Sterols in daphnia nutrition: physiological and ecological consequences

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The efficiency of carbon transfer from primary producers to higher trophic levels is crucial for aquatic food web structure. A disconnect between primary and secondary production at the phytoplankton-zooplankton interface often arises from the nutritional inadequacies of phytoplankton species. Recent findings indicate that the low carbon-transfer efficiency from cyanobacteria to Daphnia is due to a lack of sterols. This thesis further emphasizes the importance of dietary sterols for Daphnia's performance, revealing that sterol shortages significantly affect various life history traits, impacting population dynamics. Cholesterol, vital for arthropod physiology, is scarce in plant material, while eukaryotic phytoplankton offers diverse phytosterols, which differ chemically from cholesterol. Herbivorous grazers must utilize these dietary sterols to synthesize cholesterol. The research shows that the adequacy of dietary sterols varies, influencing Daphnia's growth and life history traits. Additionally, intermediary grazers like heterotrophic flagellates and ciliates can enhance the nutritional value of picocyanobacterial food sources for Daphnia by adding sterols. While ciliates cannot synthesize sterols de novo, they produce tetrahymanol, a sterol surrogate that stabilizes membranes. However, ciliates are generally less nutritious for Daphnia than algae, unless they are fed sterol-supplemented diets, highlighting the critical rol