High microbial diversity in glacial habitats uncoupled from the specialized microbiomes of resident chironomid fauna

Ecosystems associated with retreating glaciers are undergoing rapid transformation in the context of a changing climate. Invertebrate fauna, including the Chironomidae insect family (non-biting midges), is part of the active biology that characterizes glaciers and their surrounding habitats. The underlying microbiology, present in both the insects' guts and their physical habitat, represents a critical interface, controlling, on the one hand, the basis of nutrient geochemical cycling and, on the other, the health and nutritional physiology of its hosts. We aimed to assess the extent to which insect-borne bacteria resemble those found in icemelt water and the surrounding wet and terrestrial environments, in order to determine also whether the bacteria found associated with the insects could be interpreted mainly as specific dwellers, putatively involved with active physiological functions, or also as transient cells taken in for other purposes. To this end, we analyzed physical and biological samples from ten different chironomid species in two glacier-fed streams, one proglacial pond, and the surrounding habitats of three glacier systems (Agola, Amola and Mandrone) in the Italian Alps. The samples were analyzed using culture-independent amplified 16S rRNA gene bacterial metabarcoding sequencing. The bacterial diversity in glacial habitats was unexpectedly high, with numbers of sequence variants similar to those recorded in temperate, lowland, productive soils, and almost seven times higher than those found in insects. There was minimal coincidence in sequence variants between insects and habitats, amounting to just 4.9% shared cases, and the few taxa found in both insects and habitats were mostly overrepresented in the former. Additionally, there were no significant differences between insect species or between insect communities from different sites. A number of taxa occurring uniquely in insects or habitats showed peculiarities at all taxonomic levels, including specific phyla. Evidence of microevolutionary distinctness was observed in the form of sequence variants assigned to the same taxonomic name that differed in specific sequence bases and were found to be partitioned either in insect or environmental samples. Interestingly, sequence variants found in both insects and environments scored higher in bioinformatic identification, reaching deeper assigned ranks compared to variants occurring only in insects or the environment. In essence, the compared insect and environmental communities showed a very low level of symmetry and consequently a very high level of specificity to one or the other condition. The data provided limited support about the diet of chironomids in relation to the microbiota of the habitat. This suggests that the food resource for these insects likely consists mostly of dissolved organic matter and detritus of various origins, rather than intact bacteria with sequenceable genomes, as is the case elsewhere with ruminant herbivores or birds.