Organellar Genomes of Three Globally Important Nanoplanktonic Diatoms Refine Their Taxon-Specific Distribution and Succession Patterns in the Northwest Atlantic.

Nanoplanktonic diatoms (2-20 μm) are a significant yet historically understudied component of marine ecosystems. We investigated three recently isolated nanoplanktonic diatoms from the Northwest Atlantic Ocean (NWA): Minidiscus spinulatus, Mediolabrus comicus, and Minidiscus trioculatus. Using Oxford Nanopore sequencing, we assembled and annotated their complete chloroplast and mitochondrial genomes. Pangenome analyses revealed that Minidiscus species consistently clustered more closely with select Thalassiosira species, whereas M. comicus formed a sister clade with Skeletonema. Circularized chloroplast genomes allowed us to characterize the full-length 16S ribosomal RNAs for each isolate, thereby leading to higher resolution of these taxa in preexisting 16S metabarcoding data. During our study, M. spinulatus was primarily restricted to the Bedford Basin. In contrast, both M. trioculatus and M. comicus had larger geographic ranges extending to the Labrador Sea, and in the case of M. comicus, to the Canadian Arctic Gateway. Weekly metabarcoding from the coastal Bedford Basin, N.S., Canada (2014-2022), revealed a seasonal succession of nanoplanktonic taxa, with Minidiscus trioculatus dominating in the early months, followed by M. comicus and M. spinulatus. Our results highlight the critical value of phytoplankton isolations and organelle genomics for expanding our understanding of the diversity and biogeography of nanoplanktonic diatoms.