Environmental DNA COI barcoding for quantitative analysis of protists communities: A test using the Nebela collaris...

Environmental DNA surveys are used for screening eukaryotic diversity. However, it is unclear how quantitative this approach is and to what extent results from environmental DNA studies can be used for ecological studies requiring quantitative data. Mitochondrial cytochrome oxidase (COI) is used for species-level taxonomic studies of testate amoebae and should allow assessing the community composition from environmental samples, thus bypassing biases due to morphological identification. We tested this using a COI clone library approach and focusing on the Nebela collaris complex. Comparisons with direct microscopy counts showed that the COI clone library diversity data matched the morphologically identified taxa, and that community composition estimates using the two approaches were similar. However, this correlation was improved when microscopy counts were corrected for biovolume. Higher correlation with biovolume-corrected community data suggests that COI clone library data matches the ratio of mitochondria and that within closely-related taxa the density of mitochondria per unit biovolume is approximately constant. Further developments of this metabarcoding approach including quantifying the mitochondrial density among closely-related taxa, experiments on other taxonomic groups and using high throughput sequencing should make if possible to quantitatively estimate community composition of different groups, which would be invaluable for microbial food webs studies.