Disentangling the gut microbiota of Aldabra giant tortoises of different ages and environments

Background: The gut microbiota plays a pivotal role in regulating the physiological functions of its host, including immunity, metabolism, and digestion. The impact of environment and age on microbiota can be assessed by observing long-lived animals across different age groups and environments. The Aldabra giant tortoise (Aldabrachelys gigantea) is an ideal species for this study due to its exceptionally long lifespan of over 100 years. Methods: Using 16S rRNA gene amplicon analysis, we analyzed 52 fecal samples from giant tortoises in Seychelles (Curieuse and Mahé islands) and in a zoological park in Italy, from very young individuals to those of >100 years old. We performed Alpha and Beta diversity analysis, relative abundance analysis, and complex upset plot analysis, comparing the results of tortoises from different environments and age groups. Results: The diversity and overall composition of the gut microbiota of tortoises were impacted mainly by geolocation rather than their age. The greater diversity of microbiota in wild tortoises was attributed to their food variance such as wild leaves and branches, compared to captive or domesticated conditions. Beta diversity analysis also revealed the contribution of both environment and age to the variation between samples, with environments accounting for a larger proportion of this contribution. Certain bacterial families, such as Spirochaetota and Fibrobacterota, were more prevalent in environments with higher fiber intake, reflecting dietary differences. Additionally, a range of host-independent environmental bacteria was found to be specific to individuals in Curieuse and not in other geolocations. On the other hand, there were no bacterial taxa specific to centenarians, whose microbial complexity was reduced compared to adult or elderly tortoises. Conclusions: Our records showed that environment is the primary influence in the overall composition and diversity of the gut microbiota of Aldabra giant tortoises. As giant tortoises are amongst the longest-lived vertebrate animals, these findings can be utilized to monitor their health according to their ages, and enhance their conservation efforts.