Due to limited mixing capacities, heterogeneities regularly occur when scaling-up bioreactors
for large-scale production. Microbial cultures are continuously exposed to local gradients in
fundamental process parameters such as substrate, pH, temperature and dissolved oxygen DO
concentration. These micro-environmental fluctuations may have detrimental effects on
cellular growth, metabolism and morphology, depending on the nature, intensity, duration
and/or frequency of the fluctuations encountered. The aim of this study was to investigate the
impact of pH and DO fluctuations on the dynamic behavior of Yarrowia lipolytica, a
microorganism with a promising biotechnological potential, at both morphological and
metabolic levels. For this purpose, batch and continuous cultivations modes were
preferentially adopted, as it enabled respectively, the study of the stress response of yeast
populations growing at their maximum specific rate, and at various controlled specific growth
rates in physiological steady-states. In addition, an important effort was devoted to the
development and validation of morphological methods in order to acquire quantitative
characterization of the response dynamics at the population scale. The macroscopic behavior
of Y. lipolytica was assessed through examining the patterns of growth, viability, glucose
uptake, oxygen consumption, organic acid and carbon dioxide production rates. Changes in
the yeast morphology were characterized at the cell population level by means of flow
cytometry, morphogranulometry and diffraction light scattering techniques. The results
reflected no significant effect of pH and DO fluctuations on the macroscopic behavior
(specific rates, yields, viability) of the yeast. Nevertheless, mycelial growth was induced upon
exposure to both stressors, only in glucose-excess environments, suggesting therefore an
impact of glucose levels on the regulation of dimorphic transition in Y. lipolytica. Controlling
residual glucose concentrations in Y. lipolytica fermentations may contribute to a better
monitoring of its morphological changes in response to environmental stimuli. Such data
would help to optimize bioprocess performances at the industrial scale since it alleviates
physico-chemical impacts due to filamentous cells. |
Due to limited mixing capacities, heterogeneities regularly occur when scaling-up bioreactors
for large-scale production. Microbial cultures are continuously exposed to local gradients in
fundamental process parameters such as substrate, pH, temperature and dissolved oxygen DO
concentration. These micro-environmental fluctuations may have detrimental effects on
cellular growth, metabolism and morphology, depending on the nature, intensity, duration
and/or frequency of the fluctuations encountered. The aim of this study was to investigate the
impact of pH and DO fluctuations on the dynamic behavior of Yarrowia lipolytica, a
microorganism with a promising biotechnological potential, at both morphological and
metabolic levels. For this purpose, batch and continuous cultivations modes were
preferentially adopted, as it enabled respectively, the study of the stress response of yeast
populations growing at their maximum specific rate, and at various controlled specific growth
rates in physiological steady-states. In addition, an important effort was devoted to the
development and validation of morphological methods in order to acquire quantitative
characterization of the response dynamics at the population scale. The macroscopic behavior
of Y. lipolytica was assessed through examining the patterns of growth, viability, glucose
uptake, oxygen consumption, organic acid and carbon dioxide production rates. Changes in
the yeast morphology were characterized at the cell population level by means of flow
cytometry, morphogranulometry and diffraction light scattering techniques. The results
reflected no significant effect of pH and DO fluctuations on the macroscopic behavior
(specific rates, yields, viability) of the yeast. Nevertheless, mycelial growth was induced upon
exposure to both stressors, only in glucose-excess environments, suggesting therefore an
impact of glucose levels on the regulation of dimorphic transition in Y. lipolytica. Controlling
residual glucose concentrations in Y. lipolytica fermentations may contribute to a better
monitoring of its morphological changes in response to environmental stimuli. Such data
would help to optimize bioprocess performances at the industrial scale since it alleviates
physico-chemical impacts due to filamentous cells. |