海洋酸化对磷限制条件下海洋固氮蓝藻的影响及其机理

丝状束毛藻和单细胞鳄球藻等优势海洋固氮蓝藻，是大面积寡营养海区中“新氮”的重要来源，其对海洋酸化的响应将显著影响海洋初级生产力。束毛藻和鳄球藻的生长和固氮经常受限于海水中磷等营养盐的缺乏。然而，已有的酸化研究大多采用富营养条件，或在实验技术方面无法实现稳定的磷限制条件。本项目基于申请人前期研究，从酸化和磷限制均可能影响固氮蓝藻的细胞能量代谢这一新角度出发，提出磷限制将加剧酸化对固氮蓝藻的负效应的科学假设。拟通过实验室微观机制探讨与现场观测的结合，运用生物、化学与组学分析等手段，在实验室采用能有效模拟大洋表层海水磷营养盐限制条件的连续培养系统，深入探讨束毛藻和鳄球藻响应酸化的机理；在现场采用分粒级结合营养盐添加和酸化培养的方案，对磷限制条件下酸化对海洋固氮蓝藻的影响及其机理进行系统的阐释，从而提升对全球变化下海洋生物固氮及其生物地球化学效应的认知。

Marine dinitrogen-fixing cyanobacteria, such as the filamentous Trichodesmium and unicellular Crocospheara, are one of the important sources of "new nitrogen" in oligotrophic oceans. Therefore, their responses to ocean acidification can lead to significant changes in marine primary production. Growth and nitrogen fixation of Trichodesmium and Crocospheara are often limited by the deficiency of phosphorus in surface seawaters. However, previous studies are conducted either under nutrient replete conditions or using batch cultures that cannot achieve steady-state phosphorus limitation. Based on our previous research, we hypothesize that phosphorus limitation will augment the negative effect of ocean acidification on marine nitrogen fixing cyanobacteria, as both acidification and phosphorus limitation may affect the energy metabolism in the cells of the diazotrophs. We will carry out both laboratory and field in situ incubation experiments, combined with chemical, biochemical and omics techniques. Particularly, in the laboratory, we will use chemostats to culture Trichodesmium and Crocosphaera, which is challenging but can mimic actual phosphorus limitation in the oligotrophic oceans; in the field, size-fractionated nitrogen fixation rate and diazotroph community will be determined following the on-deck nutrient and acidification manipulated incubation. Our findings should provide a mechanistic insight into how nitrogen fixing cyanobacteria will respond to ocean acidification under phosphorus-limited conditions. This will improve our understandings on marine nitrogen fixation and its biogeochemical impacts in the future oceans.