Systemic analysis of the response of Aspergillus nigerto ambient pH

Abstract

Background: The filamentous fungus Aspergillus niger is an exceptionally efficient producer of organic acids, which is one of the reasons for its relevance to industrial processes and commercial importance. While it is known that the mechanisms regulating this production are tied to the levels of ambient pH, the reasons and mechanisms for this are poorly understood.

Effect of Sucrose Concentration on Oxalic Acid Biosynthesis by Aspergillus Niger

“…Only in the cultures with higher initial substrate amount, 175 and 200 g·dm -3 , citric acid concentration grew constantly to the end of the process. Production of gluconic acid by A. niger is a process of oxidation of available glucose into unavailable to other organisms compound that could be used later as a carbon source for oxalic acid synthesis in the case of lack of easier assailable sources [Andersen et al 2009, Poulsen et al 2012. Formation of gluconic acid is indicated as the main disadvantage of usage of sucrose and glucose as carbon sources in bioprocesses with A. niger [Strasser et al 1994, Cameselle et al 1998, Walaszczyk et al 2014.…”

“…This last process is the most efficient at pH 5.0-5.5, in which many bacteria have their optimum of growth. The formed gluconic acid is later used by A. niger as a carbon source [Andersen et al 2009, Poulsen et al 2012.…”

pH Signaling in Human Fungal Pathogens: a New Target for Antifungal Strategies

“…The signaling pathways mediating responses to pH therefore play a key role in the cell biology of these organisms. Neutral-alkaline sensing relies mainly on the pathway called PacC in filamentous fungi and Rim101 in yeasts (7,(13)(14)(15)(18)(19)(20)(21). This pathway is functional in deuteromycetes, ascomycetes, and basidiomycetes and is well conserved, although marked differences have been noted between these three groups of fungi (20,22,23).…”

Organic acid production in Aspergillus niger and other filamentous fungi

“…Nevertheless, the viewpoint that citrate is solely an overflow metabolite is changing, and A. niger citrate secretion might also be regarded as a response to environmental conditions, such as competition (Andersen et al, 2009), or low iron bioavailability (Odoni et al, 2017b). While large scale production of citrate can probably be attributed to a combination of different mechanisms, increased understanding of all the various aspects that lead to increased citrate production can provide tools to further control and modulate citrate production.…”

“…The lack of oxalate at lower pH levels, on the other hand, can be explained by the finding that when the culture medium is below pH 2.5, oxalate decarboxylase, the enzyme that degrades oxalate to CO 2 and formate ( Figure 3.1), is synthesised (Emiliani and Bekes, 1964). In a systems level approach, Andersen et al (2009) formulated the hypothesis that the sequential production of organic acids by A. niger, and specifically oxalate and citrate, leads to the most efficient acidification of the medium based on the external pH. As stated by the authors, continuous acidification of the environment provides a means to effectively outcompete other organisms that are not able to thrive at a low pH, thereby providing an evolutionary advantage for the fungus.…”