Abstract
Organic acid accumulation is probably the best-known example of primary metabolic overflow. Both bacteria and fungi are capable of producing various organic acids in large amounts under certain conditions, but in terms of productivity-and consequently, of commercial importance-fungal platforms are unparalleled. For high product yield, chemical composition of the growth medium is crucial in providing the necessary conditions, of which the concentrations of four of the first-row transition metal elements, manganese (Mn2+), iron (Fe2+), copper (Cu2+) and zinc (Zn2+) stand out. In this paper we critically review the biological roles of these ions, the possible biochemical and physiological consequences of their influence on the accumulation of the most important mono-, di- and tricarboxylic as well as sugar acids by fungi, and the metal ion-related aspects of submerged organic acid fermentations, including the necessary instrumental analytics. Since producing conditions are associated with a cell physiology that differs strongly to what is observed under “standard” growth conditions, here we consider papers and patents only in which organic acid accumulation levels achieved at least 60% of the theoretical maximum yield, and the actual trace metal ion concentrations were verified.
Highlights
Among a list of over 300 chemicals that can be produced from biomass and have the potential to be transformed into new type of useful molecules, the top twelve include eight organic acids [1]
Many investigations on the effect of trace metals on organic acid fermentations did not consider the fact that the carbon source and the inorganic medium components all introduce traces of metal ions and their results do not clearly demonstrate whether the further addition of a trace metal ion had an effect or not. For all these reasons we will in this review only consider those papers in which organic acid production was studied under conditions that allowed acid accumulation at least at 60% of the theoretical maximum yield, and in which the actual trace metal ion concentration was verified by chemical analysis
The model case, in which the concentration of certain divalent transition metal ions is critical for organic acid accumulation by fungi, is citric acid
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. For the citric acid fermentation and its impairment by manganese ions it has later been clearly documented that surface cultivation is much less sensitive to the negative influence of trace elements [17] Most of these early studies (and even many of the contemporary ones) were not performed under conditions in which product formation rate and yield could be compared to the industrial organic acid production processes and do not allow conclusions to be drawn as to the occurrence of the observed effects under true producing conditions. For all these reasons we will in this review only consider those papers in which organic acid production was studied under conditions that allowed acid accumulation at least at 60% of the theoretical maximum yield, and in which the actual trace metal ion concentration was verified by chemical analysis
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