Phycosphere bacterial diversity in green algae reveals an apparent similarity across habitats

Phytoplankton and bacteria play the foremost role in primary production and often act in unison in biogeochemical cycling. Studies conducted so far are inconclusive on species specificity of phycosphere bacteria as the overarching function of specific clades of algae-associated bacteria, for instance Roseobacter in sulfur cycling, is widely held. In this study, we attempt to demonstrate the diversity of phycosphere bacteria in phylogenetically divergent unialgal green algae from vastly different environmental samples like soil, freshwater, marine, and wastewater with diatom and cyanobacteria as an outgroup. Diversity analyses using Differential Gel Gradient Electrophoresis (DGGE) revealed the predominant presence of bacteria belonging to Bacteroidetes phylum (46% of all strains). 454 pyrosequencing of selected strains from different habitats not only confirmed the presence of Bacteroidetes (33.1% of total reads) but also revealed the presence of bacteria belonging to α-Proteobacteria (52.6%), all in close association with their host. Majority of those symbiotic bacteria have been classified as Plant Growth Promoting Bacteria (PGPB) including prominent Sphingomonads and Rhizobacter. Results suggest that although host algae might encourage species specific interactions, specific functional traits are prerequisite for proximal adhesion in nutrient-rich phycosphere. While Bacteroidetes is known to have significant role in nutrient cycling through degradation of plant and algal macromolecules and for its attached growth, PGPB have proven symbiosis with plants and the overwhelming presence of these bacteria in green algae points to possible co-evolution.