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In the microbial loop, organic matter, Irrespective of its origin and composition, is incorporated in the food chains only after it has been converted into bacterial biomass. The absence of polyunsaturated fatty acids (PUFAs) in bacteria leads to the loss of these components in the initial h k of the microbial loop. Thls results in significant differences in the nutritional value of the initial links of food webs and the microbial loop with respect to essential fatty acids. We investigated the fatty acid composition and the capability of biosynthesizing PUFAs in marine zooflagellates and ciliates that constitute links of the microbial loop. The composition of marine zooflagellate Bodo sp. varied with substrate but in all experiments flagellates contained PUFAs. Bodo sp. is capable of synthesizing PUFAs, and 22:6(n-3) synthesis rates were especially high. The ciliate Euplotes crassus was found to contain essential fatty acids. Despite the fact that synthesis rates of PUFAs in ciliates were markedly lower compared to that in zooflagellates, the occurrence of essential fatty acids in c l a t e s further enriches the biochemical composition of the microbial loop. It was shown that flagellates and ciliates are the source of PUFAs in the microbial loop. This compensates for the drawbacks of the microbial loop, connected with the specific biochemical composition of bacteria.

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