Evaluation of productive biofilms for continuous lactic acid production.

In white biotechnology research, the putative superiority of productive biofilms to conventional biotransformation processes based on planktonic cultures has been increasingly discussed in recent years. In the present study, we chose lactic acid production as a model application to evaluate biofilm potential. A pure culture of Lactobacillus bacteria was grown in a tubular biofilm reactor. The biofilm system was cultivated monoseptically in a continuous mode for more than 3 weeks. The higher cell densities that could be obtained in the continuous biofilm system compared with the planktonic culture led to a significantly increased space-time yield. The productivity reached 80% of the maximum value 10 days after start-up and no subsequent decline was observed, confirming the suitability of the system for long-term fermentation. The analysis of biofilm performance revealed that productivity increases with the flow velocity. This is explained by the reduced retention time of the liquid phase in the reactor, and, thus, a minor pH drop caused by the released lactic acid. At low flow velocities, the pH drops to a value where growth and production are significantly inhibited. The biofilm was visualized by magnetic resonance imaging to analyze biofilm thickness. To deepen the understanding of the biofilm system, we used a simple model for cell growth and lactic acid production.

Magnetic resonance imaging reveals detailed spatial and temporal distribution of iron-based nanoparticles transported through water-saturated porous media.

The application of engineered nanoparticles (ENP) such as iron-based ENP in environmental systems or in the human body inevitably raises the question of their mobility. This also includes aspects of product optimization and assessment of their environmental fate. Therefore, the key aim was to investigate the mobility of iron-based ENP in water-saturated porous media. Laboratory-scale transport experiments were conducted using columns packed with quartz sand as model solid phase. Different superparamagnetic iron oxide nanoparticles (SPION) were selected to study the influence of primary particle size (d(P)=20 nm and 80 nm) and surface functionalization (plain, -COOH and -NH2 groups) on particle mobility. In particular, the influence of natural organic matter (NOM) on the transport and retention behaviour of SPION was investigated. In our approach, a combination of conventional breakthrough curve (BTC) analysis and magnetic resonance imaging (MRI) to non-invasively and non-destructively visualize the SPION inside the column was applied. Particle surface properties (surface functionalization and resulting zeta potential) had a major influence while their primary particle size turned out to be less relevant. In particular, the mobility of SPION was significantly increased in the presence of NOM due to the sorption of NOM onto the particle surface resulting in a more negative zeta potential. MRI provided detailed spatially resolved information complementary to the quantitative BTC results. The approach can be transferred to other porous systems and contributes to a better understanding of particle transport in environmental porous media and porous media in technical applications.