Polyunsaturated fatty acids:: Evidence for non-substitutable biochemical resources in Daphnia galeata

The factors that determine the efficiency of energy transfer in aquatic food webs have been investigated for many decades. The plant-animal interface is the most variable and least predictable of all levels in the food web. In order to study determinants of food quality in a large lake and to test the recently proposed central importance of the long-chained eicosapentaenoic acid (EPA) at the pelagic producer-grazer interface, we tested the importance of polyunsaturated fatty acids (PUFAs) at the pelagic producer-consumer interface by correlating sestonic food parameters with somatic growth rates of a clone of Daphnia galeata. Daphnia growth rates were obtained from standardized laboratory experiments spanning one season with Daphnia feeding on natural seston from Lake Constance, a large pre-alpine lake. Somatic growth rates were fitted to sestonic parameters by using a saturation function. A moderate amount of variation was explained when the model included the elemental parameters carbon (r(2) = 0.6) and nitrogen (r(2) = 0.71). A tighter fit was obtained when sestonic phosphorus was incorporated (r(2) = 0.86). The nonlinear regression with EPA was relatively weak (r(2) = 0.77), whereas the highest degree of variance was explained by three C-18-PUFAs. The best (r(2) = 0.95), and only significant, correlation of Daphnia's growth was found with the C-18-PUFA alpha -linolenic acid (alpha -LA; C18:3n-3). This correlation was weakest in late August when C:P values increased to 300, suggesting that mineral and PUFA-limitation of Daphnia's growth changed seasonally. Sestonic phosphorus and. some PUFAs showed not only tight correlations with growth, but also with sestonic alpha -LA content. We computed Monte Carlo simulations to test whether the observed effects of alpha -LA on growth could be accounted for by EPA, phosphorus, or one of the two C-18-PUFAs, stearidonic acid (C18:4n-3) and linoleic acid (C18:2n-6). With > 99 % probability, the correlation of growth with alpha -LA could not be explained by any of these parameters. In order to test for EPA limitation of Daphnia's growth, in parallel with experiments on pure seston, growth was determined on seston supplemented with chemostat-grown, P-limited Stephanodiscus hantzschii, which is rich in EPA. Although supplementation increased the EPA content 80-800X, no significant changes in the nonlinear regression of the growth rates with alpha -LA were found, indicating that growth of Daphnia on pure seston was not EPA limited. This indicates that the, two fatty acids, EPA and alpha -LA, were not mutually substitutable biochemical resources and points to different physiological functions of these two PUFAs. These results support the PUFA-limitation hypothesis for sestonic C:P < 300 but are contrary to the hypothesis of a general importance of EPA, since no evidence for EPA limitation was found. It is suggested that the resource ratios of EPA and alpha -LA rather than the absolute concentrations determine which of the two resources is limiting growth.