Biogeography of six species in the planktonic diatom genus Bacteriastrum (Bacillariophyta)

ABSTRACT Marine planktonic diatom species can exhibit contrasting distribution patterns, from endemic to cosmopolitan. Endemicity is counter-intuitive for planktonic species given their potentially large population sizes and ample migration opportunities by means of ocean currents. Here, we analyse the biogeography of six species of the diatom genus Bacteriastrum with apparently contrasting distribution patterns. Occurrence data obtained from metabarcode reads in samples from Ocean Sampling Day (OSD) sites and from Tara Oceans stations as well as from their observational records included in biodiversity data repositories of GBIF and OBIS were plotted in occurrence maps. According to metabarcoding data, the six species examined here occur all over the temperate and tropical parts of the oceans. Observational records corroborate this finding for B. elegans, B. furcatum, B. hyalinum and B. mediterraneum. Instead, atypical colony former B. jadranum is encountered at just a few distant sites, and solitary B. parallelum only in the Gulf of Naples. The metabarcoding data reveal that the latter two species are far more widely distributed than their actual sightings according to GBIF and OBIS, and that B. hyalinum also occurs in the Arctic. The most likely reasons for the discrepancies among the different data sources are the relatively recent description of B. jadranum and B. parallelum, their diminutive and inconspicuous habitus, and their possible misidentification in routine plankton counting. HIGHLIGHTS • Metabarcoding data of Tara Oceans and OSD discern Bacteriastrum species.• Pan-oceanic distribution of Bacteriastrum species according to global metabarcoding data.• Distribution patterns of phytoplankton species can be inferred from global metabarcode datasets.