Microscale profiling of photosynthesis‐related variables in a highly productive biofilm photobioreactor

Abstract

In the present study depth profiles of light, oxygen, pH and photosynthetic performance in an artificial biofilm of the green alga Halochlorella rubescens in a porous substrate photobioreactor (PSBR) were recorded with microsensors. Biofilms were exposed to different light intensities (50-1,000 μmol photons m(-2) s(-1) ) and CO2 levels (0.04-5% v/v in air). The distribution of photosynthetically active radiation showed almost identical trends for different surface irradiances, namely: a relatively fast drop to a depth of about 250 µm, (to 5% of the incident), followed by a slower decrease. Light penetrated into the biofilm deeper than the Lambert-Beer Law predicted, which may be attributed to forward scattering of light, thus improving the overall light availability. Oxygen concentration profiles showed maxima at a depth between 50 and 150 μm, depending on the incident light intensity. A very fast gas exchange was observed at the biofilm surface. The highest oxygen concentration of 3.2 mM was measured with 1,000 μmol photons m(-2) s(-1) and 5% supplementary CO2. Photosynthetic productivity increased with light intensity and/or CO2 concentration and was always highest at the biofilm surface; the stimulating effect of elevated CO2 concentration in the gas phase on photosynthesis was enhanced by higher light intensities. The dissolved inorganic carbon concentration profiles suggest that the availability of the dissolved free CO2 has the strongest impact on photosynthetic productivity. The results suggest that dark respiration could explain previously observed decrease in growth rate over cultivation time in this type of PSBR. Our results represent a basis for understanding the complex dynamics of environmental variables and metabolic processes in artificial phototrophic biofilms exposed to a gas phase and can be used to improve the design and operational parameters of PSBRs.

Microalgae production in a biofilm photobioreactor

“…This results in the accumulation of hydroxide (OH -) and an increase of the pH inside the biofilm (Li et al 2015a;Li et al 2016). Using DIC, however, the shifts in the chemical equilibrium of DIC species can be ignored, significantly decreasing the number of balances.…”

“…This is caused by continuous biofilm growth and the concomitant increase of self-shading (Li et al 2015a;Li et al 2016). Based on the observation that the respiration rate decreases upon prolonged starvation for microalgae (Ruiz-Martinez et al 2016), plants (Gary et al 2003) and bacteria (Hoehler and Jorgensen 2013;Riedel et al 2013); we believe that prolonged darkness will decrease maintenance related respiration as a starvation response.…”

“…These studies focused on nutrient limitations inside the biofilm by including diffusion and convection driven nutrient transport through the biofilm (Li et al 2015a;Li et al 2016). Other studies have included phototrophic growth in multispecies biofilm models, studying inter-species interactions in mixed biofilms under continuous light (Cole et al 2014;Munoz Sierra et al 2014) and day/night light regimes (Wolf et al 2007).…”

Biotechnological potential of an isolated and a known green microalgal strain and their cultivation in biofilm based reactors

“…These showed that light penetrates only until a certain depth into the biofilm and light intensity decreases steeply in the top layers (100-200 µm) [72,114]. Based on these findings we could suspect that the top cell layers absorbed the major part of light, which caused astaxanthin production and accumulation in these cell layers, while the inner cell layers continue dividing due to the optimal (decreased) light intensities.…”

“…Based on these findings we could suspect that the top cell layers absorbed the major part of light, which caused astaxanthin production and accumulation in these cell layers, while the inner cell layers continue dividing due to the optimal (decreased) light intensities. Based on the findings of Li et al [114], which demonstrated that light penetration depth is independent of the light intensity, consequently further increase of the light would not cause drop in the biomass production either. However, the so called "dark zones" can form when the biofilm thickness is over 400 µm, where respiration and consequently biomass loss can occur.…”