Uses of molecular taxonomy in identifying phytoplankton communities from the Continuous Plankton Recorder Survey

In the last 10 years there has been an explosion of new taxa (from species to phyla) observed using DNA-based identification tools. These tools are useful for identifying morphologically challenging and “hidden” taxa to complement light microscopy observations. Frequently, DNA tools, such as metabarcoding are used to identify whole communities of thousands of taxa from a single water sample, at exceptional speed. Here, we show how DNA tools can be used to identify plankton taxa that are challenging to identify microscopically or are impossible to identify from partially degraded, archival continuous plankton recorder (CPR) samples, numbering 500,000, dating back up to 1958. Such methods now allow us to enhance microscopy-based taxonomy by identifying these hidden taxa, and observing how plankton communities have changed over decades. We detail several case studies that have used DNA tools on CPR samples. We discuss future challenges and opportunities for this unique sample archive.