Bioprocess Engineering
1) Enhancing highly viscous fermentations by multiple-phase cultivation technology
By suspending an aqueous culture in an immiscible oil phase, we confine the viscosity-causing mechanisms in aqueous droplets. This mechanism keeps the bulk viscosity manageable, although the aqueous phase may approach semi-solid gel particles. However, the complex multiple-phase characteristics of the new cultivation technology pose serious fundamental and process challenges to bioengineers, especially in predicting the change in droplet size and the phase inversion from W/O to O/W under certain culture conditions. These studies have been mostly limited to xanthan fermentation.
(2) Exploring nitrate-respiring fermentation technology
Oxygen supply often limits the performance of aerobic bioprocesses, due to the poor solubility of oxygen in aqueous media. Many microorganisms are, however, capable of anaerobic respiration using chemical oxidants such as nitrate, nitrite, sulfate, carbonate, ferric ion, etc. Nitrate respiration (such as denitrification and dissimilative ammonification) is among the most well known and energy favorable. In this research we examine the feasibility of enhancing the productivity of oxygen-limiting fermentations by combining aerobic and anaerobic respiration mechanisms. Current example studied is the rhamnolipid production by Pseudomonas. Its highly foaming characteristics restricts the functional aeration rate. The ability of Pseudomonas’s active denitrification is used to improve the fermentation. Medium design has to be modified significantly to accommodate the fundamental change in biochemistry.
(3) Photobioreactor for algal and cyanobacterial culture
We are examining ways of improving light delivery in photobioreactors. Among the many unique products from algal and cyanobacterial culture, the focus of our current study is the protein-layered, sub-micron, hollow, gas vesicles. We have demonstrated their function as ideal gas carriers, e.g. for oxygenation in shear sensitive culture. Other medical applications are being identified.
Environmental Bioengineering
(1) Study of environmental bioprocesses using culture fluorescence
Many fluorophores, intracellular and extracellular, are present in biological processes. The variations of their concentrations are closely linked with the biological activities and, therefore, can be used as effective metabolic and/or process indicators. Among the best studied biofluorophores are NAD(P)H, i.e., the reduced coenzyme nicotinamide adenine dinucleotide and its phosphorylated form. Having both oxidized NAD(P)+ and reduced NAD(P)H forms, coenzymes NAD(P) are the key intermediate electron acceptors in cellular metabolism. Because only the reduced coenzymes are fluorescent, the NAD(P)H fluorescence reflects the intracellular redox state, in addition to the cell concentration. We have used on-line NAD(P)H fluorescence in various environmental studies including biological wastewater treatment, microbial nitrate respiration (i.e. denitrification and dissimilative ammonification) and nitrification. We have studied its application in sludge digestion processes. Systematic scanning for other extracellular biofluorophores is also performed to generate a more complete understanding of the culture fluorescence profile and its relationship to the biological activities.
(2) Microbial hydrocarbon metabolism and bioremediation
Much is known about aerobic hydrocarbon metabolism under laboratory conditions. It is, however, extremely complex and sensitive to the change of environmental conditions. For example, we have observed varied production of rhamnolipids (biosurfactants), poly-3-hydroxyalkanotes, lipases, and even extracellular waxy particles in aerobic culture of Pseudomonas aeruginosa growing on hexadecane. Each of them affects the subsequent bioremediation differently. Some are beneficial, while others are potentially harmful. We thus focus our study on the effects of varying, uncontrollable, environmental conditions on the complex hydrocarbon metabolism. As anaerobic condition inevitably occurs in soil/aquifer systems, we also examine the effects of nitrate respiration, occurring sequentially or simultaneously with the aerobic respiration. Besides advancing our knowledge in the natural biodegradation, we try to develop bioremediation strategies that incorporate the microbial ability in anaerobic respiration using either naturally present or easily delivered chemical oxidants. Because of the significantly higher solubility in water of these oxidants (such as nitrate) than oxygen, the strategies are expected to be more efficient and implementable in the field than the conventional efforts to render the whole contaminated site aerobic.