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

Highlights

• Phycosphere bacterial diversity of 11 algal strains from different habitats by DGGE.

• DGGE & pyrosequencing revealed the dominance of α-Proteobacteria and Bacteroidetes.

• Algal-bacterial interactions might not be species specific.

• Algae might prefer certain bacterial clades with specific functional traits.

• Results of this study has ecological and biotechnological implications.

Abstract

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.

Introduction

Axenic cultures only exist under strict laboratory conditions and even in relatively non-sterile environment, algae would get contaminated, in other words, find some associates to live with [2], [4]. In nature, algae depend on bacteria and bacteria exploit algae resulting in a dynamic role in biogeochemical cycling [3]. In specific cases, algae completely depend on bacteria for growth and division [4], [11]. Most algae, especially green algae, can survive on their own but depend on bacteria to grow faster as the growth rates of axenic cultures are much slower than xenic cultures [6]. Noticeably, in each study, the mechanism of interaction has been found to be different. This leads us to question the nature of algal–bacterial interactions. Some studies reveal that algal–bacterial interactions are species specific [10], [12], while other studies indicate that certain communities like Bacteroidetes and Proteobacteria are dominant among diatoms and dinoflagellates [1], [13], [25]. Amidst the inconclusiveness of the nature of interactions, either species specific or generic, in diatoms and dinoflagellates, there is little evidence of nature of bacterial diversity in green algae, considering their commercial importance.

Furthermore, most studies on bacterial diversity in algae are rightly focused on marine environment, considering their importance to biogeochemical cycling [1], [28]. In studies on marine algae alone, there have been suggestions of species specific interactions yet clades like Roseobacter have been identified as dominant clade [25], [28]. Our recent studies on bacterial assemblages in phycosphere of green algae from freshwater sources revealed their role in algal growth and metabolism, and in wastewater treatment, and thereby their applicability to microalgal biotechnology [6], [7], [16], [17]. Hence in this study, we have focused on the phycosphere bacterial diversity of phylogenetically and ecologically diverse green algae, isolated from lentic and lotic freshwater systems, soil, wastewater, and marine waters. The endeavor is to investigate whether these interactions are species specific and habitat influenced or dominated by certain functional or phylogenetic clades, irrespective of the ecosystem they have been isolated from. Furthermore, this study might throw light on some important questions on a possible co-evolution by bacteria with both algae and land plants as the latter also harbor bacteria in its roots for nutrient exchange [24].