Relationships between Sediment Microbial Communities and Pollutants in Two California Salt Marshes

Abstract

Salt marshes are important ecosystems whose plant and microbial communities can alter terrestrially derived pollutants prior to coastal water discharge. However, knowledge regarding relationships between anthropogenic pollutant levels and salt marsh microbial communities is limited, and salt marshes on the West Coast of the United States are rarely examined. In this study, we investigated the relationships between microbial community composition and 24 pollutants (20 metals and 4 organics) in two California salt marshes. Multivariate ordination techniques were used to assess how bacterial community composition, as determined by terminal restriction fragment length polymorphism and phospholipid fatty acid analyses, was related to pollution. Sea urchin embryo toxicity measurements and plant tissue metabolite profiles were considered two other biometrics of pollution. Spatial effects were strongly manifested across marshes and across channel elevations within marshes. Utilizing partial canonical correspondence analysis, an ordination technique new to microbial ecology, we found that several metals were strongly associated with microbial community composition after accounting for spatial effects. The major patterns in plant metabolite profiles were consistent with patterns across microbial community profiles, but sea urchin embryo assays, which are commonly used to evaluate ecological toxicity, had no identifiable relationships with pollution. Whereas salt marshes are generally dynamic and complex habitats, microbial communities in these marshes appear to be relatively sensitive indicators of toxic pollutants.

Microbial Community Composition and Denitrifying Enzyme Activities in Salt Marsh Sediments

“…In this study, elevation appeared to account for the extensive variations in microbial community composition. This was similar to what was found in one of our previous studies (12) but different from what was found in another study regarding TRFLP profiles across several marshes (18). The contrast occurs because prevailing explanatory variables will change depending on the spatial and temporal context of a study.…”

“…Sodium concentrations (Table 1) in sediments, after adjustment for that in ASW, were used as a proxy for in situ salinity. Concentrations of metals varied widely (Table 1), without respect to either elevation or station, and were comparable to those reported for a previous year for this marsh (12). Thus, metal concentrations at individual stations and elevations are not distinguished in Table 1.…”

“…Salt marshes differ fundamentally from soils and marine sediments in that marsh sediments are tidally influenced with associated redox potential, salinity, and inundation fluctuations (50). Studies of total bacterial communities in salt marsh sediments indicate that while they are highly spatially variable, they are significantly influenced by geographic location, elevation, and selected pollutants, such as metals (12,18). Yet, rhizosphere diazotroph communities were shown to be relatively stable and persistent (46) in the face of short-term carbon and nutrient variability (61,62).…”

“…How denitrifier communities and their functions may vary with environmental and pollutant factors in salt marshes remains largely unknown. Therefore, the objective of this study was to examine, for a salt marsh system with high nitrate loading (57), the spatial gradients of microbial community composition and denitrification enzyme activities (DEAs), the relationships of community composition and function to environmental factors as well as to each other, and the potential effects of metals considered here to be sedimentary pollutants that could affect denitrifiers and denitrification (12,18).…”

“…A third technique, partial CCA (pCCA), was used to investigate the effects of metals on microbial communities after adjustment for natural variability due to elevation and carbon content. Briefly, variables representing elevation and carbon content were designated covariables, and the residual variabilities in the TRFLP profiles were correlated with metal concentrations, as previously described (12).…”