Quantifying terrestrial carbon in freshwater food webs using amino acid isotope analysis: Case study with an endemic cavefish

Abstract

Flow of terrestrial carbon though aquatic ecosystems (allochthony) is an important but underestimated component of the global carbon cycle. A lack of clear consensus about the importance of allochthonous (terrestrial) organic carbon is sometimes attributed to uncertainties associated with conventional ‘bulk’ isotope data, the most widely used ecological tracer. Amino acid‐specific isotope analysis is an emerging research method promising to address existing limitations of bulk C and N isotope analyses. We tested the efficacy of amino acid δ13C data as a generalizable measure of allochthony by analysing an aggregated dataset (n = 168) of primary and secondary data of carbon sources from disparate geographical locations across the globe. We found the δ13C fingerprints amino acids to be consistently distinct between allochthonous (terrestrial) and autochthonous (aquatic) carbon sources. We also found that our approach is most effective when we use only essential amino acid tracers (i.e. isoleucine, leucine, phenylalanine, threonine and valine). Predictive trends in δ13C fingerprints appear to be largely compatible across studies and/or laboratories. As a case study, we used this approach to quantify the contribution of terrestrial carbon to an endemic cavefish, Cryptotora thamicola, and found that its biomass was comprised largely of autochthonous carbon (~75%).

Effects of ethanol preservation and formalin fixation on amino acid stable isotope analysis (δ13C and δ15N) and its ecological applications

“…Results from δ 13 C profiles are more variable across amino acids, with essential amino acids and glutamic acid being less susceptible to preservation effects. Crucially, however, amino acids that are commonly used as ecological tracers (δ 15 N: glutamic acid and phenylalanine; δ 13 C: isoleucine, leucine, phenylalanine, and valine) (Chikaraishi et al ; Liew et al ) appear sufficiently robust, which validates the direct ecological application of data from preserved samples without mathematical correction factors.…”

“…Data inputs to the mixing models comprised δ 13 C profiles from four essential (vertebrate) amino acids, namely isoleucine, leucine, phenylalanine, and valine. These amino acids have been shown to effectively distinguish between terrestrial and aquatic carbon sources owing to their negligible levels of trophic fractionation (McMahon et al ; Liew et al ). Threonine was excluded from the analysis because δ 13 C readings of this essential amino acid fell below detection limits in our samples.…”

“…First, primary producer groups have been found to exhibit less variation in δ 13 C profiles of individual amino acids compared to bulk δ 13 C profiles (Larsen et al 2013), and appear to have characteristic patterns that are relatively consistent. This facilitates applications across taxonomic groups, time, space, and trophic positions (Liew et al 2019). Second, amino acid δ 15 N profiles are largely conserved across tissue types within an organism (Chikaraishi et al 2014), and trophic positions can be estimated with unprecedented precision because trophic fractionation values are more predictable than in bulk SIA (Chikaraishi et al 2009;Steffan et al 2013;Bowes and Thorp 2015).…”

Reweaving river food webs through time

“…González-Bergonzoni et al, 2014, Liew et al, 2019. González-Bergonzoni et al, 2014, Liew et al, 2019.…”

Terrestrial contributions to Afrotropical aquatic food webs: The Congo River case

“…On the other hand, carbon isotope data are important tracers to distinguish C 3 and C 4 plant sources (Abrantes et al, ; Cerling et al, ) and methanotrophic inputs (Grey, ; Whiticar, ). Overall, new advances in analytical techniques such as stable hydrogen isotope analysis (Soto, Koehler, Wassenaar, & Hobson, ) and compound‐specific stable isotope approaches (e.g., amino acids, Liew et al, ) can potentially solve the common inability of separating aquatic and terrestrial sources.…”