Fatty acid metabolism and modifications in Chironomus riparius

Abstract

A priori knowledge of fatty acid modifications in consumers is essential for studies using fatty acids as biomarkers. We investigated fatty acid metabolism and possible modification pathways in benthic invertebrate Chironomus riparius larvae (Diptera). We conducted diet manipulation experiments using natural food sources (two chlorophyte algae, a diatom and a non-toxic cyanobacterium). We also did a diet-switch experiment on two different resources, fish food flakes TetraMin ® and cyanobacterium Spirulina , to study fatty acid turnover in Chironomus . Results of the diet manipulation experiments indicate that Chironomus larvae have a strong tendency to biosynthesize 20:5n-3 and 20:4n-6 from precursor fatty acids, and that the dietary availability of polyunsaturated fatty acids (PUFA) does not control larval growth. Fatty acid modifications explain why low dietary availability of PUFA did not significantly limit growth. This has ecologically relevant implications on the role of benthic chironomids in conveying energy to upper trophic level consumers. A diet-switch experiment showed that the turnover rate of fatty acids in Chironomus is relatively fast––a few days. The compositional differences of algal diets were large enough to separate Chironomus larvae into distinct groups even if significant modification of PUFA was observed. In summary, fatty acids are excellent dietary biomarkers for Chironomus , if modifications of PUFA are considered, and will provide high-resolution data on resource use. This article is part of the theme issue ‘The next horizons for lipids as ‘trophic biomarkers': evidence and significance of consumer modification of dietary fatty acids'.

The critical importance of experimentation in biomarker-based trophic ecology

“…It has been shown for both pinnipeds [27] and fish [28] that consumer assimilation and modification of dietary fatty acids varies according to the food consumed. Several articles in the present issue have further demonstrated diet specificity of fatty acid trophic assimilation in new model consumers [29][30][31]. In addition, Budge et al [32, this issue] also demonstrate that the fat content of the diet affects consumer assimilation of dietary fatty acids.…”

“…With primary consumers, it is relatively easy to provide fresh, single species of phytoplankton or macroalgae as foods that are likely representative of nature since many short-lived primary consumers feed on temporally limited algal blooms. In this special issue, numerous authors employed this approach and focused on feeding pure algal diets [20,21,29,30,33,34]; with their springtail experiments Kühn et al [35, this issue] were also able to offer mixed dried diets of bacteria, microalgae, fungi and plants. At intermediate trophic levels, it can be difficult to meet the nutritional requirements of consumers by feeding single species, particularly for longer-duration experiments, since such predators are rarely so specialized [28,31].…”

Warming Decreases Bioconversion of Polyunsaturated Fatty Acids in Chironomid Larvae Maintained on Cyanobacterium Microcystis

“…The presented fatty acids accounted for more than 99% of all fatty acids in Microcystis diets (Micro20 and Micro25), and more than 92% of all fatty acid in TetraMin ® . The lower percentage in TetraMin ® is due to the exclusion of long-chain saturated (≥C 20 LC-SFA) and monounsaturated fatty acids (≥C 20 LC-MUFA) from the table, which are prevalent in TetraMin ® but not in Microcystis, see also Strandberg et al [16]. The mean EPA w% decreased with increasing temperature in both diet treatments, but the decrease was statistically significant only for the Micro25 diet.…”

“…Generally, the abundance of C 20 and/or C 22 PUFA increases with each trophic step [ 14 ], but exceptions are common. For instance, retroconversion of C 22 PUFA to C 20 PUFA has been detected in, for example, cladocerans and chironomids [ 15 , 16 ]. Chironomids (Diptera: Chironomidae) are nonbiting midges whose life cycle includes aquatic egg, larval and pupal stages, and an aerial adult stage [ 17 ].…”

“…We have previously shown that Chironomus riparius (hereinafter referred to as Chironomus ) maintained on cyanobacterium Microcystis have surprisingly high levels of EPA and ARA, considering that these PUFA are not found in Microcystis [ 16 ]. Dietary C 18 precursors alone could not explain the higher-than-expected ARA and/or EPA levels in Chironomus [ 16 ]. We concluded that most likely the C 20 precursors in Microcystis , i.e., 20:3n-6 and 20:4n-3, were Δ5-desaturated to ARA and EPA, respectively ( Figure 1 ).…”