Current strategies and future prospects for enhancing microbial production of citric acid.

Aspergillus niger and Yarrowia lipolytica are highly important in citric acid (CA) production. To further minimize the cost of CA bio-production using A. niger and Y. lipolytica, some strategies (e.g., metabolic engineering, efficient mutagenesis, and optimal fermentation strategies) were developed to enhance CA production and low-cost carbon sources were also utilized to decrease CA bio-production cost. In this review, we summarize the recent significant progresses in CA bio-production, including metabolic engineering, efficient mutagenesis and screening methods, optimal fermentation strategies, and use of low-cost carbon sources, and future prospects in this field are also discussed, which could help in the development of CA production industry.

[High-resolution Distribution Characteristics of Phosphorous, Iron and Sulfur Across the Sediment-Water Interface of Aha Reservoir].

In situ two-dimension, high-resolution distribution of phosphorus(P), iron(Fe), sulfur(S) from the sediment-water interface was investigated in different areas of Aha Reservoir, a typical sub-deep water lake, using diffusive gradients in thin films(DGT) technique, combining with water chemistry and sedimentary phosphorus speciation analysis, aimed to discuss the distribution and control factors of P-Fe-S in sediments. DGT-P ranged from 0.00 mg·L-1 to 0.43 mg·L-1. DGT-Fe rangd from 0.00 mg·L-1 to 2.83 mg·L-1. DGT-S ranged from 0.00 mg·L-1 to 0.10 mg·L-1. There was no significant correlation among the concentrations of P, Fe, S in the sediment pore water of Aha Reservoir, which is markedly different from the results of previous studies. We hypothesize that the high ratios of Fe/P and Fe/S induced the direct reaction of surplus Fe(Ⅲ) with S2-, yielding FeS and/or FeS2, while P was firmly fixed in sediments by Fe(Ⅲ). NaOH-SRP(ranging between 192 and 604 mg·kg-1) and BD-P(ranging between 143 and 524 mg·kg-1) were the two major fractions of P in sediments. Since the environment for the bottom layer of water in Aha Reservoir is anoxic, the geochemical cycle of Fe and S potentially has a massive impact on the activation and migration of sedimentary P.