Utilization of Dietary Protein in the Litter-Dwelling Larva of Bibio marci (Diptera: Bibionidae)

Abstract

As plant litter is typically N limited, protein digestion and amino acid assimilation are key processes in saprophagous invertebrates. The recalcitrance of dietary protein may be increased by their binding to phenolic compounds in humic substances. Binding phenolic compound to protein may be also important in humus acid formation and stabilization of nitrogen bounded protein in soil organic matter. Consequently, better understanding of interaction of protein and phenolic compounds in invertebrate gut is important not only from ecophysiology perspective but also in terms of soil organic matter dynamics. In this study, we evaluated the digestive efficiency of proteins during gut passage in the litter-dwelling larva of Bibio marci. Anterior part of the midgut is highly alkaline, which agrees with the protease pH optimum. We found large differences in protease activities of the leaf litter, gut, and excrements. However, individual gut compartments were close to one another in their protease activities. The same was true for the contents of amino acids and ammonia. Also, amino acid composition differed between compartments. Litter had the high content of glutamine and serine, whereas hindgut and excrement were characterized by the high content of methionine, and the remaining gut segments had the high content of valine. Experiment with 14C-labelled protein demonstrated that gut passage improved the digestive efficiency of proteins, especially for a humic acid stabilized protein.