12 Plant–Animal–Microbe Interactions in Coastal Sediments: Closing the Ecological Loop Roberta L. Marinelli and George G. Waldbusser Abstract Plant–animal–microbe associations are common in coastal marine sedimentary environ- ments. A review of the literature suggests that many of these interactions involve the creation of structures and subsequent alteration of the physical and biogeochemical environment, and represent examples of ecosystem engineering. We consider three common nearshore plant systems (salt marshes, mangroves, and seagrasses) for which plant–animal interactions have been documented and evaluate them in the context of their structural modifications, the attendant biogeochemical significance, and the extent to which positive and negative interactions dominate. We argue that the strength of observed interactions appears to vary substantially both within and among systems, due to spatial and temporal variance in physical and biotic processes and nonadditive relationships between macroorganisms and microbially mediated biogeochemistry. These conclusions, however, are tentative, as the role of microbial population dynamics and metabolic processes remains poorly under- stood. Several studies suggest that a fuller understanding of microbiological processes may provide the mechanistic underpinnings of plant–animal–microbial consortia, and how they impact productivity and system function on local and broad scales. We suggest that strong linkages between community composition and ecosystem function are attained in highly developed plant–animal–microbe associations and suggest an alternative framework for evaluating the strength and ecosystem-level effects of such consortia. Tests of these interactions, and their relationship to biodiversity and landscape-scale processes, should provide fertile ground for future research. Introduction Plant–animal–microbe associations are ubiquitous in highly productive coastal marine sedimentary environments. Generally, these associations are identified by the plant compo- nent, e.g., seagrass meadows, salt marsh habitats, and mangroves, often because the plant is the more conspicuous member of the community and provides a significant structural entity. Inconspicuous autotrophs such as the microphytobenthos are also highly productive [Pinckney and Zingmark, 1993] but their structural influence, e.g., promotion of binding and stability, is often less obvious. The microphytobenthos occupy vast areas of the coastal 1 Macro- and Microorganisms in Marine Sediments Coastal and Estuarine Studies 60 Copyright 2005 by the American Geophysical Union 10.1029/60CE13 9743.qxd 5/23/05 9:34 PM Page 1