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

Laboratory experiments with stable isotopes (C-13 and N-15) were conducted to determine the importance of bacteria and algae as food sources for free-living nematodes. All tested bacterivorous nematodes (Caenorhabditis elegans, Acrobeloides tricomis, Poikilolaimus sp. and Panagrolaimus sp.) were found to be depleted in delta C-13 (on average by -1.71 +/- 0.56 parts per thousand) and enriched in delta N-15 (on average by 3.17 +/- 1.27 parts per thousand) 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 delta C-13 and delta N-15 values when placed on the same bacterial diet of E. colt, consistent with differences in their trophic shifts. Conversely, no differences in delta C-13 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 C-13-enriched (NaHCO3)-C-13, C-13(6)-glucose and N-15-enriched (NaNO3)-N-15 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.