Substrate composition influences amino acid carbon isotope profiles of fungi: implications for tracing fungal contributions to food webs

Abstract

Fungi link detrital resources and metazoan consumers through their role as decomposers. However, fungal contributions to metazoans may be misestimated in amino acid isotope studies because fungi are capable of both synthesizing amino acids (AAs) de novo and absorbing AAs from their environment. While fungi cultured in AA-free media have been used to represent fungi in studies of natural environments, fungi likely gain energetic benefits by taking up substrate AAs directly in situ. Consequently, fungi cultured on AA-free media may not be representative of the true variability of natural fungal δ 13 C AA profiles. Therefore, the objective of this experiment was to determine the effect of substrate AA availability on yeast δ 13 C AA profiles. We found that yeasts cultured in media of relatively higher AA content had different δ 13 C AA profiles than yeasts grown in AA-free media, in part because yeasts utilized two essential AAs (Leu and Val) directly from media substrates when available in sufficient amounts. Furthermore, these differences among yeast δ 13 C AA profiles remained after normalization of δ 13 C AA values. We recommend further characterization of the variation in fungal δ 13 C AA profiles and the incorporation of this potential variability into interpretations of basal resource use by metazoans.

Intercontinental analysis of temperate steppe stream food webs reveals consistent autochthonous support of fishes

“…It is unknown whether the heterotrophic microbial energy pathway to higher‐order consumers is sustained by autochthonous or allochthonous carbon (or what proportion of AAs are routed through gastrointestinal tract microbes; Kelly & Martínez del Rio, 2010, Newsome et al, 2011). However, our observation of a distinct heterotrophic signal in consumer tissues (as opposed to a δ 13 C EAA profile characteristic of terrestrial carbon) suggests that any contribution of allochthonous carbon is likely reworked and facilitated into metazoan food webs primarily via the microbial loop (Arsenault et al, 2022; France, 2011). In fact, the δ 13 C EAA profiles of terrestrial resources did not overlap with those of fish communities, or with any other resource category, despite being the most thoroughly characterised resource group.…”

The coupling of green and brown food webs regulates trophic position in a montane mammal guild

“…Although plant and bacterial sources were identifiable with ≥95% accuracy, fungal characterization lagged at 86%, thereby inflating the error associated with proportional estimates. Ecologically, this likely reflects the fact that our sampled fungi are mixotrophs that can directly acquire EAAs from external sources like plants, and the amino acid composition of substrates can dictate fungal fingerprints (Arsenault et al, 2022). Three fungal samples in our data set overlapped with plant fingerprints (Figure 2), all ectomycorrhizal symbionts ( Boletus , Lactarius , and Russula spp.…”

Autochthonous production sustains food webs in large perialpine lakes, independent of trophic status: Evidence from amino acid stable isotopes

“…As a result, fungi might overlap with algae on the LDA‐space. In addition, fungi show complex metabolism, and can directly incorporate EAAs from the environment (Arsenault, Liew, & Hopkins, 2022), stressing that brown food webs might be hard to track using the C fingerprint.…”