In the present study, a comprehensive investigation of the dynamic processes in an artificial algal biofilm immobilized on a porous substrate has been conducted. Experimental investigations including microsensor measurements were carried out. For this purpose, the microsensor setup used for profiling submerged biofilms was modified to enable measurement on the investigated biofilms. To achieve an accurate evaluation of the data acquired through microsensor measurements, a new mathematical method was developed, and a 20 μm depth resolution has been suggested for future photosynthetic activity measurements.
After the establishment of the microsensor methods, a systematic microsenser investigation was carried out: the distribution of dissolved oxygen, pH value and photosynthetic productivity profiles of algal biofilms in a porous substrate biofilm photobioreactor (Twin-Layer photobioreactor) exposed to different surface irradiance and/or exposed to different gas phase CO2 concentrations were measured. The results acquired from these experiments offered important insights into the processes in such biofilms: E.g. light penetration depth, maximal dissolved oxygen concentration and pH distribution. The results show, as expected, photosynthesis in the biofilms occurs only near the biofilms surface (i.e. in the illuminated zones), and dark respiration in the inner part of the biofilm could be the reason of the observed biomass productivity decrease with prolonged cultivation. Also, increases in surface irradiance and/or gas phase CO2 concentrations led to an increase in photosynthetic productivity of the investigated biofilm. No photoinhibition was observed in the studied biofilms, although exceptionally high dissolved oxygen concentrations (12 times of that in normal atmosphere) have been recorded.
The model (as described in the 3rd manuscript, Li et al. 2015d) developed in the study has proven to be very effective in predicting experimental observations. The results show clearly the importance of taking into account not only adsorption and scattering, but also the adaptation of the pigment content of the biomass for investigating radiative transfer in PSBR biofilms. Also, through the development of the model, important insights into the dynamic processes in the investigated biofilm were acquired: E.g., it is very likely that the facilitated CO2 transfer plays an important role in inorganic carbon transport in the studied biofilms when the CO2 concentration supplied in the gas phase is low; macronutrients (N and P) do not limit growth even at high surface irradiance and high gas phase CO2 concentrations as long as they are sufficiently supplied in the medium; and the buffering of the medium with a strong buffer will have significant effects on the inorganic carbon availability in the studied biofilm.
Through this study, a solid basis has been established for future investigation on PSBR biofilms. The methods and model developed in this study are established specifically for investigating biofilms grown in the Twin-Layer porous substrate biofilm photobioreactor. However, with minor modifications and/or additional experimental measurements, they can be easily applied to other phototrophic biofilm systems or for investigation and/or optimization of commercial scale systems
