Algal associates and the evidence of cyanobacterial nitrogen fixation in the velamen roots of epiphytic orchids

Abstract

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.

Keywords

• Algal diversity
• Root-associated-algae
• Symbiotic nitrogen fixation
• Tropical epiphytic orchids
• Velamen roots