4.9 Growth kinetics

So far we have described the basic kinetics of mycelial growth in words. Converting them to algebra results in the relationships we illustrated in Figs 7-9 being expressed in the equation:

Ē = µmax G

where Ē is the mean extension rate of the colony margin;
µmax is the maximum specific (biomass) growth rate;
and G is the hyphal growth unit length.

As can be seen from Fig. 13 and Table 2 , the factors which determine the radial growth rate of the colony (Kr) are the specific growth rate of the fungus (μ) and the width of the peripheral growth zone (w); that is, Kr = wμ.

The fungal colony therefore grows outward radially at a linear rate (that is, an arithmetic plot of colony radius against time forms a straight line), continually growing into unexploited substratum. As it does so, the production of new branches ensures the efficient colonisation and utilisation of the substratum. For the colony as a whole, the peripheral growth zone is a ring of active tissue at the colony margin which is responsible for expansion of the colony. At the level of the individual hypha, the peripheral growth zone corresponds to the volume of hypha contributing to extension growth of the apex of that hypha (the hyphal growth unit).

The rate of change in conditions below a colony will be related to the density (biomass per unit surface area) of the fungal biomass supported. It follows from this that a profusely branching mycelium (low value of G) will develop unfavourable conditions in the medium below the colony more rapidly than a sparsely branching mycelium (high value of G). Consequently, a relationship between G and w would be anticipated , and is observed. It means, for example, that Kr can be used to study the effect of temperature on fungal growth because w is not affected appreciably by temperature, however the concentration of glucose (for example) does affect w, so Kr cannot be used to investigate the effect of nutrient concentration. The biological consequence of this is that filamentous fungi can maintain maximal radial growth rate over nutrient-depleted substrates.

Several mathematical models of fungal growth have been published, and we refer you to Bartnicki-Garcia et al. (1989), Boswell et al. (2003), Boswell (2008), Davidson (2007), Goriely & Tabor (2008), Moore et al. (2006), and Prosser (1990, 1995a & b).

Unlike colonies formed by unicellular bacteria and yeasts, where colony expansion is the result of the production of daughter cells and occurs only slowly, the ability of filamentous fungi to direct all their growth capacity to the hyphal apex allows the colony to expand far more rapidly. Importantly, the fungal colony expands at a rate which exceeds the rate of diffusion of nutrients from the surrounding substratum. Although nutrients under the colony are rapidly exhausted, the hyphae at the edge of the colony have only a minor effect on the substrate concentration and continue to grow outwards, exploring for more nutrients. In contrast, the rate of expansion of bacterial and yeast colonies is extremely slow and less than the rate of diffusion of nutrients (Table 3). Colonies of unicellular organisms quickly become diffusion limited and therefore, unlike fungal colonies, can only attain a finite size.

Table 3. Colony radial growth rate (Kr) of bacterial and fungal colonies cultivated at their optimum temperatures
Species
w (μm)
μ (h-1
Kr (μmh-1)
Bacteria
Escherichia coli
91
0.28
18

Streptococcus faecalis

Not done

0.65

18

Pseudomonas florescens

Not done

0.59

29

Myxococcus xanthus (non-motile)

Not done

Not done

20

Streptomyces coelicolor

Not done

0.32

22

Fungi  

Candida albicans (mycelial form)

119

0.39

46

Penicillium chrysogenum
496
0.16
76
Neurospora crassa
6800
0.26
2152
Data summarised from Oliver & Trinci, 1985.

Updated December 30, 2016