The microbiome at the interface between environmental stress and animal health: an example from the most threatened vertebrate group

Abstract

Nitrate pollution and global warming are ubiquitous stressors likely to interact and affect the health and survival of wildlife, particularly aquatic ectotherms. Animal health is largely influenced by its microbiome (commensal/symbiotic microorganisms), which responds to such stressors. We used a crossed experimental design including three nitrate levels and five temperature regimes to investigate their interactive and individual effects on an aquatic ectotherm, the European common frog. We associated health biomarkers in larvae with changes in gut bacteria diversity and composition. Larvae experienced higher stress levels and lower body condition under high temperatures and nitrate exposure. Developmental rate increased with temperature but decreased with nitrate pollution. Alterations in bacteria composition but not diversity are likely to correlate with the observed outcomes in larvae health. Leucine degradation decreased at higher temperatures corroborating accelerated development, nitrate degradation increased with nitrate level corroborating reduced body condition and an increase in lysine biosynthesis may have helped larvae deal with the combined effects of both stressors. These results reinforce the importance of associating traditional health biomarkers with underlying microbiome changes. Therefore, we urge studies to investigate the effects of environmental stressors on microbiome composition and consequences for host health in a world threatened by biodiversity loss.

Keywords: Rana temporaria; global warming; gut microbiome; multiple stressors; nitrate pollution; water-borne corticosterone.

Figure 1.

Comparison among model estimates for R. temporaria larvae (a) stress levels (WB-CORT), (b) body condition (SMI), (c) developmental rate given by the number of Gosner’s developmental stages advanced by the larvae divided by the number of days from hatching until the end of the experiment, (d) gut microbiome bacteria alpha diversity (Shannon entropy index) and (e) bacteria composition (1 − axis NMDS scores), controlling for the effect of aquarium. Larvae were submitted to the combination of three levels of nitrate exposure (0 [control], 50 and 100 mg l⁻¹) and five temperature regimes (18°C [control], 22°C, 26°C, 28°C and fluctuating temperature). Underlying models are presented in table 1. Different capital letters above each graph indicate differences among nitrate treatments (Tukey method adjusted p < 0.05). Different capital letters within the boxes corresponding to temperature regimes indicate differences among them for each graph (Tukey method adjusted p < 0.05). For models with significant interactive effects, different small cap letters (a–c) in colours corresponding to nitrate exposure indicate nitrate treatments that differed within temperature regimes. Different letters (d–r) in colours corresponding to temperature regimes indicate temperature treatments that differed within nitrate treatments (Tukey method adjusted p < 0.05).

Figure 2.

(a) Relative abundance of phyla and (b) NMDS distribution (Procrustes: rmse = 0.0001, max. resid. = 0.0007) of ASVs of bacteria in the guts of R. temporaria larvae submitted to the combination of three levels of nitrate exposure (0 [control], 50 and 100 mg l⁻¹) and five temperature regimes (18°C [control], 22°C, 26°C, 28°C and fluctuating temperature).