Trophic upgrading of picocyanobacterial carbon by ciliates for nutrition of Daphnia magna

Unicellular picocyanobacteria, such as species of the genus Synechococcus, are unsuitable for supporting growth and reproduction of Daphnia spp. In Synechococcus spp., long-chain polyunsaturated fatty acids (PUFAs) and sterols are absent, which leads to a low carbon transfer efficiency at the picocyanobacteria-Daphnia spp. interface. Herein, we address the question as to whether ciliates can serve as a trophic link between picocyanobacterial production and Daphnia spp. production, thereby upgrading the nutritional value of a picocyanobacterial food source by producing essential lipids such as PUFAs or sterols. In simplified experimental food chains consisting of 1 of 2 different Synechococcus strains, the ciliates Colpidium campylum or Cyclidium sp., and D. magna, we provided evidence that predation on ciliates by Daphnia spp. allows access to picocyanobacterial production. Since daphnids are primarily sterol-limited when grown on the picocyanobacteria Synechococcus spp., the observed trophic upgrading of Synechococcus food-quality by intermediary ciliates is most probably due to the addition of sterols or sterol-like compounds that (at least partly) release Daphnia spp. from sterol limitation. The absence of sterols in the ciliates used in the present study suggests that tetrahymanol and/or hopanoids provide functional equivalents of sterols not only in ciliates but also in Daphnia spp., thereby leading to enhanced growth of the cladocerans.