Stable nitrogen isotope analysis of amino acids as a new tool to clarify complex parasite–host interactions within food webs

Stable nitrogen isotope analysis of amino acids as a new tool to clarify complex parasite-host interactions within food webs

This study explores the trophic relations between parasites and their hosts by applying stable isotopes to clarify parasite feeding strategies. By examining the stable isotopes of nitrogen, we can infer the feeding status of an organism within an ecosystem. The method tracks the ratio between the abundant nitrogen-14 and the relatively rare nitrogen-15 and identifies changes that occur throughout food webs. When an organism feeds and incorporates nitrogen from proteins, for instance, the relative abundance of the rare nitrogen-15 increases (e.g. isotope fractionation) and this increase was believed to be similar across most feeding interactions. However, the changes measured between parasites and their hosts are extremely varied and do not cleanly fit into our previous understanding of feeding interactions.

Previous stable isotope work has focused on all (bulk) of the organic nitrogen compounds contained in the tissue that is examined. Here, we isolate and measure individual amino acid nitrogen values from field samplings ranging from secondary consumers to top predators to provide more precise feeding inferences and help us to further understand the varied isotope fractionation of parasites acquired with bulk analyses. For example, the parasitic copepod Mytilicola orientalis lives inside the intestine of the Pacific oyster for a part of its life, but it remains unclear whether M. orientalis feeds solely on host tissue or gut content, or a mixture of the two resources. Our bulk isotope analysis shows a similar trophic (feeding) position between host and parasite indicating parasitic feeding on the gut content of its host (kleptoparasitism). However, analysis of amino acids reveals that M. orientalis has a roughly half position higher trophic position than its host indicating that M. orientalis has a mixed feeding style relying upon both host tissue and gut contents.

Analysis of amino acid nitrogen isotopes gives a clearer depiction of parasite-host relationships than bulk isotope analysis largely due to the removal of underlying nitrogen baseline variability. The integration of this underlying variability allows for better resolution of small trophic position differences between parasite-host pairs as a result of mixed feeding strategies. This research shows how compound specific isotope analysis can be used to clearly determine the feeding styles of parasites and to help us further understand their ecological position within food webs.

Image Caption: Harbour porpoise lung and lung nematodes, Tijs Joling busy with dissections, and Wadden Sea sampling associated with this project.

NIOZ twitter handle: @NIOZnieuws / David Thieltges: @DavidWThieltges / Philip Riekenberg: @phrieken

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