Modelling hydrolysis of leaf litter by digestive enzymes of the snail Melanopsis praemorsa: combination of response surface methodology and in vitro assays

Abstract

The objective of the present study was to test the application of an in vitro assay simulating the digestive hydrolysis of leaf litter by the freshwater snail M. praemorsa, as well as to determine the possible influence of different factors in the efficiency of such process to release biologically available C and N under the forms of reducing sugars and amino acids from two different substrates. A novel approach to construct a model explaining the effect of three main factors (temperature, total reaction time and enzyme:substrate ratio) in the digestive hydrolysis of cellulose and protein present in leaf litter of different nutritive value is used. The methodology combines a factorial design based in the response surface methodology (RSM) and in vitro digestibility assays adapted to the physiology of both plant substrates used (alder and poplar leaves). The model revealed a different influence of the factors in the hydrolysis of two plant substrates, poplar and alder leaves and the main effect was produced by the time available for hydrolysis. A compensation response based in a longer gut retention time for the lower quality substrate was observed in the feeding assays. The use of in vitro assays and RSM provides a useful insight on the effect of factors and mechanisms underlying the observed differences in nutritional value of leaf litter for an aquatic invertebrate, being such differences linked to the whole bioavailability of carbon and nitrogen in headwater streams.