Photosynthetic capacity and pigment distribution of a siphonous green alga, Dichotomosiphon tuberosus.

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.