<p><strong>Abstract.</strong> Microbial nitrogen processing in direct association with marine animals and seaweeds is poorly understood. Macrobiota supply a substrate for microbes to reside, and a source of excreted nitrogen and dissolved organic carbon (DOC). We tested the role of a mussel (<i>Mytilus californianus</i>), a red alga (<i>Prionitis sternbergii</i>) and an inert substrate for microbial activity using enclosed chambers and enriched ammonium and nitrate. Chambers with seawater from

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... he same environment served as a control. We found that mussels and <i>Prionitis</i> elevated ammonium oxidation and nitrate reduction two orders of magnitude over that of seawater, while the effect of simply an inert substrate had relatively little effect. Extrapolating to a square meter of shoreline, microbial activity associated with mussels could oxidize 2.5<span class="thinspace"></span>mmol of ammonium and reduce per 1.2<span class="thinspace"></span>mmol of nitrate per day. A square meter of seaweed could produce even higher rates, at 135.2 and 320.5<span class="thinspace"></span>mmol per day for nitrification and nitrate reduction, respectively. Seawater collected from the shore versus 2&amp;ndash;5<span class="thinspace"></span>km offshore showed no difference in ammonium oxidation or nitrate reduction. Microbial nitrogen metabolism associated with mussels did not change whether we measured it at night or during the day. When we experimentally added DOC (glucose) as a carbon source, there was no change to nitrification rates. Added DOC did increase DIN and phosphorus uptake, indicating that elevating the concentration of DOC stimulated heterotrophic microbial activity, and suggests potential competition for DIN between heterotrophic and chemolithotrophic microbes and their seaweed hosts. Our results indicate that microbes in direct association with coastal animals and seaweeds greatly enhance nitrogen processing, and likely provide a template for a diversity of ecological interactions.</p>