Microbial Food Webs in Marine Sediments: Spatial Motives Link Bacterivory and Biogeochemistry

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*** 9618135 Epstein This project addresses trophic relations between sediment micrograzers and bacteria of particular biogeochemical importance. It is suggested that proto- and micrometazoan grazers balance production of sediment chemo- and photolithotrophs and thus become important players in the dynamics of reduced and oxidized forms of sulfur, nitrogen, and carbon, and possibly Mn and Fe. This idea is supported by the results of previously work, pilot data, and literature information. All these data suggest the following ideas. 1. Major sediment microbacterivores (flagellates, ciliates, nematodes, and others) are concentrated in a specific sediment layer, at and immediately below oxic/anoxic interface. 2. It is exactly in this sediment layer that most sulfide, ammonium, and methane oxidizing chemolithotrophs, as well as anaerobic photosynthesizers, are known to concentrate. 3. In this particular layer, which exhibits increased grazing pressure over bacteria, micrograzers appear to remove daily up to 50% of bacterial standing stock. Based on the above evidence, the following hypothesis is advanced. Proto- and micrometazoan grazing controls bacterial dynamics in a specific sediment layer at and immediately below the oxic/anoxic interface. The hypothesis will be tested by comparing the rates of bacterivory by sediment flagellates, ciliates, nematodes, and other micrograzers, with bacterial standing stock and bacterial production in the identified sediment layer. This will be done by a combination of classical, modern, and novel approaches. Seasonality will be examined by conducting the experiments in two different seasons, which were previously shown to contrast with each other in the microbial food web structure. At each seasonal time point, experiments will account for daily dynamics in grazing activity and vertical migrations of prospective bacterivores. Many key techniques to b e used here have been substantially improved, and supporting evidence collected, under prior NSF support. The model community (fine intertidal sands, Boston Harbor) has been under study for several years and is uniquely suited for the planned research. This work is one of the first to introduce spatial motives into studies of sediment microbial food webs. The target of this research are those functional types of bacteria that are particularly important in sediment biogeochemistry, nutrient recycling, and sediment-water exchange. The significance of studying mechanisms governing these bacteria dynamics goes therefore beyond its intrinsic value for detailing benthic microbial food web structure, and is important in a much larger ecological context. ***