Metabarcoding Reveals Temporal Patterns of Community Composition and Realized Thermal Niches of Thalassiosira Spp. (Bacillariophyceae) from the Narragansett Bay Long-Term Plankton Time Series

Abstract

Diatoms generate nearly half of marine primary production and are comprised of a diverse array of species that are often morphologically cryptic or difficult to identify using light microscopy. Here, species composition and realized thermal niches of species in the diatom genus Thalassiosira were examined at the site of the Narragansett Bay (NBay) Long-Term Plankton Time Series using a combination of light microscopy (LM), high-throughput sequencing (HTS) of the 18S rDNA V4 region and historical records. Thalassiosira species were identified over 6 years using a combination of LM and DNA sequences. Sixteen Thalassiosira taxa were identified using HTS: nine were newly identified in NBay. Several newly identified species have small cell diameters and are difficult to identify using LM. However, they appeared frequently and thus may play a significant ecological role in NBay, particularly since their realized niches suggest they are eurythermal and able to tolerate the >25 °C temperature range of NBay. Four distinct species assemblages that grouped by season were best explained by surface water temperature. When compared to historical records, we found that the cold-water species Thalassiosira nordenskioeldii has decreased in persistence over time, suggesting that increasing surface water temperature has influenced the ecology of phytoplankton in NBay.

Irradiance modulates thermal niche in a previously undescribed low-light and cold-adapted nano-diatom

“…When BLASTed, the most common diatom genera were Thalassiosira (eight), Skeletonema (four), and Minidiscus (two)( Table S2). These are consistent with previous observations at Narragansett Bay, where Thalassiosira and Skeletonema often dominate the diatom community (15,40). Of these 20 most abundant recovered ASVs, one was a perfect match (100% ID and 100% coverage) to the 18S rRNA gene sequence of our isolate.…”

“…This diatom was isolated from water collected at the Narragansett Bay Time Series (15) site (latitude 41.47, longitude -71.40) in March, 2018. SST was 2 °C at the time of collection.…”

“…Replicates were kept in exponential phase by diluting cultures with sterile medium when biomass reached a predetermined threshold. Cultures were acclimated to each combination of irradiance (15,30,50,60, 70, 100, and 120 µmol photons m -2 sec -1 ) and temperature (2,8,12,14,16,18,20,22,24,26 °C) for two weeks.…”

“…For these experiments we used ~638 µmol photons m -2 sec -1 , which approximated the highest value measured in a 50-year dataset of surface irradiance from the Narragansett Bay Time Series (https://web.uri.edu/gso/research/plankton/data/). Triplicate cultures acclimated to either 4 °C or 16 °C and 30 µmol photons m -2 sec -1 were exposed to this extreme light level for one, three, or six hours, and compared to triplicate cultures that were not exposed (negative control), or were continuously exposed (positive control Six years of amplicon sequencing data from our study site using diatom-specific primers matching the V4 hypervariable region of the 18S rRNA gene (33) were obtained from (15). Raw sequence data were downloaded from NCBI (PRJNA327394) and quality filtered as for the Illumina sequencing.…”

“…Interactive effects between light and thermal niche for this Chaetoceros isolate from the thermal performance curves (TPCs) at three different irradiance levels (15,30, and 50 µmol photons m -2 sec -1 ) are shown in Figure 3a and Table 1. Here we abbreviate the light treatments as low, optimal, and high light.…”