The influence of mangrove saplings (Avicennia marina) and fiddler crabs (Uca vocans) on carbon, iron, and sulfur biogeochernistry 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.
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