Fungal Growth Energetic Constraints

Saprotrophic fungi utilize energy to grow, reproduce, and substrate digestion. To understand the mechanism of how this takes place, researchers Luke L. M. Heaton from the University of Oxford, Nick S. Jones from the Imperial College London, and Mark D. Fricker from the University of Oxford developed a model that given any growth rate one can identify the strategy that maximizes the fraction of energy that could be spent on reproduction. The predictions of growth rates and bioconversion efficiencies achieved by this model are consistent with empirical findings, and it predicts the optimal investment in reproduction, resource acquisition, and biomass recycling for a given environment and timescale of reproduction.

Thus, if the timescale of reproduction is long compared to the time required for the fungus to double in size, this model suggests that the total energy available for reproduction is maximal when a very small fraction of the energy budget is spent on reproduction. The model also suggests that fungi growing on substrates concentrated with a high low-molecular-weight compounds will not benefit from recycling rather they should be able to grow utilizing more energy on reproduction without recycling. In contrast, recycling offers considerable benefits to fungi growing on recalcitrant substrates, where the individual hyphae are not crowded and the time taken to consume resource is significantly longer than the fungus doubling time.

Fungi are ubiquitous and have ecological importance. It’s widely known that fungi are involved in the decomposition of woody debris. Levels of carbon and mineral nutrient found in ecosystems depend partly on this rate of decomposition. These levels of carbon and mineral nutrient have a significant impact on the ecosystems. Subsequent decomposition involves growth in or on the resource, breakdown of the substrate through digestive enzymes, and active absorption of the solubilized products to fuel further growth or reproduction. Each fungal species adopts a different strategy in this process depending on the relative investment in growth, resource acquisition, or reproduction.

Forest Fungus

Because fungal colonies do not fit well into existing models for either plants or animals and though they exhibit features that characterize both, the researchers adopted a model with similar features with traditional, dynamic energy-budget models. This model was based on first principles appropriate for a fungal system. Of interest is the habitation of new terrain through spore’s transport which is similar to seed dispersal in plants. Local foraging for resources is typical of animals, although in the case of fungi “foraging” involves growth as an interconnected mycelial network rather than movement. This mycelial network is quite dynamic and has to balance discovery of new resource through exploration with exploitation of existing resource while maintaining transport between changing sources and sinks in the face of predation and competition. The levels of resource also impact the trade-off between growth and reproduction. Larger fungi can produce more spores, and many species require a mycelium of some minimal size before they can grow reproductive structures.

The researchers found out that the species that grow on patchy, ephemeral substrates like fruit, phloem sap, or dung exhibit rapid growth and reproduce within a short period. The model suggested that the relatively fast growth of such species simply reflects the fact that nutrient-rich substrates that are easily digested can support higher rates of growth, particularly if the fungus is well adapted to growing on the substrate of interest. The time a patch of resource is available determines the period of reproduction for fungi that grow on that resource. The model implied that growth strategies maximizing the energy spent on reproduction over timescales that are similar to doubling time, commit a large fraction of the available energy to reproduction. The opposite is that when the timescale of reproduction is extended more than the doubling time, strategies that maximize the energy available for reproduction are very similar to strategies that maximize the growth rate, with only a small fraction of energy spent on reproduction.

Further, it was found out that bioconversion efficiency doesn’t vary with the rate of growth. Slow growth was weak because of the need to expend energy to sustain the existing colony while at the same time new growth taking place. Conversely, there are inevitable additional costs associated with fast growth, such as high requirements for transport and resource consumption.
With the recycling parts of the mycelium, the existing hyphal tips obtain an additional source of nutrients but that recycling will tend to reduce the fungus’s capacity to synthesize and transport new cell constituents. Consequently, it is not obvious when recycling will be beneficial or why some species but not others recycle extensively. The model implied that there was a relationship between the extent of recycling and the quality of resource on which the fungi grow. Maintaining hyphae that have digested the surrounding resource is beneficial only because it helps to reduce the cost of transport.

The full study report can be found at the University of Chicago Press Journals.