Seki Soichiro, Kobayashi Koichi, Fujii Ritsuko
Graduate School of Science, Osaka City University, 3-3-138, Sugimoto, Sumiyoshi, Osaka, 558-8585, Japan.
Institute for Protein Research, Osaka University, 3-2, Yamadaoka, Suita, Osaka, 565-0871, Japan.
Photosynth Res. 2025 May 21;163(3):30. doi: 10.1007/s11120-025-01148-3.
Dichotomosiphon tuberosus is one of the Bryopsidales, a siphonous, unicellular multinucleate ulvophyte. Bryopsidales typically occur in the ocean and contain unique carbonyl carotenoids siphonaxanthin (Sx) and its ester siphonein (Sn) in their major light-harvesting pigment-protein complexes, allowing them to utilize the green light available in the deep ocean for photosynthesis. However, unlike other Bryopsidales, D. tuberosus occurs in fresh water and is reported to contain Sn but not Sx. D. tuberosus inhabits deep lakes around the world, but in Okinawa, Japan, it inhabits very shallow waterways. Here, we measured the photosynthetic capacity of D. tuberosus collected from Okinawa waterway and compared it with another intertidal Bryopsidale Codium fragile. D. tuberosus had higher photosynthetic electron transport capacity and stronger non-photochemical quenching than C. fragile, consistent with the brighter growth environments for D. tuberosus than C. fragile. We also measured the absorption spectra and the pigment compositions within the photosynthetic pigment-protein complexes from D. tuberosus. Green light absorption of each complex in D. tuberosus was weaker than that in C. fragile. In contrast, Chl b absorption in LHCII was stronger in D. tuberosus than in C. fragile, whereas the opposite was true in photosystems. This implies that a large proportion of the irradiated energy is absorbed by LHCII complex and quenched more efficiently. Our results indicate that the photosynthetic capacity of D. tuberosus is further optimized for higher light environments compared with C. fragile.
块茎二歧管藻是羽藻目植物之一，属于管状体、单细胞多核的石莼类绿藻。羽藻目植物通常生长在海洋中，在其主要的光捕获色素 - 蛋白质复合物中含有独特的羰基类胡萝卜素——管藻黄素（Sx）及其酯类管藻素（Sn），这使得它们能够利用深海中可用的绿光进行光合作用。然而，与其他羽藻目植物不同的是，块茎二歧管藻生长在淡水中，据报道它含有管藻素（Sn）但不含管藻黄素（Sx）。块茎二歧管藻分布于世界各地的深水湖泊中，但在日本冲绳，它生长在非常浅的水道中。在这里，我们测量了从冲绳水道采集的块茎二歧管藻的光合能力，并将其与另一种潮间带羽藻目植物——脆弱仙掌藻进行了比较。与脆弱仙掌藻相比，块茎二歧管藻具有更高的光合电子传递能力和更强的非光化学猝灭，这与块茎二歧管藻比脆弱仙掌藻生长环境更明亮一致。我们还测量了块茎二歧管藻光合色素 - 蛋白质复合物的吸收光谱和色素组成。块茎二歧管藻中每个复合物对绿光的吸收比脆弱仙掌藻弱。相反，块茎二歧管藻中光系统II捕光复合物（LHCII）中叶绿素b的吸收比脆弱仙掌藻强，而在光系统中情况则相反。这意味着大部分照射能量被LHCII复合物吸收并更有效地猝灭。我们的结果表明，与脆弱仙掌藻相比，块茎二歧管藻的光合能力针对更高光照环境进行了进一步优化。
