Hydrodynamics, Fungal Physiology, and Morphology

Abstract

Filamentous cultures, such as fungi and actinomycetes, contribute substantially to the pharmaceutical industry and to enzyme production, with an annual market of about 6 billion dollars. In mechanically stirred reactors, most frequently used in fermentation industry, microbial growth and metabolite productivity depend on complex interactions between hydrodynamics, oxygen transfer, and mycelial morphology. The dissipation of energy through mechanically stirring devices, either flasks or tanks, impacts both microbial growth through shearing forces on the cells and the transfer of mass and energy, improving the contact between phases (i.e., air bubbles and microorganisms) but also causing damage to the cells at high energy dissipation rates. Mechanical-induced signaling in the cells triggers the molecular responses to shear stress; however, the complete mechanism is not known. Volumetric power input and, more importantly, the energy dissipation/circulation function are the main parameters determining mycelial size, a phenomenon that can be explained by the interaction of mycelial aggregates and Kolmogorov eddies. The use of microparticles in fungal cultures is also a strategy to increase process productivity and reproducibility by controlling fungal morphology. In order to rigorously study the effects of hydrodynamics on the physiology of fungal microorganisms, it is necessary to rule out the possible associated effects of dissolved oxygen, something which has been reported scarcely. At the other hand, the processes of phase dispersion (including the suspended solid that is the filamentous biomass) are crucial in order to get an integral knowledge about biological and physicochemical interactions within the bioreactor. Digital image analysis is a powerful tool for getting relevant information in order to establish the mechanisms of mass transfer as well as to evaluate the viability of the mycelia. This review focuses on (a) the main characteristics of the two most common morphologies exhibited by filamentous microorganisms; (b) how hydrodynamic conditions affect morphology and physiology in filamentous cultures; and (c) techniques using digital image analysis to characterize the viability of filamentous microorganisms and mass transfer in multiphase dispersions. Representative case studies of fungi (Trichoderma harzianum and Pleurotus ostreatus) exhibiting different typical morphologies (disperse mycelia and pellets) are discussed.

The filamentous fungal pellet—relationship between morphology and productivity

“…They observed predominantly pellet morphology. High agitation in combination with high aeration flow rates led to increased oxygen mass transfer and decreased pellet size, respectively (Serrano-Carreon et al 2015 ). Consequently, growth rate and maximum biomass concentration were increased.…”

Scale-up agitation criteria for Trichoderma reesei fermentation

“…In addition, T. reesei is strictly aerobic and the increase of viscosity also strongly reduces the rate of oxygen mass transfer (Gabelle et al 2012). To ensure a sufficiently high rate, the power input has to be increased, which results in an increase of the fluid dynamic stresses on the mycelial structures, which because of their size and filamentous nature are prone to size reduction (Serrano-Carreón et al 2015). Thus, the fungus morphology impacts process parameters and the process conditions affect in return the 4 fungus morphology.…”

Study on the submerged fermentation process producing perylenequinones derivatives by shiraia sp. AL18

“…It is a plausible speculated that the excessive DO level inhibited the synthesis of PQDs possibly due to the adverse effect of oxidative stress on metabolism of the mycelin caused by oxygen free radicals which can be produced by PQDs (23), but the results of oxygen stress improved the yield of hypocrellin A make the conjecture become suspicious (24). A reasonable explanation is that the decline of the yield of PQDs possibly due to the effect of the mycelial morphology, physiology and the damage of the agitator under the conditions of the stirring fermentation tank (25)(26)(27). A genetically-engineered strain carrying an hemoglobin gene from vitreoscilla sp.…”