Mangrove forests are highly productive but globally threatened coastal ecosystems, whose role in the carbon budget of the coastal zone has long been debated. Here we provide a comprehensive synthesis of the available data on carbon fluxes in mangrove ecosystems. A reassessment of global mangrove primary production from the literature results in a conservative estimate of ∼218 ± 72 Tg C a⁻¹. When using the best available estimates of various carbon sinks (organic carbon export, sediment burial, and mineralization), it appears that >50% of the carbon fixed by mangrove vegetation is unaccounted for. This unaccounted carbon sink is conservatively estimated at ∼112 ± 85 Tg C a⁻¹, equivalent in magnitude to ∼30–40% of the global riverine organic carbon input to the coastal zone. Our analysis suggests that mineralization is severely underestimated, and that the majority of carbon export from mangroves to adjacent waters occurs as dissolved inorganic carbon (DIC). CO₂ efflux from sediments and creek waters and tidal export of DIC appear to be the major sinks. These processes are quantitatively comparable in magnitude to the unaccounted carbon sink in current budgets, but are not yet adequately constrained with the limited published data available so far.

The influence of mangrove saplings (Avicennia marina) and fiddler crabs (Uca vocans) on carbon, iron, and sulfur biogeochemistry in mangrove sediment was studied using outdoor mesocosms with and without plants (21 m⁻²) and crabs (68 m⁻²). Saplings grew more leaves and pneumatophores in the presence of crabs. Dense microalgal mats lead to two to six times higher benthic production and about two times higher benthic respiration in the absence of crabs. Particle mixing by crabs increased the reactive oxidized iron (Fe(III)) in the upper 2 cm of the sediment, whereas oxygen leaching by roots maintained the deeper rhizosphere oxidized and enriched in Fe(III). The highest microbial activity, measured as carbon dioxide production and iron reduction, occurred within the upper 2 cm of ungrazed sediment and was fueled by the large near‐surface biomass of microalgae. Leaching of dissolved organic carbon (DOC) from roots stimulated bulk sulfate reduction and caused an upward cascading reduction of the sediment as indicated by low Fe(III) and high Fe(II) between 2‐cm and 6‐cm depth. The effect DOC was also evident as increased microbial abundance at all depths in the sediment. Fe(III) was the most important electron acceptor for microbial carbon oxidation in ungrazed sediment (63–70%), whereas sulfate reduction was more important in grazed sediment (36‐44%), particularly in the presence of plants. Aerobic respiration always accounted for <20%. Fiddler crabs and roots of A. marina have complementary effects on the biogeochemistry of mangrove sediment. Their association seems to be mutually beneficial with respect to growth and food availability.