(a) Fraction of unigenes within each habitat which are multi-habitat genes (for all unigenes, or when considering only mobile elements or antibiotic resistance genes). (b) A total of 7,443 MGSs were built, across all the habitats as species proxies to reliably assess their habitats. Each circle shows the number of metagenomic species for each habitat, x-axis represents the number of genes in the catalogue specific to each habitat, the y-axis represents the number of samples. Note that differing sampling depth and habitat-specific biodiversity impact those numbers.