Photoautotrophic microalgae based biorefinery concepts are currently not competitive compared to other established production systems. Therefore, innovative upstream processes need to be developed to increase the competitiveness of photoautotrophic microalgae biorefinery concepts. Abiotic sub-lethal stress induction via nanosecond pulsed electric field (nsPEF) treatment might be a viable process to increase the efficiency of photoautotrophic microalgae cultivation. In this work, an increased cell growth after nsPEF treatment was observable. Application of nsPEF to highly proliferating cells in a repetitive process resulted in a statistical significant increase in cell growth (p = 0.009). The effect was most pronounced after five days wherefore cellular structures and processes were analyzed to reveal a possible mechanism. Within this work, a protocol for increased cell proliferation with a possible mechanism was derived, which improves competitiveness of photoautotrophic microalgae biorefineries in the future. However, based on the derived mechanism, the results are also relevant for other microorganisms.

Compared to mechanical extraction methods, pulsed electric field (PEF) treatment provides an energy-efficient and gentle alternative. However, the biological processes involved are poorly understood. The unicellular green microalga Chlorella vulgaris was used as model organism to investigate the effect of PEF treatment on biological cells. A viability assay using fluorescein diacetate measured by flow cytometry was established. The influence of developmental stage on viability could be shown in synchronised cultures when applying PEF treatment with very low specific energies where one part of cells undergoes cell death, and the other part stays viable after treatment. Reactive oxygen species generation after similar low-energy PEF treatment could be shown, indicating that PEFs could act as abiotic stress signal. Most importantly, a cell-death inducing factor could be extracted. A water-soluble extract derived from microalgae suspensions incubated for 24 h after PEF treatment caused the recipient microalgae to die, even though the recipient cells had not been subjected to PEF treatment directly. The working model assumes that low-energy PEF treatment induces programmed cell death in C.vulgaris while specifically releasing a cell-death inducing factor. Low-energy PEF treatment with subsequent incubation period could be a novel biotechnological strategy to extract soluble proteins and lipids in cascade process.

In the last decays pulsed electric field techniques gained increasing importance in biotechnology and food production. Application of electric pulses to cells leads to increased permeability and conductivity of cell membranes and formation of pores in cell membrane - phenomenon called electroporation. Electroporation could be a very suitable technique for improvement of extraction or dying in both food processing and biorefinery. Non-thermal plasma treatment generates reactive species in the samples causing damage of the cell membrane and could be used for wastewater treatment, microbial inactivation and as a treatment prior to open fermentations on complex substrates. In this study, recent advances in pulsed electric field treatment of food, algal biomass and hospital wastewater as well as role of non-thermal plasma treatment in bio-decontamination of complex substrates will be addressed. Also, some environmental and economic aspects of these novel techniques in comparison to conventional treatments will be discussed.

Cultivation-based and DNA-based methods for determining the bacterial load and the composition of the bacterial spectrum have been successfully established for media in electrodip painting, and used for the detailed analysis of the contamination situation in an E-coating system of an automobile plant in Germany. Dominating representatives of the genus Microbacterium spp., the orders Burkholderiales and Pseudomonadales, the family Cytophagaceae and the genera Corynebacterium spp., Sphingomonas spp., and Stenotrophomonas spp. were used for inactivation experiments. Different pulsed electric field (PEF) parameters were studied for an effective and target-directed inactivation of defined bacterial suspensions containing mixtures of Gram-positive as well as Gram-negative bacteria, but also single species suspensions in adequate liquids. PEF treatment with pulse durations longer than 1.0 µs effectively killed bacteria even in low conductivity media, regardless of whether the pulses were unipolar or bipolar, indicating that the choice of pulse shape does not limit the design of the PEF system. Model calculations showed that for efficient treatment in bypass mode, a high treatment flow rate is required rather than a high inactivation efficiency of the PEF treatment. By using specific treatment parameters, such as bipolar pulses of 50 k Vcm⁻¹ and a treatment energy of 40 J mL⁻¹, a significant reduction in both Gram-negative and Gram-positive bacteria (> 2 log₁₀ reduction) can be achieved while minimizing electrode corrosion and coating degradation. PEF treatment proves to be an effective alternative to the use of biocides in an E-coating system and can help maintain a bacteriostatic environment in the system by operating at different points, in transfer flow or bypass mode, ensuring biocide-free operation.