Starling Flock Networks Manage Uncertainty in Consensus at Low Cost

Flocks of starlings exhibit a remarkable ability to maintain cohesion as a group in highly uncertain environments and with limited, noisy information. Recent work demonstrated that individual starlings within large flocks respond to a fixed number of nearest neighbors, but until now it was not understood why this number is seven. We analyze robustness to uncertainty of consensus in empirical data from multiple starling flocks and show that the flock interaction networks with six or seven neighbors optimize the trade-off between group cohesion and individual effort. We can distinguish these numbers of neighbors from fewer or greater numbers using our systems-theoretic approach to measuring robustness of interaction networks as a function of the network structure, i.e., who is sensing whom. The metric quantifies the disagreement within the network due to disturbances and noise during consensus behavior and can be evaluated over a parameterized family of hypothesized sensing strategies (here the parameter is number of neighbors). We use this approach to further show that for the range of flocks studied the optimal number of neighbors does not depend on the number of birds within a flock; rather, it depends on the shape, notably the thickness, of the flock. The results suggest that robustness to uncertainty may have been a factor in the evolution of flocking for starlings. More generally, our results elucidate the role of the interaction network on uncertainty management in collective behavior, and motivate the application of our approach to other biological networks.     

Optimized fed-batch fermentation of L-β-hydroxy isobutyric acid by Yarrowia lipolytica

Yarrowia lipolytica KCCM50506, which transforms isobutyric acid to L-#-hydroxy isobutyric acid (L-#-HIBA), was screened. Chemostat cultures were carried out in jar fermentors at dilution rates of 0.02 h^у to 0.12 h^у. L-#-HIBA fermentation-regulating factors were determined to be specific growth rate, and concentrations of glucose and isobutyric acid in fermentor from analysis of steady-state data. The specific productivity of L-#-HIBA increased as the specific growth rate increased, apparently as a growth-associated type of product formation. A fed-batch culture was carried out under optimum conditions where the concentrations of glucose and isobutyric acid in the fermentor were maintained at 23 g l^у and 9 g l^у, respectively. The concentrations of cells and L-#-HIBA obtained at the end of fermentation were 20 g l^у and 49 g l^у, respectively, corresponding to 2.0 and 2.7 times more than concentrations in batch culture.     

Formic acid: Possible prebiotic reducing agent under photochemical conditions

The experiments were conducted to explore the possibility that formic acid could serve as a reducing agent for carbonyl compounds on the prebiotic earth, since it has been shown that formic acid is a product of numerous chemical evolution experiments. The photoreduction of solutions 0.33 M in acetaldehyde or acetone at 2537 Å in 0.167 M formic acid, sodium formate, or both formic acid and sodium formate in a nitrogen atmosphere has been conducted. The formation of the corresponding reduction product, ethanol or isopropyl alcohol, and the disappearance of the reactant were followed as a function of irradiation time by using gas chromatography. Product confirmation was done by NMR analysis and mass spectrometry.