The Rhizosphere Zone: A Hot Spot of Microbial Activity and Methylmercury Production in Saltmarsh Sediments of San Francisco Bay, California
Abstract
Tidal marshes of varying hydrology and salinity have been shown to have high rates of microbial methylmercury (MeHg) production, especially the periodically flooded, higher elevations which are densely vegetated with shallowly rooted plants. The specific influence of emergent wetland plants and their active rhizosphere (root zone) on mercury (Hg) biogeochemistry, however, is poorly understood. Seasonal and spatial patterns of Hg biogeochemistry were examined in 2005 and 2006 at three marshes along a salinity gradient of the Petaluma River, in Northern San Francisco Bay, California. In addition, to directly examine the influence of rhizosphere activity on MeHg production, a suite of devegetation experiments was conducted in 2006 within each marsh using paired vegetated and devegetated plots in two marsh subhabitats: poorly- drained interior sites and well-drained "edge" sites near slough channels. Surface sediment (0-2cm) was sampled in both April and August from these plots, as well as from 1st and 3rd order slough channels that were naturally free of vegetation. Vegetated marsh sites produced 3- to19-fold more MeHg than did slough sites, and MeHg production rates were greater in marsh interior sites compared to more oxic marsh "edge" sites. Microbial biomass (ng DNA gdrysed) was greater in vegetated marsh settings, compared to slough channels, and increased significantly between April and August at all marsh sites. Despite this seasonal increase in microbial biomass, MeHg concentrations and production rates decreased from April to August in vegetated surface sediments. Microbial indicators of methylation also decreased from April to August, including rates of microbial sulfate reduction and the abundance of iron- and sulfate- reducing bacterial DNA. Results from the devegetated plots suggest that root exudation of fermentative labile carbon to surface soils is responsible for the higher microbial biomass, and the higher relative abundance of iron- and sulfate-reducing bacterial DNA in vegetated sites. In contrast to microbial indicators of mercury methylation, no effects of devegetation or seasonal sampling were observed on sediment pools of reactive Hg (Hg(II)R), although the relative abundance of mercuric reductase functional genes (MerA) was reduced by devegetation and was correlated with live root density. These experiments, among others, demonstrate that the presence and activity of emergent wetland plants directly influence Hg cycling in densely rooted surface soils. Development of this shallow and dense rhizosphere is likely to be a primary reason as to why periodically flooded, high elevation marsh sites are a "hot spot" for mercury methylation.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.B11G..08W
- Keywords:
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- 0442 Estuarine and nearshore processes (4235);
- 0465 Microbiology: ecology;
- physiology and genomics (4840);
- 0476 Plant ecology (1851);
- 0489 Trace element cycling (4875);
- 0497 Wetlands (1890)