A global haptophyte 18S rRNA gene phylogeny including potentially novel lineages, and a curated reference sequence database for taxonomic assignment of metabarcodes.

Microalgae of the division Haptophyta are a major component of the marine nanoplankton and present in all seas. They are important primary producers and grazers of picoplankton in the ocean, and their metabolic products can have an impact on global climate. Due to their small size, species are mostly difficult to distinguish by microscopy. Environmental sequencing studies have revealed a high marine haptophyte diversity. We have constructed a curated haptophyte 18S rRNA-gene reference database comprising c. 970 sequences, mostly >800 bp for taxonomic assignment of environmental sequences and metabarcoding reads. We have taxonomically assigned all sequences by phylogenetic analyses, removed chimeras, updated the taxonomy according to recent revisions, and defined clades without cultured and sequenced representatives. Haptophyta embrace numerous clades potentially representing novel lineages from genus to class level. Here we present current knowledge on their distribution based on available 18S rRNA gene sequences.