Dilution rates and their transition modes influence organic acid productivity and bacterial community structure on continuous meta-fermentation using complex microorganisms.

We investigated the effect of dilution rates (D) (0.05, 0.15, and 0.4 h) and its transition mode strategies (constant, up, and down modes) on organic acid productivity and bacterial community structure on continuous meta-fermentation using complex microorganisms. The number of bacterial species decreased with increasing D in the constant mode while up and down modes maintained high and low values, respectively, regardless of the changing D values. Caldibacillus hisashii was the predominant species in all modes at all D values, while other bacterial species, including Anaerosalibacter bizertensis and Clostridium cochlearium were predominant in only certain modes and D values. The highest total organic acid productivity of 3.16 g L h was obtained with 82.2% lactic acid selectivity at D = 0.4 h⁻ in constant mode. Heterofermentation occurred in the up mode, while the down mode exhibited the maximum butyric acid productivity of 0.348 g L h with 43.8% selectivity at D = 0.05 h. The constant, up, and down modes showed the distinct main products of lactic, acetic and formic, and butyric acids, respectively. In this study, we proposed a new parameter of species-specific productivity (SSP) to estimate which species and how much a bacterium quantitatively contributes to the targeted organic acid productivity in continuous meta-fermentation. SSP was determined based on the abundance of functional genes encoding key enzymes from the results of 16S amplicon analysis. In conclusion, D values and their transition modes affect productivity by changing the bacterial community structure, and are a significant factor in establishing a highly productive process in continuous meta-fermentation.