Selective feeding in nematodes: a stable isotope analysis of bacteria and algae as food sources for free-living nematodes

Laboratory experiments with stable isotopes (13C and 15N) were conducted to determine the importance of bacteria and algae as food sources for free-living nematodes. All tested bacterivorous nematodes (Caenorhabditis elegans, Acrobeloides tricornis, Poikilolaimus sp. and Panagrolaimus sp.) were found to be depleted in δ13C (on average by −1.71 ± 0.56‰) and enriched in δ15N (on average by 3.17 ± 1.27‰) relative to their bacterial diets of Escherichia coli and Matsuebacter sp. The nematode species showed considerable differences in their stable isotope composition with respect to food sources. Moreover, they differed significantly in δ13C and δ15N values when placed on the same bacterial diet of E. coli, consistent with differences in their trophic shifts. Conversely, no differences in δ13C values were observed among nematode species placed on the same Matsuebacter sp. diet. In mixed food sources of E. coli and Matsuebacter sp., E. coli contributed 71% of the carbon to C. elegans and Matsuebacter sp. more than 90% of the carbon to A. tricornis. An enrichment experiment based on 13C-enriched NaH13CO3, 13C6-glucose and 15N-enriched Na15NO3 tracers in a freshwater periphytic community showed the importance of micro-algae and diatoms over heterotrophic bacteria as the main food sources of free-living periphytic nematodes. In this respect, direct grazing may predominate, possibly together with the use of extracellular polymeric substances (EPS) from diatoms. In general, the use of stable isotopes to study nematode feeding ecology can be useful to investigate directly the type of ingested food item(s), different bacteria and algae, and the contribution to nematode diet, in addition to the conventional feeding type scheme.

Affiliations: 1: 1University Bielefeld, Animal Ecology, Morgenbreede 45, D-33615 Bielefeld, Germany