Role of Rhizobium, a plant growth promoting bacterium, in enhancing algal biomass through mutualistic interaction

Rhizobium plays the pivotal role in mutualistic interactions with plants and this study extends this mutualism to several species of green algae. Denaturing gradient gel electrophoresis (DGGE) and 16S rRNA gene clone library experiments of Chlamydomonas reinhardtii, Chlorella vulgaris, Scenedesmus sp., and Botryococcus braunii revealed that the dominant phycosphere bacteria hosted by these green algae were Rhizobium, Mesorhizobium, and Shinella within Rhizobiales, Flavobacterium within Flavobacteriales, and Pseudomonas within Pseudomonadales. When Rhizobium sp., most prevalent and dominant bacterium isolated from C. vulgaris, was co-cultured with green algae, it promoted algal cell count by ∼72%. The qPCR analysis of 16S rRNA, Rhizobium specific rirA and rhtA genes, performed to understand the effect of green algae on growth of Rhizobium sp., showed a significant increase in copy numbers indicating sustained growth of Rhizobium upon co-culture with green algae. Likewise, growth rates of algae and Rhizobium increased by an average of ∼11% and ∼110%, respectively, confirming mutualistic interaction. Considering the presence of Rhizobium sp. in a variety of green algae, this must be a major mutualistic relationship among green algae and this ubiquitous association might serve as a model for elucidating the mutualistic mechanism between green algae and rhizobacter. This interaction could be also utilized in enhancing microalgal biomass, especially slow growing organisms like B. braunii, to augment their bioenergy productivity.