Hidden diversity: Resolving annual and seasonal community composition of a diatom genus in Narragansett Bay through long-term data and molecular analysis

Diatoms generate 40-50% of oceanic primary production. Understanding the ecology of individual diatom species is essential for generating predictions of phytoplankton community composition, production, export and phenology. Here, we examine the diatom genus Thalassiosira, using data from the Narragansett Bay Long Term Phytoplankton Time Series. This data set is comprised of weekly microscope counts of phytoplankton, physical water column data and nutrient and Chlorophyll a concentrations. We analyzed these data in conjunction with newly generated high-throughput sequencing data obtained from archived DNA samples. Due to limitations of taxonomic identification using light microscopy, an average of 47% of the Thalassiosira species counted over the past 15 years (1999-2014) could not be identified to the species level. Just three species of Thalassiosira could be identified to the species level in this data set through visual counts. In contrast, taxonomic studies have identified 11 different species from Narragansett Bay. Preliminary results utilizing molecular barcoding at the highly-variable V4 region of the 18S ribosomal gene successfully identified multiple Thalassiosira species from the long term time series over a 6 year time period (2009-2014). The incorporation of new technologies into long-term time series requires extensive validation. A set of experiments designed to examine the extent of 18S copy number variation in the genus Thalassiosira may allow for the integration of microscopy counts and high-throughput sequencing data in a quantitative fashion. Inclusion of methods that provide high-resolution species identification into long term time series has the potential to reveal annual and seasonal variations in species composition that were previously “hidden,” allowing for new insights into the factors that drive both short- and long-term variation in phytoplankton communities.