Phototrophic biofilms are mixed aquatic photo-/heterotrophic microbial communities growing on submerged surfaces exposed to sunlight. Attached photosynthetic microbes generate energy from captured light energy, reduce carbon dioxide, and provide the organic substrates and oxygen that sustain (fuel) the growth of heterotrophic microbes in the biofilms. Phototrophic and heterotrophic microbes produce an extracellular matrix of polymeric substances (EPS). This matrix of adhesives provides the attachment of the micro-organisms to the surfaces they are growing on (substratum), and holds the biofilm together.

Important applications of phototrophic biofilms are found in wastewater treatment, anti-fouling coatings for ship hulls, and CO2-sequestration.