Exploring the potential of algae/bacteria interactions

Highlights

• Complex interactions exist between algae and bacteria.

• Algae suppress excess biofilm formation on their surfaces.

• Algae and bacteria communicate with each other using chemicals.

• Self-organized algae/bacteria biofilms enable light/electricity conversion.

• Studies on designed consortia will create new biotechnology.

Algae are primary producers in aquatic ecosystems, where heterotrophic bacteria grow on organics produced by algae and recycle nutrients. Ecological studies have identified the co-occurrence of particular species of algae and bacteria, suggesting the presence of their specific interactions. Algae/bacteria interactions are categorized into nutrient exchange, signal transduction and gene transfer. Studies have examined how these interactions shape aquatic communities and influence geochemical cycles in the natural environment. In parallel, efforts have been made to exploit algae for biotechnology processes, such as water treatment and bioenergy production, where bacteria influence algal activities in various ways. We suggest that better understanding of mechanisms underlying algae/bacteria interactions will facilitate the development of more efficient and/or as-yet-unexploited biotechnology processes.

Introduction

Algae are phototrophs that occur in freshwater and marine environments. They vary from small unicellular microalgae, such as cyanobacteria and diatoms, to large multicellular macroalgae, such as giant kelp. They are primary producers that synthesize organic compounds from carbon dioxide, thereby supporting heterotrophic organisms (consumers) that decompose organics and recycle elements. Ecological studies have identified specific phylogenetic groups of heterotrophic bacteria to occur in close association with specific algae [1]. For instance, Lachnit et al. analyzed biofilm communities attaching onto three different species of macroalgae in different seasons, suggesting that marine macroalgae harbor species-specific and temporally adapted epiphytic bacterial biofilms on their surfaces [2]. In addition to biofilm bacteria, algal exudates may also influence planktonic organisms (e.g., heterotrophic bacteria) in the vicinity of algae, and the term ‘phycosphere’ has been coined to describe a region where algal exudates are influential upon co-occurring organisms [3]. A number of studies have demonstrated specific combinations of algae and bacteria to occur in phycospheres, suggesting the presence of their specific interactions. To cite an instance, two heterotrophic bacterial phyla, Proteobacteria (e.g., Roseobacter and Sulfitobacter) and Bacteroidetes (e.g., Cytophaga and Flavobacterium), appear to be consistently associated with diatoms [1, 4]. On the other hand, it has also been shown that heterotrophic bacteria influence algal behaviors in various ways, including the stimulation of growth, morphogenesis, spore germination, and colonization [5]. These interactions are of ecological and biogeochemical importance, since they are considered to be fundamental factors in shaping aquatic communities.

Algae are also the focus of research for their application to biotechnology processes, such as water treatment and bioenergy production [6]. Bacteria may co-occur in these processes and affect biotechnological outcomes in various ways [7]. Moreover, studies have attempted to exploit algae/bacteria interactions in designed consortia, such as those for microbial solar cells (MSCs) [8^••]. The design of microbial consortia is a recently proposed direction in biotechnology; for instance, an article has suggested that a synthesis of knowledge from studies of sophisticated natural interactions is expected to create an exciting tool for synthetic biology in which the assembled parts are not just genes, but organisms and communities [9]. Although such examples are currently very limited, complex algae/bacteria interactions are considered to provide attractive opportunities for synthetic biotechnology.

In the present article, we first review algae/bacteria interactions in the natural environment that are potentially relevant to biotechnology processes. We next summarize biotechnology processes that exploit consortia of algae and bacteria with a particular focus on designed consortia for MSCs. On the basis of these instances, this article suggests that a better understanding of mechanisms underlying algae/bacteria interactions will facilitate the development of more efficient and/or as-yet-unexploited biotechnology processes.