Modeling microalgae biofilms morphology using a 2-D cellular automaton approach to reveal the combined effect of substrate and light

The microalgal biofilm morphology can influence the bulk transport of nutrients and subsequently affect the growth process of the microalgal biofilm and its final biomass. Light is the energy base for microalgae and profoundly affects their growth, but how light intensity affects the morphology of microalgae biofilms has not been well studied. Here, we have confirmed experimentally and by a 2-D cellular automata model that light and nutrient concentrations together determine the morphology of microalgal biofilms. From the experiments conducted, at a NO3-N concentration of 2 mg/L, the biofilm morphology produced at high light intensities was very rough and granular, whereas at low light intensities, the biofilm was flat and smooth. Meanwhile, at sufficiently high concentrations of NO3-N, a flatter, smoother biofilm was produced regardless of the light intensity. This conclusion was also confirmed using the 2-D cellular automata model. Using the model, we established that the effect of light intensity on biofilm morphology is mainly twofold: the effect on the maximum growth rate of the microalgae and the depth of light penetration into the biofilm. The significance of these two effects changes as the bulk concentration varies, which in turn, leads to the biofilm morphology responding differently to changes in light intensity at different bulk concentrations. Based on the effects of light intensity and bulk concentration, a redefined Thiele module (Th) is proposed for microalgal biofilms. We demonstrate that the compactness of the biofilm decreased logarithmically while roughness increased logarithmically as the Th number increased when Th > 3.