Light Variability Illuminates Niche-Partitioning among Marine Picocyanobacteria

Prochlorococcus and Synechococcus picocyanobacteria are dominant contributors to marine primary production over large areas of the ocean. Phytoplankton cells are entrained in the water column and are thus often exposed to rapid changes in irradiance within the upper mixed layer of the ocean. An upward fluctuation in irradiance can result in photosystem II photoinactivation exceeding counteracting repair rates through protein turnover, thereby leading to net photoinhibition of primary productivity, and potentially cell death. Here we show that the effective cross-section for photosystem II photoinactivation is conserved across the picocyanobacteria, but that their photosystem II repair capacity and protein-specific photosystem II light capture are negatively correlated and vary widely across the strains. The differences in repair rate correspond to the light and nutrient conditions that characterize the site of origin of the Prochlorococcus and Synechococcus isolates, and determine the upward fluctuation in irradiance they can tolerate, indicating that photoinhibition due to transient high-light exposure influences their distribution in the ocean.

原绿球藻（Prochlorococcus）与聚球藻（Synechococcus）这两类微微型蓝细菌（picocyanobacteria）是全球大面积海域海洋初级生产的核心贡献者。浮游植物细胞会被裹挟于水层环流中，因此常暴露于海洋上层混合层内辐照度的快速变化之中。当辐照度出现向上波动时，光系统II（photosystem II）的光失活速率会超过通过蛋白质周转实现的修复速率，进而引发初级生产力的净光抑制，甚至可能导致细胞死亡。本研究表明，各类微微型蓝细菌的光系统II光失活有效截面保持保守，但不同菌株的光系统II修复能力与蛋白特异性光系统II光捕获能力呈显著负相关，且菌株间差异极大。修复速率的差异与两类蓝细菌分离株的来源生境所特有的光照及营养盐条件相匹配，并决定了它们所能耐受的辐照度向上波动幅度，这表明短暂高光暴露引发的光抑制会影响其在海洋中的分布格局。