As a burrowing deposit-feeder, the brittlestar Amphipholis gracillima feeds on particulate organic matter in surface sediments. These sediments are actively colonized by microbial communities and can be rich with their cellular biomass and exudates. Microbial exopolymer secretions (EPS) are carbohydrate-like polymers produced by microalgae and bacteria that form dense biofilms at the sediment/water interface. In addition to being a source of carbon, the adsorptive heteropolysaccharides that comprise EPS aggregate particulate matter. They are negatively charged, causing them to bind macromolecules and divalent cations. As such, sediment biofilms may also be vectors for sediment-bound pollutants.

EPS are assimilable by some benthic infauna and may be utilized as a significant carbon source. Recent research suggests that EPS are assimilated by some deposit-feeders, including a holothurian, and may be supplemental sources of nutrition. The burrowing brittlestar A. gracillima inhabits the upper 10-15 cm of sandy mud and was used in a mass balance approach to model the incorporation of radiolabeled EPS by a typical deposit-feeder.

Feeding and enzyme assays were conducted with laboratory-cultured microbial EPS to determine if the brittlestar was able to digest and assimilate representatives of this class of microbial exudates. Gut tissue from the brittlestar was homogenized and used in enzyme assays to determine whether EPS-degrading enzymes were present. Enzyme assays showed that A. gracillima possesses soluble and membrane-bound enzymes capable of hydrolyzing the polysaccharide laminarin (commonly produced by brown algae and diatom EPS) and alginate (also produced by brown algae). Hydrolases of bacterially produced dextran, polygalacturonic acid, pseudomonad EPS and amylose were weakly detected. Soluble enzymes provided the greatest amount of activity; membrane bound enzymes were less active.

Lab-cultured EPS from the marine bacterium Pseudoalteromonas atlantica and a benthic diatom (Nitzschia sp.) were labeled with 14C and fed to brittlestars in sediment-bound and dissolved form. Comparison of the absorption efficiencies show that both polymer types are highly assimilable by A. gracillima (AE=86-99%) and that polymer type and exposure both have a significant effect on how well EPS can be utilized by the brittlestar. Brittlestars fed 14C-labeled EPS from both microbial sources assimilated both EPS with high efficiencies (86-99%). Polymer type and exposure both had significant effects (p-value < 0.05) on how well EPS was absorbed by brittlestars.

These findings suggest that EPS may represent a previously unidentified energy source for this A. gracillima and other organisms with similar feeding habits. They also suggest that A. gracillima may utilize some microbial polymers as food and that it may be more adept at utilizing algal resources like diatom communities than previously believed.