Response of Spring Diatoms to CO2 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 CO2 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 CO2 levels. A short-term (3-day) incubation experiment was carried out on-deck using surface Oyashio waters under different pCO2 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 pCO2 levels. These results suggest that CO2 fixation capacity of diatoms decreased rapidly under elevated CO2 levels. In the high CO2 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 CO2 treatment, contributions of Bacillariaceae also increased together with other eukaryotes. These suggest that changes in CO2 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 CO2 levels in terms of their community composition, diversity, and photosynthetic physiology.下一代测序技术（Next-generation Sequencing, NGS）能够以高分辨率解析浮游植物群落组成。然而，目前鲜有研究采用该技术探究自然浮游植物群落对环境变化的响应。本研究通过对硅藻特异性rbcL基因（DNA）进行NGS分析，评估了北太平洋西部亲潮区域春季硅藻对不同CO₂水平的响应，该基因编码核酮糖二磷酸羧化酶/加氧酶（RubisCO）的大亚基；同时还检测了硅藻中rbcL转录本（cDNA）的丰度与组成，以探究它们对CO₂浓度变化的生理响应。2011年5月，我们使用亲潮区域表层海水，在不同CO₂分压（180、350、750和1000 μatm）条件下开展了为期3天的甲板原位培养实验。培养期间，硅藻特异性rbcL基因的转录本丰度随海水CO₂分压升高而降低。上述结果表明，在CO₂浓度升高的条件下，硅藻的CO₂固定能力会快速下降。在高CO₂处理组（750和1000 μatm）中，相较于对照组（350 μatm），硅藻特异性rbcL基因及其转录本的多样性均出现下降，角毛藻科、海链藻科和脆杆藻科在总群落中的占比也随之降低，但棒杆藻科的占比有所上升；在低CO₂处理组中，棒杆藻科与其他真核生物的占比均有所上升。上述结果表明，CO₂浓度变化能够改变亲潮区域春季硅藻的群落组成。总体而言，NGS技术使我们能够从群落组成、多样性以及光合生理等多个维度，更深入地理解硅藻对CO₂浓度变化的响应。