Phototrophic Microbial Communities and Root Anatomical Adaptations of the Leafless Epiphytic Orchid Chiloschista lunifera (Rchb.f.) J.J.Sm

Epiphytic and leafless orchids possess specialized root structures and host diverse associated microorganisms, which may contribute to their adaptation to limited access to water, nutrients, and photosynthetic tissues. Microscopic, anatomical, and molecular analyses of aerial and substrate roots of Chiloschista lunifera (Rchb.f.) J.J.Sm. revealed consistent phototrophic microbial biofilms on the velamen surface. These biofilms comprised filamentous and unicellular cyanobacteria, singular bacterial cells, and green algae. Morphological characterisation identified cyanobacteria belonging to five taxonomic orders, which are Chroococcales, Chroococcidiopsidales, Nostocales, Leptolyngbyales, and Synechococcales. 16S rRNA amplicon sequencing confirmed cyanobacterial dominance, with Chroococcidiopsis thermalis PCC 7203 strongly prevalent in root wash samples (up to 99.99% relative abundance), while root homogenate samples harboured a more diverse assemblage including Phormidiaceae, Leptolyngbya, Scytonema, and Calothrix. In addition, a green alga from Watanabeales (Jaagichlorella sp.) was identified based on morphological characteristics. TEM showed diverse cyanobacterial forms and unicellular green algae with well-developed photosynthetic structures. Root anatomy differed between root types. Substrate roots exhibited an inverted kidney-shaped transverse profile, whereas aerial roots were circular, with differences in velamen distribution reflecting adaptation for water retention and substrate attachment. Branched root hairs occurred on substrate roots, while aerial roots possessed unbranched root hairs, indicating functional specialization. Autofluorescence analysis revealed lignified tissues and abundant cortical chloroplasts, suggesting a photosynthetic role of the roots that may compensate for leaflessness. These findings enhance our understanding of the anatomical and ultrastructural features of epiphytic leafless orchid roots and may support future conservation and propagation efforts.