Phototrophic biofilms: diversity, ecology and applications

Phototrophic biofilms are complex conglomerations of light-driven microorganisms as autotrophs with heterotrophs embedded in a mucilaginous matrix comprising EPS (extracellular polymeric substances), attached to a solid surface. EPS provide structural and functional integrity and is essential for the physico-chemical and biological properties of the biofilms. These biofilms thrive in simple to extreme environments and comprise cyanobacteria, diatoms, microalgae, fungi, bacteria and protozoa. Formation of flocs/biofilms is an essential facet of bioremediation and wastewater treatment. Biofilm formation is a scourge in medical sciences, but in agriculture, they can be potent candidates for integrated nutrient and disease management or soil structure improvement, as they aid in better and effective colonization in soil and around roots, enabling a network from soil to the plant. The process of biofilm formation is intriguing and presents a challenge for understanding the signals and metabolites involved and the orchestration of multiple biochemical pathways. Metagenomic analysis of biofilms has unveiled complex genomic data and molecular diversity among culturable and non-culturable microbial communities dwelling in such biofilms; however, information on phototrophic biofilms is scanty. Particle-tracking techniques have shown the significance of water channels in mediating water flow, nutrient cycling and exchange of metabolites within the biofilm community. Proteomic analyses and their bioinformatic delineation have illustrated that in these biofilms, the phototrophic partner is involved in intercellular signaling, aggregation, carbohydrate and amino acid metabolism. The present review focuses on phototrophic biofilm formation, their diversity, applications and ecological roles with special emphasis on agriculture and allied sectors.