Does sterol availability in a forested headwater stream constitute a nutritional constraint for macroinvertebrates?

ABSTRACT

Headwater streams are characterised by predominantly heterotrophic functioning resulting from leaf litter input, but autochthonous primary production can also contribute to energy fluxes and the supply of nutrients. While much work has focused on the role of polyunsaturated fatty acids as essential nutrients, less attention has been paid to the significance of sterols in stream food webs. Yet these molecules are essential to arthropods that can only synthesise cholesterol from a limited range of dietary sterols. In a headwater stream, we tracked the transfer of dominant sterols from allochthonous and autochthonous resources to 5 benthic macroinvertebrates. Despite their formal functional feeding group, all the taxa tended to be opportunistic omnivores that relied on all available resources. These behaviours could be attributed to nutritional constraints in the stream food web, but these constraints were not related to the sterol supply from the different basal sources. Dominant sterols from detrital sources (leaf litter, fine benthic organic matter [FBOM]) and primary producers (epilithic biofilms, bryophytes) were all Δ5-sterols (β-sitosterol, campesterol, stigmasterol, epibrassicasterol). The levels of cholesterol detected in macroinvertebrates, as well as the presence of desmosterol, indicate efficient dealkylation activity of these Δ5-sterols. Leaf litter has consistently shown high β-sitosterol/stigmasterol ratios, giving it greater nutritional value than usually accepted. But bryophytes, and especially epilithic biofilms, contained cholesterol, providing a direct supply to invertebrates, thereby promoting their growth. Detrital particles (FBOM) colonised by microalgae could be the best-balanced diet to avoid sterol deficiencies due to their cholesterol content and high β-sitosterol/stigmasterol ratios.

KEYWORDS:

• biofilms
• bryophytes
• headwater streams
• leaf litter
• macroinvertebrates
• sterols

Acknowledgements

We thank Théo Aurand and Floriane Martin, 2 MSc students, for their help in the field and during the analyses. We also thank Ms Miranda Dickenson for proofreading the English.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The data that support the findings are available upon reasonable request.

Author contribution statement

Conceptualisation: TL-V and CD Data analysis: TL-V, FP, and CD. Preparation of figures and tables: TL-V. Data interpretation and writing: TL-V, CD, AB, and MD.

Additional information

Funding

This work was supported by Centre National de la Recherche Scientifique [Grant Number EC2C0 2019].