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Title Benthic Microbial Food Webs: Spatial and Temporal Variations and the Role of Heterotrophic Protists in Salt Marsh Sediments
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In order to determine if the loss of bacterial biomass varies over time, I investigated the microbial food web structure at two time scales in the salt marsh sediments of Sapelo Island, Georgia. Samples were collected monthly for one year at three contrasting subtidal locations: a high energy sandy beach, a muddy Spartina marsh, and a tidal creek bed. Concentrations of benthic microalgae (BMA), bacteria, heterotrophic protists, and metazoan meiofauna were measured at each location. Additionally, short-term dynamics of sediment microbial populations and bacterivory rates were investigated over a diel period in an intertidal creek bed to determine if variable rates of protist grazing could significantly impact bacterial standing stock. Although bacterivory rates were variable throughout the day, there were no periods of the day when protists could effectively reduce bacterial biomass. Yearly sampling revealed high variation in the microbial food web structure, mostly among sample locations. However, I observed a shift from a BMA-dominated community in the spring/ early summer months to a bacterial-dominated food web in the late summer/fall at all locations. Bacteria and heterotrophic protist concentrations were significantly related to porewater volume (6.9±1.2 x 10 and 5.1±1.1 x 10 -1 cells ml, respectively, ±SE). The low abundance of protists (relative to high bacterial concentrations) may be due to top-down pressure by large ciliates and nematodes. This top-down pressure in the autumn months (when BMA are proportionally less abundant) may contribute to high concentrations of bacteria during this season. Microbenthos displayed a clumped distribution pattern in Fluorescently Labeled Embedded Cores (FLEC). The aggregation of microbenthos indicates that the importance of protists in microenvironments may be overlooked by their total concentrations in the sediments. The common benthic ciliate, Uronema marinum, was capable of ingestion of fluorescently-labeled dextran (a high molecular weight carbohydrate) at low concentrations (3 microM DOC). However, the ingestion of starch, glucose and acetate did not significantly improve the growth or biomass production for this ciliate. Instead, I hypothesize that the initial breakdown of recalcitrant organic carbon compounds (and egestion of fermentation byproducts) can stimulate sulfate reducing bacteria and indirectly benefit bacterivorous protists through increased bacterial production.

Contributor Matthew R. First

First, M.R. 2008. Benthic Microbial Food Webs: Spatial and Temporal Variations and the Role of Heterotrophic Protists in Salt Marsh Sediments. Ph.D. Dissertation. University of Georgia, Athens, Georgia.

Key Words microbial ecology, protists, salt marsh, sediments, Student Publication, UGAMI Publication
File Date 2008
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This material is based upon work supported by the National Science Foundation under grants OCE-9982133, OCE-0620959, OCE-1237140 and OCE-1832178. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.