Digestion of cellulose by stomach homogenates of green abalone (Haliotis fulgens)

Three experiments were conducted to study the cellulase activity of stomach homogenates of green abalone, with and without antibiotics added. Forty-ei ght previously frozen stomachs from wild abalone adults were thawed and ind ividually homogenized. Four cultured abalone adults maintained under a bala nced diet containing cellulose were killed and the fresh stomach samples pr ocessed. Cellulase activity was estimated through the production of glucose , incubating the homogenate from each organism with cellulose. To determine the effect of bacteria, each stomach was incubated (at 25 degreesC for 20 h) in a completely randomized design, with and without an antibiotic mixtur e. For a time series measurement (exp. 2), four homogenates from wild abalo ne were used; the incubation mixture was followed up taking samples at 0, 1 , 2,4, 8, 16, 20, 32, 64 and 72 h. The latter samples were also used to mea sure the enzymatic activity before and after a 72-h incubation, in the abse nce of antibiotics (exp. 3). Initial bacterial counts in the wild abalone h omogenates were 459 colonies per mL. In the presence of the antibiotic mixt ure, no bacterial growth was observed. Initial cellulase activity was 1.6 U nits. Cellulose degradation figures were low, even in the absence of antibi otics, indicating the low dependence of abalone on the nutrients derived fr om the digestion of the cell walls. Still, the addition of antibiotics depr essed the digestion of cellulose (14.5 vs. 10.2%), suggesting the importanc e of live bacteria in the digestion process, and that the majority of the d egraded cellulose is perhaps acted upon by endogenous enzymes. The time ser ies experiment of samples from wild abalone, with and without antibiotics, showed quadratic effects on cellulose digestion y = 7.55 + .29h - .005 h(2) (r(2) = .2212), but no significant differences between treatments. The gra dual increase in the digestion of cellulose, followed by a plateau is simil ar to what happens in ruminant digestion. The initial and final enzyme acti vity without antibiotics showed a significant reduction of 41.1% for the wi ld abalone whereas in cultured abalone a 50.9% significant reduction was ob served. It seems that the cellulase complex enzymes present in the homogena tes remain active after 72 h of incubation, even though their activity is l owered by about one half. Although the use of antibiotics assure the absenc e of live bacteria in the incubation mixture, the cellular enzymes possibly freed by the effect of the antibiotics could still be active for up to 72 h after the microbes are no longer viable. Thus the enzymatic activity attr ibuted to the stomach itself could still be confounded.