Ulrich Soltmann

Freeze gelation: a new option for the production of biological ceramic composites (biocers)

Abstract Freeze gelation was used for immobilization and conservation of living microorganisms in inorganic solids. The irreversible sol–gel transition of a mixture of colloidal silica, ceramic powder and biocomponent by freezing (freeze gelation or freeze casting) enables the production of low-cost, porous, crack-free green bodies with nearly zero shrinkage in which microorganisms are immobilized safely. First investigations to the survival rate, activity and storage ability will be presented for biocers with Bacillus sphaericus and Saccharomyces cerevisiae under the selected freeze and drying conditions. The structure of these new biocers were characterized by SEM micrographs, physical adsorption and mercury porosimetry.

Algae biocers: astaxanthin formation in sol–gel immobilised living microalgae

New biocer materials consisting of the living microalgae cells Haematococcus pluvialis immobilised within modified sol–gel silica layers can produce the metabolic carotinoid dye astaxanthin after being coated on glass then cultivated. The layer stability and viability of the embedded microalgae depends on the preparation, storage, cultivation, stressing and extraction conditions. For example the use of a new optimised culture medium ensures the viability of the immobilised microalgae cells for more than 40 days. The combined application of Fe2+ compounds with NaCl or hydrogen peroxide as stress factors causes a strong increase in astaxanthin formation within the biocer coatings during cultivation. The dye can then be extracted with various organic solvents. When combined with cell-protective additives there is only a moderate decrease in viability after solvent extraction, allowing the immobilised microalgae to be recultivated. In this way the continuous biotechnological production of astaxanthin seems possible in principle.

Alginate/silica hybrid materials for immobilization of green microalgae Chlorella vulgaris for cell-based sensor arrays

Thin layers and patterned dot arrays of sodium alginate containing living microalgal cells were deposited onto glass carriers which were subsequently gelled using amino-functionalized silica sol to obtain reinforced alginate hydrogels. The resulting alginate/silica hybrid materials showed improved stability in salt-containing solutions compared to alginate gels gelled by traditional methods using Ca2+-ions. Cell arrays were patterned by printing nanolitre-scale drops of sodium alginate/cell suspension using a non-contact micro-dosage system which allows the printing of solutions of high viscosity. Cultures of the green microalga Chlorella vulgaris were immobilized within the newly developed alginate/silica hydrogels in order to demonstrate the potential of the hybrid matrix for the design of cell-based detection systems. The herbicide atrazine as well as copper ions have been used as model toxicants. Short-term toxicity tests (exposure time: 1 h) have been carried out using atrazine and changes in chlorophyll a (Chl a) fluorescence were measured by imaging pulse amplitude modulated-fluorometry (Imaging-PAM). C. vulgaris cells immobilized within alginate/silica hydrogels demonstrated a highly reproducible response pattern and compared well to freely suspended cells. Activity and response sensitivity of immobilized cells to atrazine was largely maintained for up to 8 weeks, especially when stored under cool conditions in the dark. Furthermore, immobilized cells could be repeatingly used for short-term toxicity tests as atrazine produces a reversible inhibition of photosynthesis.

Long-term activity of biohybrid coatings of atrazine-degrading bacteria Pseudomonas sp. ADP

The atrazine-degrading bacterial strain Pseudomonas sp. ADP was immobilized by the sol–gel process within thin silica layers coated onto water-retaining carrier materials (expanded clay pellets and scoria). The performance of the obtained biohybrid material has been investigated concerning long-term activity under non-growth conditions. Experiments were run in phosphate buffer containing atrazine (20 mg l−1) as the sole source of carbon and nitrogen. Even after one year of consecutive batch tests, P. ADP immobilized onto expanded clay pellets showed a high atrazine degradation activity. In the course of long-term batch experiments, the average amount of removed atrazine was about 94% during each assay cycle. Staining with CTC revealed that in spite of cultivation under non-growth conditions over a period of one year, immobilized cells were still vital and showed respiratory activity.