Improving the outcome of metabarcoding methods with taxonomic expertise

Planktonic organisms are highly diverse and critical for aquatic ecosystem function. However, plankton tend to be small, fragile, difficult to collect, and hard to identify by traditional taxonomic methods. Metabarcoding is a fast, relatively cheap tool for species identification that assigns a taxonomic identification based on DNA sequences obtained from a mixed sample. The key for the metabarcoding approach to work is having a reliable and complete sequence database for taxonomic matching. We subsampled plankton tows, sorted, identified and barcoded one portion (StreamCode database) while processing another for metabarcoding. We compared the outcome of metabarcoding analysis with and without the new barcodes and taxonomic identifications, to determine the extent a targeted effort by a group of taxonomists can impact the outcome of metabarcoding. Phylum assignment and number of families identified for each phylum was improved by the use of the StreamCode database. Additionally, we compared the identity of the families found with each method, or by both. For some well-studied groups, such as copepods, metabarcoding analyses found a higher number of families than the ones identified by the experts. However, for other groups not that well represented in GenBank such as pteropods (sea butterflies) or polychaetes, StreamCode experts were more successful in identifying unique families than metabarcode analyses. The StreamCode effort will contribute with new sequences for families previously not represented in GenBank or other curated databases. Our results highlight that targeted efforts to improve the number of sequences available could go a long way to make metabarcoding methods more effective for assessment of planktonic diversity.