Macrophytes and microbes: Spartina alterniflora and Phragmites australis affect brackish sediment microbial community structure and function

Vegetation zonation in estuarine systems is controlled by the system hydrology. Anthropogenic alteration of estuarine hydrology has resulted in altered vegetation patterns. An example of this alteration in northeastern U.S. marshes is the spread of Phragmites australis, and regional salt marsh restoration projects commonly replace Phragmites with Spartina species.; The presence of halogenated organic contaminants in estuarine sediments, where their fate is determined in large measure by the estuarine microbial community, is a worldwide problem. The effect of specific plant species on sediment microbial community structure or functional abilities is unknown. I address the question of whether sediment microbiota in unvegetated sediments and sediments associated with Spartina alterniflora and Phragmites australis differ with respect to structure and functional capabilities.; Samples from the anthropogenically disturbed Hackensack River estuary (SMC) and the undisturbed Mullica (MUL) and Maurice (MAUR) River estuaries were compared. To study the effects of vegetation, while controlling environmental variables of salinity, nutrient inputs and hydroperiod, greenhouse tidal mesocosms were established. Sediment characteristics of bulk density, moisture content and soil organic matter were similar between sites. Spartina fine root biomass was significantly greater than that of Phragmites at all sites and belowground biomass of both species was significantly greater in the SMC site than in the undisturbed field sites.; Microbial community composition was determined by phospholipid fatty acid (PLFA) analysis. Microbial community function was determined by analysis of extracellular enzyme activity, substrate dependent respiration and debromination of tetrabromobisphenol A. Microbial PLFAs correlated with sediment ability to mineralize a range of organic substrates. Spartina sediments were found to debrominate at a faster rate than Phragmites or unvegetated sediments. Extracellular enzyme activity was greater in the undisturbed sediments than in comparably vegetated SMC sediments. Increased nutrient loads corresponded with increased plant biomass, and increased PLFA diversity in Phragmites and unvegetated sediments.; This study demonstrates that under estuarine conditions of anaerobiosis and brackish salinities the effect of plant roots interacts with hydrological influences in determining sediment microbial community structure and functional abilities. The interaction between belowground plant biomass and sediment microbiota needs to be considered when restoring contaminated estuarine sediments.