Photosynthetic organisms have various pigments enabling them to adapt to various light environments. Green plants are divided into two groups: streptophytes and chlorophytes. Streptophytes include some freshwater green algae and land plants, while chlorophytes comprise the other freshwater green algae and seawater green algae. The environmental conditions driving the divergence of green plants into these two groups and the changes in photosynthetic properties accompanying their evolution remain unknown. Here, we separated the core antennae of PSI and the peripheral antennae [light-harvesting complexes (LHCs)] in green plants by green-native gel electrophoresis and determined their pigment compositions. Freshwater green algae and land plants have high Chl a/b ratios, with most Chl b existing in LHCs. In contrast, seawater green algae have low Chl a/b ratios. In addition, Chl b exists not only in LHCs but also in PSI core antennae in these organisms, a situation beneficial for survival in deep seawater, where blue-green light is the dominant light source. Finally, low-energy Chl (red Chl) of PSI was detected in freshwater green algae and land plants, but not in seawater green algae. We thus conclude that the different level of Chl b accumulation in core antennae and differences in PSI red Chl between freshwater and seawater green algae are evolutionary adaptations of these algae to their habitats, especially to high- or low-light environments.

SUMMARY We report two red algal species, Chondracanthus saundersii C.W. Schneider & C.E. Lane (Gigartinaceae, Gigartinales) and Schottera koreana M.S. Calderon, T.H. Seo & S.M. Boo (Phyllophoraceae, Gigartinales), from Japan for the first time based on molecular and morphological analyses. A C. saundersii specimen was collected at a depth of 35 m offshore of Mageshima Island, Kagoshima Prefecture, Japan. The rbcL and COI‐5P (cox1) sequences of C. saundersii form a clade with the type specimen from Bermuda and additional specimens from Cuba and Brazil. Morphologically, C. saundersii is characterized by the presence of secondary attachments, compressed to flattened axes up to 1 mm in width, and a looser medulla. Two specimens of S. koreana were collected in lower intertidal to subtidal waters at Ohara, Chiba Prefecture, and Matsuyama, Ehime Prefecture, Japan. The rbcL sequences of S. koreana were closely related to reported sequences of specimens from South Korea including the type specimens. Morphologically, S. koreana is characterized by the presence of unicellular colorless hair developed from cortical cells.