RuBisCO (ribulose 1,5-bisphosphate carboxylase/oxygenase) is one the most abundant enzymes on Earth. Virtually all food webs depend on its activity to supply fixed carbon. In aerobic environments, RuBisCO struggles to distinguish efficiently between CO₂ and O₂. To compensate, organisms have evolved convergent solutions to concentrate CO₂ around the active site. The genetic engineering of such inorganic carbon concentrating mechanisms (CCMs) into plants could help facilitate future global food security for humankind. In bacteria, the carboxysome represents one such CCM component, of which two independent forms exist: α and β. Cyanobacteria are important players in the planet’s carbon cycle and the vast majority of the phylum possess a β-carboxysome, including most cyanobacteria used as laboratory models. The exceptions are the exclusively marine Prochlorococcus and Synechococcus that numerically dominate open ocean systems. However, the reason why marine systems favor an α-form is currently unknown. Here, we report the genomes of 58 cyanobacteria, closely related to marine Synechococcus that were isolated from freshwater lakes across the globe. We find all these isolates possess α-carboxysomes accompanied by a form 1A RuBisCO. Moreover, we demonstrate α-cyanobacteria dominate freshwater lakes worldwide. Hence, the paradigm of a separation in carboxysome type across the salinity divide does not hold true, and instead the α-form dominates all aquatic systems. We thus question the relevance of β-cyanobacteria as models for aquatic systems at large and pose a hypothesis for the reason for the success of the α-form in nature.

RuBisCO（核酮糖 1,5-二磷酸羧化酶/加氧酶）是地球上最丰富的酶之一。几乎所有食物网都依赖其活动来提供固定碳。在有氧环境中，RuBisCO 难以有效地区分 CO ₂和 O ₂。为了补偿，生物体进化出了收敛解决方案，将 CO ₂集中在活性位点周围。通过基因工程将这种无机碳浓缩机制（CCM）植入植物中，有助于促进人类未来的全球粮食安全。在细菌中，羧基体代表了这样一种 CCM 成分，其中存在两种独立的形式：α 和 β。蓝藻是地球碳循环中的重要参与者，绝大多数门都拥有β-羧基体，包括大多数用作实验室模型的蓝藻。例外的是海洋原绿球藻和聚球藻，它们在数量上占主导地位的开放海洋系统。然而，海洋系统偏爱α型的原因目前尚不清楚。在这里，我们报告了 58 种蓝细菌的基因组，它们与从全球淡水湖泊中分离出的海洋聚球藻密切相关。我们发现所有这些分离株都具有 α-羧基体并伴有 1A 型 RuBisCO。此外，我们证明 α-蓝藻在全球淡水湖泊中占主导地位。因此，跨越盐度鸿沟的羧基体类型分离的范式并不成立，相反，α-型在所有水生系统中占主导地位。因此，我们质疑β-蓝藻作为整个水生系统模型的相关性，并对α-蓝藻在自然界中成功的原因提出了假设。