Fig. 3: Origin and occurrence of spurious taxa.

a Taxonomic profile and ecological distribution. Inner ring: SILVA-based classification of all non-redundant spurious molecular species at the phylum and family level. Outer colored ring: sample type characterized by the highest prevalence for the given taxon. Outer bars: corresponding highest prevalence values. Only samples with relative abundances >0.25% for any given OTU were counted as positive for prevalence calculation. The total numbers of samples considered were: human, 46,153; soil, 29,864; freshwater, 13,977; mouse, 10,409; marine, 8478. b Distribution of the spurious taxa across sample types. The exclusivity of each OTU for any given sample type was assessed using a Z-test: those assumed to be non-specific for any given sample type appear in red (p < 0.05). The total number of IMNGS samples considered for each sample type with at least one of any spurious taxa matching sequences above 0.25% relative abundance was labeled as “Total” (equal numbers in panel a). The number of samples in each type covered by at least one spurious OTU with highest prevalence in this sample type was labeled as “Covered” (i.e., the remaining samples in that category contained also at least one spurious OTU, which was however characterized by highest prevalence in another sample type). c Redundancy of the spurious taxa across 10 sequencing runs. d Violin plots of the distribution of median relative abundances of all spurious molecular species within each sample type as shown in panel b. The average prevalence of the spurious taxa in each sample category is shown as mean ± SD below the x-axis. e The ZymoBIOMICS DNA Standard was sequenced as such or in combination with DNA extracts of cecal contents from germfree mice with or without pre-treatment for free DNA removal as described in detail in the methods. The stacked bar plots indicate the number of spurious taxa and positive hits in the different sample treatment categories with or without relative abundance filtering following the color codes presented in the figure panel.