxidative damage in mangrove plants, thereby reducing photosynthetic activity, while sediment β-glucosidase activity increased, collectively resulted in a 1.20 mg/g decrease in SOC. Our results indicate that elevated SPM reorganises carbon composition within mangrove wetlands, with potential implications for global-scale carbon cycling, underscoring the importance of incorporating SPM as a key agent forcing in the management of blue carbon ecosystems.