Growth kinetics of microalgae in microfluidic static droplet arrays.

We investigated growth kinetics of microalgae, Chlorella vulgaris, in immobilized arrays of nanoliter-scale microfluidic drops. These static drop arrays enabled simultaneous monitoring of growth of single as well as multiple cells encapsulated in individual droplets. To monitor the growth, individual drop volumes were kept nearly intact for more than a month by controlling the permeation of water in and out of the microfluidic device. The kinetic growth parameters were quantified by counting the increase in the number of cells in each drop over time. In addition to determining the kinetic parameters, the cell-size distribution of the microalgae was correlated with different stages of the growth. The single-cell growth kinetics of C. vulgaris showed significant heterogeneity. The specific growth rate ranged from 0.55 to 1.52 day(-1) for different single cells grown in the same microfluidic device. In comparison, the specific growth rate in bulk-scale experiment was 1.12 day(-1). It was found that the average cell size changes significantly at different stages of the cell growth. The mean cell-size increased from 5.99 ± 1.08 to 7.33 ± 1.3 µm from exponential to stationary growth phase. In particular, when multiple cells are grown in individual drops, we find that in the stationary growth phase, the cell size increases with the age of cell suggesting enhanced accumulation of fatty acids in older cells.

A whole cell biocatalyst for cellulosic ethanol production from dilute acid-pretreated corn stover hydrolyzates.

In this research, a recombinant whole cell biocatalyst was developed by expressing three cellulases from Clostridium cellulolyticum--endoglucanase (Cel5A), exoglucanase (Cel9E), and ß-glucosidase--on the surface of the Escherichia coli LY01. The modified strain is identified as LY01/pRE1H-AEB. The cellulases were displayed on the surface of the cell by fusing with an anchor protein, PgsA. The developed whole cell biocatalyst was used for single-step ethanol fermentation using the phosphoric acid-swollen cellulose (PASC) and the dilute acid-pretreated corn stover. Ethanol production was 3.59 ± 0.15 g/L using 10 g/L of PASC, which corresponds to a theoretical yield of 95.4 ± 0.15%. Ethanol production was 0.30 ± 0.02 g/L when 1 g/L equivalent of glucose in the cellulosic fraction of the dilute sulfuric acid-pretreated corn stover (PCS) was fermented for 84 h. A total of 0.71 ± 0.12 g/L ethanol was produced in 48 h when the PCS was fermented in the simultaneous saccharification and co-fermentation mode using the hemicellulosic (1 g/L of total soluble sugar) and as well as the cellulosic (1 g/L of glucose equivalent) parts of PCS. In a control experiment, 0.48 g/L ethanol was obtained from 1 g/L of hemicellulosic PCS. It was concluded that the whole cell biocatalyst could convert both cellulosic and hemicellulosic substrates into ethanol in a single reactor. The developed C. cellulolyticum-E. coli whole cell biocatalyst also overcame the incompatible temperature problem of the frequently reported fungal-yeast systems.