Under certain growth conditions some microorganisms secrete organic acids into the extracellular medium to relieve the accumulation of excess energy carriers, and/or to reduce toxic concentrations of organic acids. For example, a glycogen-deficient ∆glgC mutant of the cyanobacterium Synechococcus sp. PCC 7002 secretes pyruvate, acetate, α-ketoglutarate, α-ketoisocaproate and succinate (Davies et al., 2014; Jackson et al., 2015). Secretion of these organic acids functions as a putative energy-spilling mechanism in the absence of glycogen, the major carbon and reductant sink in this organism. Identification of secreted organic acids can facilitate the design of metabolic engineering strategies that funnel over-accumulating organic acids towards metabolic pathways that make a product of interest (such as a biofuel). Here, we describe a method for analyzing secreted organic acids in the extracellular media using high-performance liquid chromatography (HPLC). This method was developed for analysis of organic acids secreted by photosynthetic microbes (cyanobacteria and algae) into media, but could be used to analyze organic acids secreted by any microorganism cultivated in liquid medium.
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