Sediment phosphorus speciation changes by extracellular enzymatic activity (EEA) of three phosphatase pH-dependent isoforms

Abstract

Salt marshes are key ecosystems for biogeochemical cycling of nutrients in coastal areas, providing essential sequestration of contaminants and elemental organic forms while recycling these last to more bioavailable forms. Extracellular phosphatase-mediated phosphorus biogeochemical cycling acquires this way a reinforced role. In terms of speciation, salt marsh sediments exhibited a large pool of P in a bioavailable form (> 60%), being this pool characterized by being readily accessible to plants by weak chemical reactions. Regarding phosphatase activity, the acid isoform was found to be the most abundant pH-isoform present in salt marsh rhizosediments. A positive correlation could be found between humic acid bound P and total and acid phosphatase activities, possibly driven by the action of acid phosphatase on the acidic molecules of humic acids, using organic P as substrate, and releasing inorganic P into the desorbable P-pool. This process is more evident during the cold seasons, where the largest necromass input is observed entering the sediments, due to seasonal plant senescence. It could also be observed a positive correlation between inorganic phosphorus forms and phosphatase activity, consistent with phosphatases-driven inorganic phosphorus production from the recycling of organic forms, and thus a possible direct effect of phosphatase, as it was described for other ecosystems. This biogeochemical cycling is key for the ecosystem nutrient recycling, while passive remediator of eutrophication, but also for maintaining the salt marsh areas high primary productivity and their role as habitat, shelter and feeding ground of many animal species.