Seasonal abundance of major phytoplankton groups in Fram Strait
Since the primer pairs of Bradley and Wolf938 performed similarlyin silico and in mock community tests, we then only used the Wolf938 for subsequent investigations. Using the three primer sets (Wolf964, Stoeck, Wolf938), we generated 18S sequence-based community profiles of eukaryotic microbial communities in Fram Strait collected every spring and/or summer from 2001 to 2011 (except 2003) using sediment traps (Figure 3). Consistent with in silico and mock community tests, the three primer sets showed high variability in sequence abundances and composition. Based on relative abundances at higher taxonomic level (Phylum), the most striking differences mainly manifested on major phytoplanktonic groups, the most significant of which was in the Chlorophyta. For example, in both seasons across years, eukaryotic microbial sequence assemblages reflected by Wolf964 primers were mainly dominated by Stramenopiles (ca. 20%), specifically by diatoms, with the chlorophytes only averaging to ca. 2% to 6% in spring and summer, respectively. In contrast, chlorophytes (Mamiellophyceae, Chlorophyceae, Prasinophyceae, Trebouxiophyceae) were the most dominant taxa in the sequence assemblages obtained by both Stoeck and Wolf938, ranging from 35-42% in spring and both 47% in summer. This major difference in chlorophytes among the datasets could have resulted in the heterotrophic-mixotrophic taxa (i.e., Alevolates, Rhizaria) becoming more dominant in the sequence assemblages generated by Wolf964 primers. For example, large Rhizarian taxa was the third most abundant group in the Wolf964 primers especially during summer (29%), while it only ranged from 2-5% for both Stoeck and Wolf938 datasets across seasons. Interestingly, in contrast to results of in silico test, all primers were able to amplify sequences from Haptophyceae and with no significant differences in relative abundances (t-test, p >0.01) across years and seasons (2-6%), majority of which were classified belonging to Phaeocystales.