Diet-dependant incorporation of biomarkers from microbial food sources: Implications for food-web studies that use stable isotope and fatty acid analyses

Background/Question/Methods

Metazoan consumption of Bacteria and Archaea may impact key microbially-mediated processes, yet techniques to quantify trophic linkages require laboratory evaluation before robust application to field observations are possible. In a laboratory-based study, an annelid, Ophryotrocha labronica, was raised on eukaryotic, bacterial, or archaeal food sources to test whether fatty acid and stable isotope paradigms are appropriate to quantitatively model metazoan consumption of microbial food sources.

Results/Conclusions

 This annelid’s carbon tissue-diet shift was neither zero nor constant and was a function of the δ¹³C value and the C:N ratio of its food. Nitrogen isotope tissue-diet shifts were also a function of the food source and often negative when raised in antibiotic-laden seawater . However, the antibiotics themselves were likely augmenting the diet of this species and caused this negative isotopic shift, pointing to the potential roll for assimilation of dissolved nitrogen food sources by annelids. Fatty acid profiles within this species were largely independent of diet. Polyunsaturated fatty acids, commonly used as a metric for phytoplankton consumption and not provided by the food sources, were present in O. labronica tissues. This suggests that O. labronica can synthesize these fatty acids. These lab-based results were then applied to more accurately characterize the diet of field-collected annelids.  This combined lab- and field- approach allowed a novel trophic relationship to be identified between archaeal-bacterial aggregates and methane-seep polychaetes.