ORIGINAL ARTICLE Spartina maritima (cordgrass) rhizosediment extracellular enzymatic activity and its role in organic matter decomposition processes and metal speciation Bernardo Duarte 1 , Pedro R. Almeida 1,2 & Isabel Cac ¸ador 1 1 Institute of Oceanography, Faculty of Sciences, University of Lisbon, Campo Grande, Lisbon, Portugal 2 Department of Biology, University of E ´ vora, E ´ vora, Portugal Problem Salt marshes located in estuaries frequently receive large inputs of nutrients (Tobias et al. 2001), as well as particu- late and dissolved organic matter. This high nutrient input makes salt marshes one of the most productive eco- systems of the planet. This high biomass production has as a consequence large necromass generation due to litter senescence (Cac ¸ador et al. 2009). In highly industrialized estuaries, along with this nutrient input there is also a large input of heavy metals, which will accumulate in salt marsh sediments (Doyle & Otte 1997). These high inputs make salt marshes key zones for the biogeochemistry of the estuary, but also for metal cycling (Weis & Weis 2004). The microbial decomposer communities of salt marsh sediments play an essential role in these cycling processes by decomposing the organic matter, as well as other large complex molecules that reach to the sedi- ments, into more bioavailable forms (Ravit et al. 2003). As verified previously, salt marsh sediments are often very organic (Richert et al. 2000), providing large amounts of substrates for the proliferation of microbial decomposers. The generation of large amounts of below-ground necro- mass and the organic compounds exuded by living plants (Duarte et al. 2007) are the major contributors to the organic content of the rhizosediments. The large periods of submersion to which these sediments are subjected, lead to a low oxygenation of the sediments, with adverse effects on plant growth (Richert et al. 2000). Some salt marsh plants have the ability to pump oxygen from the atmosphere into the root zone, oxidizing the sediments and consequently promoting aerobic microbial activity (Ludemann et al. 2000). This kind of plant–microflora interaction is very variable, depending not only on the plant species but also on season (Wilczek et al. 2005; Yang et al. 2007). To process the organic matter, microbes produce extracellular enzymes which degrade these large complex molecules into smaller ones, which Keywords Extracellular enzymatic activity; metal speciation; salt marsh; sediment. Correspondence Bernardo Duarte, Institute of Oceanography, Faculty of Sciences, University of Lisbon, Campo Grande, 1749-016 Lisbon, Portugal. E-mail: baduarte@fc.ul.pt Conflicts of interest The authors declare no conflicts of interest. doi:10.1111/j.1439-0485.2009.00326.x Abstract Seasonal monitoring was carried out to investigate the influence of extracellular enzymatic activity (EEA) on metal speciation and organic matter cycling in the rhizosediment of Spartina maritima. Heavy metal speciation was achieved by the Tessier scheme, and showed a similar pattern of variation of the organic- bound fraction, indicating a decomposition process in progress. Both humic acid and organic matter showed the same seasonal pattern. The basal respira- tion of the rhizosediments also presented a similar seasonal pattern, indicating a microbial degradation of organic matter. The high organic-bound fraction found in the summer gradually decreased towards the winter. This decrease was found to be related to the increase of activity of peroxidase, b-N-acetylglu- cosaminidase and protease. Also the activity of sulphatase was found to be related to the depletion on the exchangeable fraction, probably due to sulphide formation and consequent mobilization. The results show an interaction between several microbial activities, affecting metal speciation. Marine Ecology. ISSN 0173-9565 Marine Ecology 30 (Suppl. 1) (2009) 65–73 ª 2009 No claim to original government work 65