Lake size is associated with the relationship between body mass and gut microbiota alpha diversity in threespine stickleback (Gasterosteus aculeatus)

Abstract

Lake size is associated with ecological conditions that can affect the microbiome of hosts, such as diet and habitat characteristics. However, whether host-microbe interactions are affected by lake size remains poorly understood. We characterized the gut microbiota of 191 wild threespine stickleback fish ( Gasterosteus aculeatus ) from six Alaskan lakes using 16S rRNA gene sequencing. We tested how gut microbiota varied across lakes with different surface areas. We then examined whether host ecotype and host traits, including sex, body mass, gravidity, and Schistocephalus solidus ( S. solidus ) infection were associated with gut microbial composition across lakes differing in surface area. Fish from intermediate-sized lakes harbored significantly more diverse microbiota than those from small and large lakes, independent of stickleback ecotype. We examined other factors commonly associated with fish microbiota. Model-predicted marginal effects from the mass and lake surface area interaction analysis indicated that the association between fish mass and microbial alpha diversity was strongly negative in the smallest lakes, weakest in intermediate-sized lakes, and strongly positive in the largest lakes. Differential abundance analysis revealed lake size-dependent associations between body mass and individual taxa. Sex and S. solidus infection were significantly associated with gut microbial composition, whereas fibrosis and gravidity had minimal effects. Together, these findings suggest that host-microbe interactions are not fixed within a species but depend on environmental conditions. By linking environmental variation with individual traits, our findings highlight how gut microbiota diversity varies in natural ecosystems. Understanding these patterns is important for predicting how gut microbiota respond to environmental change and for disentangling the effects of habitat and host biology on gut microbial communities. These insights improve our ability to understand how environmental differences shape animal health and ecosystem function across natural populations.