Biogeochemical drivers of phosphatase activity in salt marsh sediments
Introduction
Salt marshes are highly productive areas located in the interface between freshwater and marine systems, often subjected to high nutrient loadings due to anthropogenic activity that can lead to systems eutrophication (Tobias et al., 2001). Salt marshes have been regarded as intertidal wastelands, but currently their important role in coastal defense, as a biodiversity pool, as nursing, shelter and feeding areas for several animals and as a source of organic materials and nutrients for marine communities is widely recognized (Boorman, 1999).
Another important function of salt marshes is their role as a sink of pollutants like heavy metals or excess nutrients. Conversely, salt marshes could also become a source of pollutants due to exportation of dead biomass, although the amount of contaminants exported is lower than the amount of contaminants retained (Caçador et al., 2009, Duarte et al., 2008).
Excess nutrients such as nitrogen (N) and phosphorus (P) can have damaging effects, leading to eutrophication of coastal waters (Andrieux-Loyer et al., 2008; Howarth et al., 2011). Although nitrogen is considered to be a major concern when dealing with eutrophication, phosphorus is also considered to be one of the key limiting nutrients to primary productivity and therefore it is one of the nutrients responsible for eutrophication (Correl, 1998, Paerl, 2009). Phosphorous is delivered to aquatic systems in several forms such as longer chain poly-phosphates, pyrophosphates, organic phosphate esters and phosphodiesters, and organic phosphonates. This element may also be delivered both in the dissolved and particulated form being deposited in the salt marsh sediments (Correl, 1998). However, P is only biologically available when it is in the inorganic form, as orthophosphate.
Sediment microbial communities are an essential component in trophic dynamics and biogeochemical processes in coastal ecosystems. The microbial community synthesizes extracellular enzymes that are responsible for the decomposition processes. Phosphatases are extracellular hydrolases that catalyze the mineralization of organic phosphorus into inorganic and more easily metabolized forms of phosphorus. One large and important group of these enzymes are the phosphomonoesterases and their pH-isoforms: alkaline, acid and neutral phosphatases, being therefore active in several kinds of sediments (Alef et al., 1998).
Although phosphatases are not exclusively produced by the microflora (being also produced as exudates by plant roots), microbial growth is favored by salt marsh plants that release oxygen from the root system into the sediments promoting reactions with reduced species (Caetano et al., 2011, Lillebø et al., 2006). In fact, Spartina maritima marshes have been shown to have an important role on phosphorus bioavailability decreasing total P, probably due to uptake for growth purposes (Lillebø et al., 2007). Also, due to the release of oxygen by plant root systems into the sediment, P efflux is reduced (Lillebø et al., 2007). Plant roots can also affect microbial communities due to root exudates input to the rhizosphere (Hartman et al., 2009). Plant type has also been shown to alter microbial community structure and function affecting microbial activity (Garbeva et al., 2008).
Due to the elevation gradient from the upland marsh to the lower mudflats, a physic-chemical gradient is generated allowing the colonization by different halophytic species (Halimione portulacoides, Sarcocornia fruticosa, Sarcocornia perennis and S. maritima) providing differential cycling areas along the marsh. This way, in the present work the authors aim to clarify the influence of the biogeochemical marsh environment on the activity extracellular phosphatases and thus how this modulates the phosphorous recycling in marsh sediments.
Section snippets
Site description and sampling
Rosário (38°40′N, 9°01′W) is a mature salt marsh located in the southern part of the Tagus estuary, in the vicinity of various urbanized and industrialized zones (Valiela et al., 2000). The upper marsh is mainly colonized by H. portulacoides (Amaranthaceae) and S. fruticosa (Amaranthaceae) and undergoes short submersion episodes during high tide. The middle marsh is colonized mainly by S. perennis (Amaranthaceae), which can also be found although in smaller extents in the lower marsh where S.
Sediment physicochemical characteristics
Bare sediments presented the lowest water, humic acid and organic matter contents, while S. perennis, H. portulacoides and S. fruticosa rhizosediments showed respectively the highest value of water, humic acids and organic matter contents. Concerning these parameters, bare sediments differed significantly (p < 0.01) from plant colonized sediments (Table 1). Regarding the rhizosediment pH it varied between 6 and 7 while in the bare sediments was higher (8.18). Again, bare sediments differed
Discussion
Enzymatic activity seems to be modulated by sediment physicochemical features such as pH and humic acid concentration, leading to changes in phosphorus biogeochemical cycling. In fact, the main contributor to total phosphatase activity in the sediments was acid phosphatase activity due to slightly acidic pH in the plant-colonized sediments. As for the bare sediments, which had higher pH, there was a smaller difference between acid and alkaline phosphatase activities. Also, in sediments
Conclusions
Halophytic vegetation is known to act as ecosystem engineer's changing the sedimentary environment mostly due to physic-chemical changes of the sediments. This physic-chemical sedimentary environment appears as a modulating factor constraining phosphatase activity in sediments and thus has an evident role in phosphorous. Phosphatase activity is higher in vegetated sediments than in bare sediments. Humic acid content and pH seem to be the most important factors influencing phosphatase activity.
Acknowledgments
The authors would like to thank to the “Fundação para a Ciência e Tecnologia (FCT)” for funding the research in the Centre of Oceanography throughout the project PEst-OE/MAR/UI0199/2011 and this specific work throughout the projects ECOSAM (PTDC/AAC-CLI/104085/2008).
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