No impact of a short-term climatic El Nino fluctuation on gut microbial diversity in populations of the Galapagos marine iguana (Amblyrhynchus cristatus)

Gut microorganisms are crucial for many biological functions playing a pivotal role in the host's well-being. We studied gut bacterial community structure of marine iguana populations across the Galapagos archipelago. Marine iguanas depend heavily on their specialized gut microbiome for the digestion of dietary algae, a resource whose growth was strongly reduced by severe El Nino-related climatic fluctuations in 2015/2016. As a consequence, marine iguana populations showed signs of starvation as expressed by a poor body condition. Body condition indices (BCI) varied between island populations indicating that food resources (i.e., algae) are affected differently across the archipelago during 'El Nino' events. Though this event impacted food availability for marine iguanas, we found that reductions in body condition due to El Nino-related starvation did not result in differences in bacterial gut community structure. Species richness of gut microorganisms was instead correlated with levels of neutral genetic diversity in the distinct host populations. Our data suggest that marine iguana populations with a higher level of gene diversity and allelic richness may harbor a more diverse gut microbiome than those populations with lower genetic diversity. Since low values of these diversity parameters usually correlate with small census and effective population sizes, we use our results to propose a novel hypothesis according to which small and genetically less diverse host populations might be characterized by less diverse microbiomes. Whether such genetically depauperate populations may experience additional threats from reduced dietary flexibility due to a limited intestinal microbiome is currently unclear and calls for further investigation.

Automated summary

The diversity and richness of gut microorganisms in marine iguana populations are closely related to the genetic diversity of the host individuals, rather than being affected by El Nino-related nutrition deficiencies. During El Nino events, marine iguana populations face limited food resources, resulting in poor body condition, but with no significant differences in gut microbial structure.