ARTICLE

Phototrophic biofilms and their applications: towards a unifying concept

What are these brown-green biofilms on submerged surfaces about?

Periphyton has been studied for decades, but never in such detail, with so many analytical tools in parallel. We wanted to learn how complex consortia of small photosynthesizing microbes and bacteria organize themselves and cooperate. Why they prefer to stick together, instead of living free as a swimming individual. How they benefit mutually?

With this knowledge we wanted to predict how phototrophic biofilms develop and behave. For example: behaviour in the sense that such biofilms can be made useful in cleaning (waste) water. While thinking about the modelling concepts, the PHOBIA team carried out experiments in a special reactor.

Now we know how these solar light-driven biofilms react to light, temperature and flow speed of the water. A high flow speed has an adverse effect on the adhesion of biofilm organisms, and in the shade bacteria will adhere rather than photosynthesizing cells. Lag phases in growth become shorter, and growth rates increase as irradiance increases. Biofilm thickness ranges from 50 microns to 2 millimetres, and maximum dry biomass can attain 80 grams per m².

About 80% of the light is absorbed in the top 0.3 millimetres of the biofilm; species composition of the phototrophs has a great influence on the spectral composition inside the films. Oxygen saturation can easily reach 300% saturation in strong light. pH values inside may become 10.

As expected, photosynthesis rates decrease from top to base in the biofilms but the oxygen gradient is inversed, the highest concentration measurable at the base (near the substratum) using microsensors. Respiration, measured with the isotope 18-oxygen, increases with temperature. In the dark, biofilms, thicker than a millimetre, become anoxic. The carbon that the phototrophs synthesize in the light is utilized by them for a major part, for cell synthesis; a small part is passed on to the bacteria in the films, who utilize it very fast (in a few hours). Bacteria are a minority in these biofilms and, as the light intensity increases, bacterial biomass decreases. A part of the photosynthesized carbon is transformed into extracellular polymeric carbon, the glue. Marine biofilms contain far more adhesive substances than freshwater biofilms; these are mostly polysaccharides exuded by the organisms which make a firm cement for the biofilm). The marine biofilms also contain twice as much bacterial biomass. Low light and high temperature increase the biodiversity of bacteria. Diatoms prefer low light. In freshwater biofilms, high light favours coccoid green algae; high temperature favours cyanobacteria. In marine biofilms a succession from green algae towards cyanobacteria occurs, as biofilms mature. In our experiments phototrophic biofilms are intertwined networks of filamentous cyanobacteria, globular and linear colonies of green algae, and chains and tufts of diatoms.

The huge amount of microscope images and analytical data that we collected were used for a conventional mechanistic model, and an artificial neural network model, both newly developed for the project. The latter model uses the PHOBIA data and is trained to learn how the biofilms behave in response to the environment. The mechanistic model assigned most weight to irradiance, depth and temperature, and fitted well the oxygen and pH dynamics, in space and time. The neural network approach concluded that the time to reach half-maximum biomass depends most on irradiance and that maximum growth rates were influenced most by temperature.

The waterboard monitored the efficiency of the nutrient and metal retention; NIOO (the project coordinator) studied reed-stem phototrophic biofilms.

With the field results, and with what was learned from the PHOBIA-incubator experiments and modelling, we conclude that the reed biofilms and the incubator have the same capacity of removing nutritional elements.

With these figures recommendations can be made as to how such constructed wetlands (and reactors) may contribute to the final treatment of nutrient-rich effluent and open waters. This is an important issue in the EU with reference to the Water Framework Directive that demands drastic measures to improve the water quality all over Europe. Thus, bio-fouling can be taught to be useful.