Trophic transfer of fatty acids, sterols, and a triterpenoid alcohol between bacteria, a ciliate, and the copepod Acartia tonsa

The incorporation of lipids into the copepod Acartia tonsa and its eggs was measured when it was fed either a bacterivorous ciliate ( Pleuronema sp.) or a diatom ( Thalassiosira weissflogii ). Egg production was 10‐fold higher on the diatom diet, whereas hatch success of eggs was the same for algal and ciliate diets. Adult copepods fed diatoms contained more total fatty acid and sterols than copepods fed the ciliate diet, and individual lipids reflected the dietary source. Eggs from diatom‐fed copepods had fewer fatty acids but more sterols than eggs from copepods on a ciliate diet. Ciliate‐fed copepods and their eggs contained significant amounts of odd chain‐length and branched fatty acids diagnostic of bacteria. These fatty acids, in particular the iso C 15 and C 17 , were also elevated in ciliates feeding on bacteria in culture, suggesting the direct transfer of bacterial fatty acids from ciliates to copepods and their eggs. We also observed the assimilation of tetrahymanol, a triterpenoid alcohol specific to ciliates, into adults and eggs when copepods were fed a ciliate diet. Tetrahymanol accounted for 6.6±1.9% of total neutral lipids in adults and 35.4 ± 6.5% in eggs. These results suggest that bacterivorous ciliates may not provide copepods with adequate nutritional requirements for long‐term survival, but that lipids unique to bacteria and ciliates can be assimilated by and may provide useful tracers of consumption by copepods.