Ego & Ergosterol
Previously I wrote about fungal physical plasticity as a source of developmental opportunity, and how plasticity itself sits at the foundation of effective work with filamentous fungi. But that same plasticity can just as easily confound, and what appears at first to be a clean and useful relationship can unravel under the weight of context, becoming messy, conditional, or even misleading.
The problem of fungal biomass quantification is fascinating and humbling because it’s a corner of fungal biotechnology where a seemingly fundamental question (‘How much mycelium do I have?’) meets the complexities of fungal physical plasticity. It’s a corner where the mycelium engineer, not minding the nuances of physical plasticity, can easily find their ego bruised. And the specific example of ergosterol can provide a lesson and reference point for avoiding pitfalls.
The Potential of Ergosterol
Ergosterol is the primary sterol component of fungal cell membranes, serving many of the same functions that cholesterol serves in animal cells. It helps maintain membrane structure and fluidity, ensuring appropriate rigidity, permeability, and activity of membrane-bound proteins. Because ergosterol is largely specific to fungi and absent in animal cells (and most other organisms), it is a unique signature of fungal biomass in mixed biological systems. The promise of ergosterol, then, is a practical and fungus-specific proxy for estimating living fungal biomass.
The root of ergosterol concentration variation in fungal biomass lies in physical plasticity; the ability to modulate membrane composition in response to environmental and metabolic conditions. Ergosterol synthesis is significantly influenced by factors such as growth phase, nutrient availability, oxygen levels, and the physical mode of growth (pellet morphology, liquid vs. solid substrate, etc). As fungi adapt to changes in aeration, substrate composition, and developmental stage, they adjust the proportion of ergosterol within their cells to maintain membrane integrity and function. This means that even within a single genetic individual, ergosterol content can vary by more than an order of magnitude depending on the surrounding conditions and the morphological or metabolic state.
The Difficulty with Physical Plasticity
Accurate use of ergosterol as a fungal biomass proxy, then, depends on calibration under the specific growth conditions of interest. Because ergosterol content per unit of biomass is not fixed, any attempt to convert ergosterol concentration to biomass must rely on a calibration curve tailored to the system being studied. This becomes especially problematic in solid-state or mycelium-based composites, where intrinsic heterogeneity of moisture, oxygen, and nutrients lead to spatially eccentric growth. Ergosterol levels can vary drastically within a single sample, and without spatially resolved or condition-specific calibration, biomass estimates can be exceptionally inaccurate. Moreover, while generally considered a marker of living fungal biomass, ergosterol can persist after cell death under dark or anaerobic conditions, leading to potential overestimation of living biomass. Conversely, it degrades rapidly when exposed to UV light, risking underestimation. Holistically, this makes ergosterol a conditionally reliable biomass proxy, requiring careful calibration and validation in each specific application.
Depending on the level of control and calibration, or degree of baseline appreciation for the degree of plasticity in ergosterol responsiveness, it is easy to imagine a scenario where process decisions made based on ergosterol responses as a biomass proxy could be seen as questionable, or outright result in bad conclusions and lost capital or time. Appreciating the complexities and limitations of ergosterol as a fungal biomass marker provides a rich example of navigating the risks and opportunities of working with fungal physical plasticity. And it’s a tale of fungal biotechnology that, once digested, may just help the mycelium engineer avoid pitfalls in other corners of fungal biotechnology.
References
Charcosset, J.-Y., & Chauvet, E. (2001). Effect of culture conditions on ergosterol as an indicator of biomass in the aquatic hyphomycetes. Applied and Environmental Microbiology, 67(5), 2051–2055. https://doi.org/10.1128/AEM.67.5.2051-2055.
Nout, M. J. R., Bonants-van Laarhoven, T. M. G., de Jongh, P., & Rombouts, F. M. (1987). Ergosterol content of Rhizopus oligosporus NRRL 5905 grown in liquid and solid substrates. Applied Microbiology and Biotechnology, 26(5), 456–461. https://doi.org/10.1007/BF00253532
Eliaš D, Tóth Hervay N, Gbelská Y. Ergosterol Biosynthesis and Regulation Impact the Antifungal Resistance and Virulence of Candida spp. Stresses. 2024; 4(4):641-662. https://doi.org/10.3390/stresses4040041
Mille-Lindblom, C., von Wachenfeldt, E., & Tranvik, L. J. (2004). Ergosterol as a measure of living fungal biomass: Persistence in environmental samples after fungal death. Journal of Microbiological Methods, 59(2), 253–262. https://doi.org/10.1016/j.mimet.2004.07.010