Diversity and seasonal dynamics of small dinoflagellates in the Skagerrak, Norway revealed by 454 pyrosequencing and microscopy

Summary

Some dinoflagellates are small and difficult to identify under the light microscope and thus little is known about their seasonal dynamics at the species level. Here we combined high-throughput sequencing and microscopy to explore their diversity and seasonal dynamics, and relate this to environmental factors. We collected 21 monthly samples of nanoplankton (3-45 μm size fraction) from outer Oslofjorden, southern Norway over two years. The 18S V4 ribosomal DNA was amplified from RNA/cDNA and 454-pyrosequenced. The reads were cleaned, denoised and clustered to 99% in Qiime and assigned to taxon. From 46250 reads assigned to Dinoflagellata we detected 367 unique operational taxonomic units (OTUs) assessed to represent species or genera. The most OTU-rich orders were Gymnodiniales (32%), Syndiniales (22%), and Peridiniales (17%), followed by Dinophysiales (8%), Gonyaulacales (7%) and Suessiales (4%). Reads assigned to Gymnodiniales dominated in all samples except June and August 2010 when reads assigned to Suessiales dominated. We observed strong seasonal variation and significant yearly periodicity in dinoflagellate richness and species composition. OTU richness was highest in late autumn and lowest in winter-spring. Seasonal dynamics of potentially toxic taxa were examined. We also assess and discuss the ability to detect potentially toxic dinoflagellate species with 18S V4 high throughput sequencing and compare this with standard microscopical methods and electron microscopy. Potentially toxic species <45 μm observed under the microscope included e.g. Alexandrium pseudogonyaulax, A. ostenfeldii, Azadinium spinosum, Karenia mikimotoi, Karlodinium veneficum and Prorocentrum minimum.