Cellulase activity in the freshwater amphipod Gammarus lacustris

Deciduous leaves provide a major allochthonous source of energy to stream f ood webs, particularly in small, shaded headwater streams. To digest cellul ose and other structural carbohydrates in plants, invertebrates must use en zymes that degrade these polymers. Our research focused on the digestion of cellulose by the freshwater amphipod Gammarus lacustris. We used a feeding experiment and laboratory enzyme assays to investigate 3 potential sources of cellulase in G. lacustris: 1) tissue-level production of cellulase by G . lacustris, 2) cellulase activity by gut endosymbionts living in G. lacust ris, and 3) ingestion of fungal cellulase exoenzymes that remain active aft er ingestion by G. lacustris. We determined that G. lacustris possessed end oglucanase and beta-glucosidase enzymes that are active on soluble cellulos e derivatives. We did not detect cellobiohydrolase activity toward crystall ine cellulose in G. lacustris. Endoglucanase and beta-glucosidase activity appeared to originate from amphipod tissues; in this experiment microbial e nzymes did not contribute to cellulase activity. These results are consiste nt with other studies indicating that many invertebrates produce endoglucan ases and beta-glucosidases but very few, if any, produce cellobiohydrolase endogenously.