Trophic Enrichment Factors of Carbon and Nitrogen Isotopic Ratios (ΔC and ΔN) in Four Marine Ciliates.

Understanding the magnitude and causes of isotopic fractionation between organisms and their dietary resources is crucial for gaining knowledge on stable isotope ecology. However, little is known regarding the diet-tissue fractionation values of marine ciliates, which play a critical role in the reconstruction of microbial food webs. In the present study, we conducted experiments on two benthic ( and ) and two pelagic ( and ) marine ciliates, where they were fed with isotopically constant foods ( and ) under laboratory culture conditions to determine their carbon and nitrogen isotopic fractionation values (ΔC and ΔN). The stable isotope values (δC and δN) of ciliates for all experiments rapidly increased after the initial feeding, with half-lives ranging from 6.1 to 23.0h for δC and from 3.1 to 24.9h for δN. The ΔC and ΔN for all ciliates represented significantly positive enrichments, with overall mean fractionations of 0.6±0.2 and 1.2±0.4, respectively. Irrespective of the dietary type, both ΔC and ΔN were very similar for the same ciliate species. These results suggest that ΔC and ΔN for marine ciliates are similar to those found in common marine organisms with very little food-dependent variation. Overall, quantifying the specific isotopic fractionation of marine ciliates is expected to provide fundamental information on the trophic transfer of carbon, nitrogen, and energy flow through the microbial pathway in marine ecosystems.