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Like many deposit-feeding organisms, the burrowing brittlestar Amphipholis gracillima feeds on particulate organic matter in surface sediments. Microbial exopolymeric secretions (EPS) are carbohydrate-enriched polymers produced by microalgae and bacteria that bind aggregates and form dense biofilms near the sediment-water interface. EPS are assimilable by some benthic infauna and may be utilized as a significant carbon source. EPS are absorbed by some deposit-feeders, including a holothurian, and may be supplemental sources of nutrition. The burrowing brittlestar A. gracillima is a deposit-feeder that was used in a mass balance approach to model the incorporation of radiolabeled EPS by bottom feeders. Brittlestars were fed 14C-labeled, laboratory cultured EPS from the marine bacterium Pseudoalteromonas atlantica and a benthic diatom (Nitzschia sp.) via sediment-bound and aqueous exposures. Comparison of absorption efficiencies (AE) showed that both polymer types are highly absorbed by A. gracillima (AE = 83 to 99%). Absorption of sediment-bound bacterial and algal EPS was similar (92.2 and 90.1%), but bacterial EPS absorption was significantly (p<0.05) higher in sediment-bound (92.2%) than aqueous (83.3%) exposures. Algal EPS absorption was significantly higher in aqueous (99.9%) exposures. These findings suggest that EPS may represent a significant energy source for this deposit-feeding ophiuroid and other organisms with similar feeding habits. Additionally, A. gracillima appears to be especially adept at utilizing EPS resources from benthic diatom communities.

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