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 weissfogii). Egg production was lo-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,, and C17, 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.

The composition and incorporation of lipids in two marine ciliates, Pleuronema sp. and Fabrea salina, was examined following growth on either an algal or bacterial diet. When allowed to feed on a natural bacterial community, Pleuronerna sp. synthesized the triterpenoid alcohol gammaceran-3P-01 (tetrahymanol) and two hopanoids (hopan-3P-01 and one uncharacterized hopanoid). When fed the marine alga fsochrysis gnlbana, F. salina contained the major algal sterol 24-methylcholesta-5, 22-dien-3P-01 and several long chain ketones specific to the alga. In both ciliates, fatty acids composition showed a general correspondence to that of the diet. Using a series of antibiotic treatments to alter the bacterial prey community, and thus fatty acid composition of the ciliate's diet, promoted changes in the fatty acid composition of Pleuronerna sp. to resemble that of the bacterial prey. The addition of a mixture of algal sterols to a bacterized culture of another scuticociliate, Parauronema acufum, inhibited tetrahymanol synthesis and resulted in the incorporation of sterols into the ciliate.Supplementary key words. Sterols, trophic transfer.ILIATED protozoans represent an important component of C estuarine and coastal marine systems as both consumers of bacteria and prey for metazoan grazers [21, 251. Bacterivorous ciliates in particular provide a critical linkage in these systems by providing a mechanism for the transfer of bacterial carbon and energy to higher trophic levels, as many larger consumers cannot effectively ingest bacteria [8, 251. The biochemical composition of ciliates is an important element in understanding the nutritional benefits for growth and reproduction that metazoans gain from the consumption of ciliates. Among the various biochemical constituents, lipids represent a key component for many metazoan grazers and are particularly important to members of the Crustacea which have only limited ability for de novo synthesis of lipids and must rely on dietary sources [9]. Compared to well studied freshwater ciliates such as Tefrahymena or Paramecium [28], little information is available on the lipid composition of marine ciliates and the response of these compounds to changes in diet. The hymenostomatid ciliate Tetrahymena [ 141 was the first ciliate in which the unique triterpenoid alcohol "tetrahymanol" (gammaceran-3P-01) was observed. The structural similarity of tetrahymanol to that of sterols such as cholesterol (cholest-5-en-3P-01) that are common to other eukaryotes ( Fig. 1) and their comparable physical properties suggest that in ciliates tetrahymanol provides functional equivalence to sterols by maintaining membrane fluidity. Work by Ferguson et a]. [7] supports this hypothesis by documenting the plant sterol ergosterol (24-methycholest-5,7,22-trien-3P-o1) added as a media supplement was readily incorporated into T. pyriformis membranes, with tetrahymanol synthesis inhibited. Recently, tetrahymanol was also observed as a conservative membrane component of several marine ciliates [12], thus exte...