Comparing Ecological and Genetic Diversity Within the Marine Diatom Genus Pseudo-nitzschia: A Multiregional Synthesis

The globally distributed marine diatom genus Pseudo-nitzschia consists of approximately 40 species, more than half of which occur in US coastal waters. Here, sensitive genetic tools targeting a variable portion of the internal transcribed spacer 1 (ITS1) region of the rRNA gene were used to assess Pseudo-nitzschia spp. diversity in more than 600 environmental DNA samples collected from US Atlantic, Pacific, and Gulf of Mexico waters. Community-based approaches employed genus-specific primers for environmental DNA fingerprinting and targeted sequencing. For the Gulf of Mexico samples especially, a nested PCR approach (with or without degenerate primers) improved resolution of species diversity. To date, more than 40 unique ITS1 amplicon sizes have been repeatedly observed in ITS1 fingerprints. Targeted sequencing of environmental DNA as well as single chains isolated from live samples indicate that many of these represent novel and known inter- and intra-specific Pseudo-nitzschia diversity. A few species (e.g., P. pungens, P. cuspidata) occur across all three regions, whereas other species and intraspecific variants occurred at local to regional spatial scales only. Generally, species frequently co-occur in complex assemblages, and transitions in Pseudo-nitzschia community composition occur seasonally, prior to bloom initiation, and across (cross-shelf, latitudinal, and vertical) environmental gradients. These observations highlight the dynamic nature of diatom community composition in the marine environment and the importance of classifying diversity at relevant ecological and/or taxonomic scales.