Genomic and meta-genomic insights into the functions, diversity and global distribution of haptophyte algae

Haptophytes are an environmentally important phylum of eukaryotic phytoplankton, forming the second most abundant algal group after diatoms in recent estimates of ocean biodiversity. Haptophytes are phylogenetically and functionally diverse, including globally distributed and bloom-forming calcifying species such as <i>Emiliania</i> and <i>Coccolithus</i>, and non-calcifying orders that may form important components of phytoplankton communities in polar (<i>Phaeocystis, Chrysochromulina</i>) through to sub-tropical latitudes (<i>Pavlova</i>). In this review, we synthesize available phylogenetic, genomic and environmental information concerning the diversity of haptophyte life, considering the origins and placement on the eukaryotic tree; the diversity of the five major orders (Pavlovophyceae, Phaeocystales, Prymnesiales, the CSZ clade, and Isochrysidales); and the contrasting biogeographical distributions of haptophyte groups across different <i>Tara</i> Oceans sampling stations and size fractions. We additionally consider outstanding questions within the fields of haptophyte diversity and biology, particularly in the context of newly discovered and largely uncultured major groups (DPL lineages and Rappemonads), and current gaps in our knowledge of genomic content and niche adaptation across the haptophyte tree.