Fig. 1: Significance of Phaeocystis spp.

a Global abundance of Phaeocystis is in the range of 1–28 % of total marine eukaryotes, on par with well-known algal groups (e.g., diatoms and coccolithophores) and zooplankton. These figures highlight their importance in marine environments as primary producers. Estimates are based on biomass (MAREDAT), total occurrence in unigene collections, and based on genome-mapping of environmental reads (this study). Box-and-whisker plots within the violin plots here and in b show median, interquartile range, and 1.5*IQR values. b Environmental expression of organosulfur compound (DMSP and DMS) biosynthetic genes shows Phaeocystis-specific expression in comparison to other eukaryotic groups. Data from MATOU. c Generalized life cycle of colony-forming Phaeocystis (P. antarctica, P. globosa, and P. pouchetii) with four main morphotypes; two types of scale-forming haploid flagellates; a diploid cell cluster embedded in extracellular matrix, a large colony, and a naked diploid flagellate. In general, colonies form under nutrient-replete conditions in sufficient light (Brussaard et al.²⁴) and enclose photosynthetic non-flagellated cells. Haploid flagellates are associated with colony senescence and decline, are probably involved in sexual reproduction, and represent the life stage that persists through nutrient-deplete conditions. Other Phaeocystis species have been only found as solitary flagellates (P. cordata, P. rex, and P. scrobiculata; reviewed in Andersen et al.⁴⁶). Ploidy and typical morphotype sizes are indicated. d Estimates of lineage divergence times (node bars: 95% HPD) based on the concatenated phylogeny of 17 proteins (10,766 positions). Note the monophyly of polar strains in blue, coinciding with the last Antarctic reglaciation 12 Mya. e Pairwise sequence identity of best blast hits for protein models from Phaeocystis and other algal groups prevalent in the marine environment. Compared to diatoms (Pt × Tp, Phaeodactylum tricornutum vs. Thalassiosira pseudonana), chlorophytes (Cr × Ol, Chlamydonomas reinhardtii vs. Ostreococcus lucimarinus), and pelagophytes (Aa × Ps, Aureococcus anophagefferens vs. pelagophyte CCMP2097), Phaeocystis are a recently divergent group (Pa, Pc, Pg, P. antarctica, P. cordata, P. globosa, respectively). f Protein orthologous group overlap between Phaeocystis reference genomes and other algal groups. Source data are provided as a Source Data file.