The hanging velamen roots in epiphytic orchids are unique kind of aerial roots in the plant world. The diverse kinds of microbial associates in it suggest its ecological and functional complexities. In this context, diversity and population characteristics of the entire algal flora in the velamen roots of four tropical epiphytic orchids Acampe praemorsa, Rhynchostylis retusa, Epidendrum radicans and Oncidium sphacelatum from different geographic locations of Kerala state, India were explored and compared. The potential nitrogen-fixing activity (NFA) of cyanobacteria in the velamen roots was analysed by acetylene reduction assay to understand the nitrogen-fixing role of cyanobacterial species in the same as a source of nitrogen to the host. The total nitrogen content of the velamen roots and leaves was compared to generate indirect evidence of accumulation of nitrogen in roots by primary addition through nitrogen fixation. Altogether 24 species of algal associates were isolated from the velamen roots of four epiphytic orchids. A majority of algae were found as cyanobacterial species; many of them are well-known nitrogen-fixing categories. The total tissue nitrogen content of the velamen roots was found significantly higher than that of leaves. The NFA of cyanobacteria in the velamen roots revealed its potential role in nitrogen fixation. This preliminary level empirical evidence is sufficient to consider the velamen roots of epiphytic orchids as an ecologically complex aerial ‘organ ecosystem’ with microbial partnerships for nutritional support, especially nitrogen availability in epiphytic orchids.

附生兰花中垂悬的芦荟根是植物界中独特的气生根。其中各种微生物伴生体表明其生态和功能复杂性。在这种情况下，四种热带附生兰花Acampe praemorsa，Rhynchostylis retusa，Epidendrum radicans和Oncidium phacelatum的芦苇根中整个藻类植物群的多样性和种群特征对来自印度喀拉拉邦不同地理位置的地图进行了探索和比较。通过乙炔还原分析法分析了芦苇根中蓝细菌的潜在固氮活性（NFA），以了解与宿主氮源相同的蓝细菌物种的固氮作用。比较了芦苇根和叶的总氮含量，通过间接固氮，间接添加了根部氮积累的间接证据。从四种附生兰花的velamen根中共分离出24种藻类同伴。发现大多数藻类是蓝细菌物种；大多数藻类是蓝细菌。其中许多是众所周知的固氮类。发现芦苇根的总组织氮含量明显高于叶。藻类根中蓝细菌的NFA揭示了其在固氮中的潜在作用。初步的经验证据足以将附生兰花的天鹅绒根视为具有生态伙伴关系的生态复杂的空中“器官生态系统”，以提供营养支持，尤其是附生兰花中的氮。