International Journal of Marine Science, 2025, Vol.15, No.2, 107-117 http://www.aquapublisher.com/index.php/ijms 111 forming the legacy markers of the chromosome. Multiple second- and third-level endosymbiosis greatly expands the diversity of algae, especially those groups containing red algae-derived chloroplasts (such as diatoms, nigra, dinoflagellate, etc.) that have acquired new metabolic abilities through endosymbiosis (Keeling, 2013). Therefore, secondary and tertiary endosymbiosis is a key driver of algae diversity, allowing photosynthesis to span multiple eukaryotic groups. 4.3 Evolutionary paths of photosynthetic pigment combinations in different algae Algae of different lineages differ significantly in photosynthetic pigment composition, which reflects their different endosymbiotic history and ecological adaptation. Green algae and terrestrial plants mainly contain chlorophyll a and b, which perform photosynthesis by capturing blue and red light; red algae contains chlorophyll a and phycoerythrin, making them often red and absorbing blue-green light (Figure 2) (Stadnichuk and Kusnetsov, 2023); brown algae and Nagyl algae contain chlorophyll a, c and the orange-brown pigment fucoxanthin, which often makes them appear brown or golden yellow; dinoflagellates have chloroplasts of various types, some retain red algae-type chloroplasts, containing chlorophyll and chlorophyll, and some contain other algae chloroplasts through tertiary endosymbiosis. Cryptocyanates contain chlorophyll a and c and co-acetic bile protein (drawing from red algae pigments); nude algae (Euglenophyta) contain chlorophyll a and b, similar to higher plants (Ponce-Toledo et al., 2018). The photosynthetic pigment combination of algae is the result of their internal symbiotic origin and environmental adaptation, and different groups optimize the light quality of their respective niches through their respective pigment combinations. Figure 2 Three types of Archaeplastida double membrane chloroplasts. In glaucophytic algae (as well as in photosynthetic amaoebae), the chloroplast retains PBSs and the peptidoglycan layer (PG). Red algae retain PBSs, but the peptidoglycan layer is eliminated. In plastids of Viridiplantae, PBSs and peptidoglycan layers are lost while thylakoids form grana regions (similarly to Prochlorophyta) (Adopted from Stadnichuk and Kusnetsov, 2023) 5 Phylogenetic Relationships of Major Algae Lineages 5.1 Red and green algae: two key branches of primary photosynthetic eukaryotes The Red Algae and the Green Algae are the two main branches of primary photosynthetic eukaryotes (Archaepastida) (the third branch has very few gray algae). On the phylogeny, red and green algae (and Charophyta) occupy independent lineages respectively. Studies have shown that red algae are mostly marine large algae, and the cells contain unique phylobilin, and their phylogenetic history is relatively closed; the green algae lineage is divided into multiple branches, including flagellar green algae, filamentous green algae and chain green algae (the latter is closely related to terrestrial plants). When Strassert et al. (2020) analyzed major algae phyla in the world, they regarded red algae, green algae and gray algae as the earliest photosynthetic branches. Red and green algae show clear separations on both the chloroplast genome and nuclear genome lineage trees (Strassert et al., 2020), and are not among the most recent common ancestors, but both originate from early primitive photosynthetic events (primary endosymbiosis). Therefore, red algae and green algae constitute two basic lineages of eukaryotic photosynthetic organisms, and their comparative analysis helps to understand the evolutionary differences after primary chloroplast acquisition.
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