As a carbon-storage compound and osmoprotectant in brown algae, mannitol is synthesized and then accumulated at high levels in Saccharina japonica (Sja); however, the underlying control mechanisms have not been studied. Our analysis of genomic and transcriptomic data from Sja shows that mannitol metabolism is a cyclic pathway composed of four distinct steps. A mannitol-1-phosphate dehydrogenase (M1PDH2) and two mannitol-1-phosphatases (M1Pase1 and MIPase2) work together or in combination to exhibit full enzymatic properties. Based on comprehensive transcriptomic data from different tissues, generations, and sexes as well as under different stress conditions, coupled with droplet digital PCR (ddPCR) and proteomic confirmation, we suggest that SjaM1Pase1 plays a major role in mannitol biosynthesis and that the basic mannitol anabolism and the carbohydrate pool dynamics are responsible for carbon storage and anti-stress mechanism. Our proteomic data indicate that mannitol metabolism remains constant during diurnal cycle in Sja. In addition, we discover that mannitol-metabolism-associated (MMA) genes show differential expression between the multicellular filamentous (gametophyte) and large parenchymal thallus (sporophyte) generations and respond differentially to environmental stresses, such as hyposaline and hyperthermia conditions. Our results indicate that the ecophysiological significance of such differentially expressed genes may be attributable to the evolution of heteromorphic generations (filamentous and thallus) and environmental adaptation of Laminariales.
甘露醇（mannitol）作为褐藻中的储碳化合物和渗透保护剂，在海带（Sja）中合成并大量积累但其潜在的控制机制尚不清晰。本研究对海带基因组和转录组数据的分析表明，甘露醇代谢是由四个不同步骤组成的循环途径。一种甘露醇-1-磷酸脱氢酶（M1PDH2）和两种甘露醇-1-磷酸酶（M1Pase）共同作用呈现出完整的酶学特性。根据不同组织、不同世代、不同性别以及不同胁迫条件下的转录组数据，结合微滴式数字PCR（ddPCR）和蛋白质组数据验证，发现SjaM1Pase1在甘露醇生物合成过程中起主要作用，而基础甘露醇合成代谢和碳水化合物池动态变化可能负责碳储存和抗压力机制。蛋白质组学数据表明，甘露醇代谢在海带的昼夜周期中保持不变。此外，研究还发现甘露醇代谢相关基因（MMA）在多细胞丝状体（配子体）和大型薄壁叶状体（孢子体）世代之间呈现表达差异，并对环境胁迫（如低盐和高温条件）有不同的响应。本研究结果表明，这些差异表达基因的生理生态意义可能与海带异形世代（丝状体和叶状体）的进化和海带目藻类的环境适应有关。





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