Cultivation of filamentous fungi in airlift bioreactors: advantages and disadvantages

Abstract

Filamentous fungi or mycelia are a valuable bioresource to produce several biomolecules and enzymes, especially because of their biodegradation potential and for their key role of enablers of a circular bioeconomy. Filamentous fungi can be grown in submerged cultivation to maximise the volumetric productivity of the bioprocess, instead of using the more established and time-consuming solid-state cultivation. Multicellular mycelia are sensitive to shear stresses induced by mechanical agitation, and this aspect greatly affects their morphology in submerged cultivation (pelletisation) and the connected volumetric productivity. An efficient compromise is the growth of filamentous fungi in airlift bioreactors (ALR) where the volumetric oxygen transfer (K_La) is optimal, but the shear stress is reduced. In this review, we critically analysed the advantages and disadvantages of ALR-based cultivation of filamentous fungi, comparing these bioreactors also with stirred tank reactors and bubble column reactors; we focused on scientific literature that highlights findings for the cultivation of filamentous fungi for both the production of enzymes and the production of myco-biomass in ALR; we included studies for the control of the pelletisation of the fungal biomass in batch and semi-continuous cultivation, highlighting the interlinked hydrodynamics; finally, we included studies regarding the modifications of ALR in order to enhance filamentous fungi production. KEY POINTS: • ALR are efficient for batch and prolonged continuous cultivation of filamentous fungi. • ALR show both optimal gas hold-up and K_La with an airflow that has high superficial velocity and critical bubble diameter (1-6 mm). • Suspended mycelia aggregates (pellet) maintain a fluidised motion in ALR if their size/density can be controlled.

Keywords: Airlift fermentation; Filamentous fungi; Fluid dynamics; Volumetric productivity.

Fig. 1

Bubble column bioreactor. Picture adapted from Singhal et al. (2018)

Fig. 2

Airlift bioreactor with an internal draft tube. Picture adapted from Prado Barragan et al. (2016)

Fig. 3

Airlift configurations as internal loop airlift reactor (ILALR) or external loop airlift reactor (ELALR). a Conventional (draft tube), b rectangular, c split-plate (rectangular/cylindrical/slanted baffles), d multistage draft tube, e multiple draft tube/multiple split-plate, f stirred, g helix, h static mixing, i packed bed, j fluidised bed (three-phase), k conventional, l hybrid D-shape, m hypotenuse/triangular, n multistage, o packed/fluidised bed, p stirred (several impellers), q inversed fluidised bed (tri-phasic). Picture adapted from Behin and Amiri (2023)

Fig. 4

Correlation between the drift velocity (J_drift, m/s) and gas hold-up (ε_g) in establishing the gas flow and bubble dynamics behaviour in a sparged column. Picture adapted from Besagni (2021)

Fig. 5

Jet-loop reactor schematic. Picture adapted from Breit et al. (2022)

Fig. 6

Modified airlift bioreactor with an internal sparger in the draft tube. DPT, differential pressure transducers; RPT, relative pressure transducers; DOT, dissolved oxygen probe. (inset = gas sparger design). Picture adapted from Olivieri et al. (2010)

Fig. 7

Air supply rotating shaft system for ALR for the cultivation of filamentous fungi (Armillaria mellea) where the individual components described as follows: (4) rotating shaft, (8) air supply inlet, (9) downward conical jet-head, (10) upward conical jet-head, (11) additional rotating blades, (12) lower jet-head, (13) bottom portion of the sparger, (14) bottom jet nozzle. Picture adapted from Peng (2022)

Fig. 8

(1) Draft tube, (2) gas sparger, (3) DO-electrodes: middle riser, (4) middle downcomer, (5) base, (6) gas separator, (7) pressure taps/perforated tubes in downcomer, (8) (9) pressure taps/perforated tubes in riser, (10) (11) inverse U-tube manometers, (12) (13) temperature probe, (14) Biostat Q DCU, (15) PC/recorder, (16) riser helix/HFPgas sparger (R), downcomer helix/HFP-gas sparger (D). Picture adapted from Räsänen et al. (2016)

Fig. 9

Helical agitator in draft tube of an airlift bioreactor. Picture adapted from Sayyar et al. (2023)

Fig. 10

Airlift bioreactor provided with a tilted low-intensity anchor-shaped agitator (13). Picture adapted from Zhang et al. (2020)

Fig. 11

(1) liquid inlet, (2) upper air inlet, (3) air outlet, (4) reaction tank, (5) air guide pipe, (6) guide tube, (7) air guide hood, (8) self-priming impeller, (9) drain, (10) seal, (11) cooling water inlet, (12) lower air inlet, (13) cooling water outlet, (14) generator, (15) screen, (16) gas–liquid flow channel, (17) baffles. Picture adapted from Li et al. (2022)

Fig. 12

Computational fluid dynamics of a CR-ILR modification with internals. (From left to right): visualisation of the design of the internals added to a CR-ILR, line integral convolution of the velocity magnitude, and vertical velocity profile of the airflow in a CR-ILR. Picture adapted from Bokelmann et al. (2025)