Delimiting operational taxonomic units for assessing ciliate environmental diversity using small-subunit rRNA gene sequences.

Delineating operational taxonomic units (OTUs) is a central element in any culture-independent analysis of environmental microbial eukaryotic diversity. Previous studies either have not justified their choice in sequence distance used to bin small-subunit ribosomal RNA (SSU rRNA) gene sequences amplified from environmental samples into OTUs, or have used a value based on the average across a broad sampling of microbial eukaryotes. Here, we analyse distances (320 922 pairwise comparisons) among sequences just from identified ciliates, and compare these with their taxonomic hierarchy. Our results show that no single sequence similarity value can always and unambiguously delineate species boundaries and higher taxa. Nevertheless, we suggest the use of 98% similarity to delineate ciliate OTUs because this threshold at least accounts for intra-specific polymorphism among multiple rRNA cistron copies. However, we suggest refraining from reconciling SSU rRNA gene-based OTUs and ciliate morphotypes; these OTUs should be used to analyse ciliate phylotype diversity, not ciliate species diversity.