Microbial nar-GFP cell sensors reveal oxygen limitations in highly agitated and aerated laboratory-scale fermentors

Background  

Small-scale microbial fermentations are often assumed to be homogeneous, and oxygen limitation due to inadequate micromixing is often overlooked as a potential problem. To assess the relative degree of micromixing, and hence propensity for oxygen limitation, a new cellular oxygen sensor has been developed. The oxygen responsive E. coli nitrate reductase (nar) promoter was used to construct an oxygen reporter plasmid (pNar-GFPuv) which allows cell-based reporting of oxygen limitation. Because there are greater than 10⁹ cells in a fermentor, one can outfit a vessel with more than 10⁹ sensors. Our concept was tested in high density, lab-scale (5 L), fed-batch, E. coli fermentations operated with varied mixing efficiency – one verses four impellers.     

Temporary suppression of the flash-induced electrical potential across the thylakoid membrane upon energization

Energization of the chloroplast thylakoid membrane causes a temporary decrease in the amplitude of the flash-induced transmembrane electrical potential as monitored by the micro-electrode technique and by the electrochromic absorbance band shift at 518 nm in chloroplasts of Peperomia metallica. This energization-dependent decrease of the flash-induced potential has a relaxation time of recovery in the dark of about 23±4 s. The phenomenon can neither be explained by a decrease of the intrinsic efficiency of photosystem I and II (PSI and PSII) nor by a partial closure of reaction centers of PSI and PSII. This leads us to propose that the energization-dependent decrease of the amplitude of the flash-induced electrical potential is caused by either the formation of a fraction of PSI and/or PSII reaction centers with fast charge recombination or by an increase of the membrane capacitance. The dark recovery after energization of the amplitude of the transmembrane electrical potential and that of non-photochemical fluorescence quenching were found to be comparable, which suggests a common cause for both phenomena.     

Life history and the fitness consequences of imperfect information

The acquisition of information incurs costs in time, energy, exposure to predation, and/or lost opportunity. Without information, however, animals will be unable to assess the costs and benefits of decisions. Obtaining perfect information may be impossible, but how close to perfect do animals need assessments of ecological factors, such as predation risk, before estimation errors affect fitness? A recent article suggested that animals should be tolerant to imperfect information about predation risk, possibly relying on simple rules of thumb. Using a dynamic state variable approach, we examine some of the assumptions underlying this work, and show that tolerance towards imperfect information is dependent on life-history strategy. By changing the relationship between energy and fitness, assumptions about life-history strategies can be modified. Calculations show that tolerance to imperfect information is sensitive to these assumptions with some life histories leading to overestimation, while other life histories result in underestimation. One consistent effect across life histories is that animals with a higher rate of increase in fitness with respect to energy should show greater tolerance to imperfect information.