Response of Spring Diatoms to CO(2) Availability in the Western North Pacific as Determined by Next-Generation Sequencing

Next-generation sequencing (NGS) technologies have enabled us to determine phytoplankton community compositions at high resolution. However, few studies have adopted this approach to assess the responses of natural phytoplankton communities to environmental change. Here, we report the impact of different CO(2) levels on spring diatoms in the Oyashio region of the western North Pacific as estimated by NGS of the diatom-specific rbcL gene (DNA), which encodes the large subunit of RubisCO. We also examined the abundance and composition of rbcL transcripts (cDNA) in diatoms to assess their physiological responses to changing CO(2) levels. A short-term (3-day) incubation experiment was carried out on-deck using surface Oyashio waters under different pCO(2) levels (180, 350, 750, and 1000 μatm) in May 2011. During the incubation, the transcript abundance of the diatom-specific rbcL gene decreased with an increase in seawater pCO(2) levels. These results suggest that CO(2) fixation capacity of diatoms decreased rapidly under elevated CO(2) levels. In the high CO(2) treatments (750 and 1000 μatm), diversity of diatom-specific rbcL gene and its transcripts decreased relative to the control treatment (350 μatm), as well as contributions of Chaetocerataceae, Thalassiosiraceae, and Fragilariaceae to the total population, but the contributions of Bacillariaceae increased. In the low CO(2) treatment, contributions of Bacillariaceae also increased together with other eukaryotes. These suggest that changes in CO(2) levels can alter the community composition of spring diatoms in the Oyashio region. Overall, the NGS technology provided us a deeper understanding of the response of diatoms to changes in CO(2) levels in terms of their community composition, diversity, and photosynthetic physiology.