Production and transfer of essential fatty acids in a man-made tropical lake ecosystem

Abstract

Essential biomolecules, such as physiologically essential fatty acids, can critically influence consumers' performance and the ecosystem's functioning. Eicosapentaenoic (EPA; 20:5ω3) and docosahexaenoic (DHA; 22:6ω3) fatty acids are physiologically crucial for consumers, and they must be either obtained from the diet or bioconverted from precursors. We monitored the synthesis of EPA and DHA by primary producers in the largest man-made ecosystem (Lake Kariba) and in situ fatty acid production, trophic transfer, and endogenous production of EPA and DHA in the tropical lake food web using ¹³C-labeling, compound-specific isotopes, and gene expression of fads2 and elovl5 genes in most abundant fish species. Seston pigment analysis and 23S rRNA sequencing revealed that cyanobacteria dominated primary producers throughout three seasons, and the biosynthesis rate of EPA and DHA was under the detection limit. Moreover, due to the low zooplankton densities and EPA and DHA content in zooplankton, the transfer of EPA and DHA from phytoplankton–zooplankton to upper trophic levels is low. The low production of EPA and DHA by primary producers is mitigated by bioconversion of α-linolenic acid to EPA and DHA in two tilapia species, especially by Nile tilapia (Oreochromis niloticus) known to feed on cyanobacteria. Compound-specific isotope analysis revealed that tigerfish (Hydrocynus vittatus), the main predatory fish on the lake, was more closely related to Nile tilapia than to lake planktivorous fish (Limnothrissa miodon). Therefore, trophic interaction between cyanobacteria and algivorous fish has replaced traditional phytoplankton and zooplankton trophic interaction in the synthesis and transfer of EPA and DHA to upper trophic levels.