Growth of Arthrobotrys superba from a birch wood resource base into soil determined by radioactive tracing

Nematode-Trapping Fungi

Nematode-trapping fungi are a unique and intriguing group of carnivorous microorganisms that can trap and digest nematodes by means of specialized trapping structures. They can develop diverse trapping devices, such as adhesive hyphae, adhesive knobs, adhesive networks, constricting rings, and nonconstricting rings. Nematode-trapping fungi have been found in all regions of the world, from the tropics to Antarctica, from terrestrial to aquatic ecosystems. They play an important ecological role in regulating nematode dynamics in soil. Molecular phylogenetic studies have shown that the majority of nematode-trapping fungi belong to a monophyletic group in the order Orbiliales (Ascomycota). Nematode-trapping fungi serve as an excellent model system for understanding fungal evolution and interaction between fungi and nematodes. With the development of molecular techniques and genome sequencing, their evolutionary origins and divergence, and the mechanisms underlying fungus-nematode interactions have been well studied. In recent decades, an increasing concern about the environmental hazards of using chemical nematicides has led to the application of these biological control agents as a rapidly developing component of crop protection.

The Development of Fungal Networks in Complex Environments

Fungi are of fundamental importance in terrestrial ecosystems playing important roles in decomposition, nutrient cycling, plant symbiosis and pathogenesis, and have significant potential in several areas of environmental biotechnology such as biocontrol and bioremediation. In all of these contexts, the fungi are growing in environments exhibiting spatio-temporal nutritional and structural heterogeneities. In this work, a discrete mathematical model is derived that allows detailed understanding of how events at the hyphal level are influenced by the nature of various environmental heterogeneities. Mycelial growth and function is simulated in a range of environments including homogeneous conditions, nutritionally-heterogeneous conditions and structurally-heterogeneous environments, the latter emulating porous media such as soils. Our results provide further understanding of the crucial processes involved in fungal growth, nutrient translocation and concomitant functional consequences, e.g. acidification, and have implications for the biotechnological application of fungi.

Translocation of carbon by Rhizoctonia solani in nutritionally-heterogeneous microcosms

Responses of Rhizoctonia solani to spatial heterogeneity in sources of carbon, and associated translocation of carbon (C), were studied in a simple microcosm system comprising two discrete domains of agar gels separated on a glass slide and overlain with a porous membrane. Two arrangements of the gel pairs were used, one containing two equally large resources (representing 'homogeneous' conditions) and one containing a large and a negligible resource (representing 'heterogeneous' conditions). The nutrient sources were a standard mineral salt medium with or without glucose as sole C source. The fungus was inoculated onto one domain and growth responses determined by direct measurement of biomass. Translocation of C was quantified by use of 13C-enriched glucose. This substrate was either added to the agar at the outset, when studying newly developing colonies, or as a pulse into already established colonies. When growing in heterogeneous conditions, the fungus actively translocated C from a glucose-containing domain to sustain growth in the adjacent region lacking such a resource. In homogeneous conditions there was evidence of passive translocation (diffusion), but the fungus preferentially used local resource to maintain growth. Active translocation was only observed in newly growing colonies, whereas passive translocation occurred in both growing and established colonies. When the fungus was pulsed with a 13C-enriched glucose solution after 10 d growth, 2.5 times more 13C was taken up by the fungus grown in heterogeneous than homogeneous conditions, suggesting uptake exceeded local demands. In heterogeneous conditions, the total amount of 13C enriched glucose taken up by the fungus was independent of the location of the enriched glucose in the underlying medium. When the nylon membrane was replaced by Cellophane (an additional C source), degradation of the membrane and an increase in biomass occurred only in the heterogeneous system. The possible implications for these results in soil systems are discussed.

An evolutionary scenario for the transition to undifferentiated multicellularity

The evolutionary transition from single cells toward multicellular forms of life represents one of the major transitions in the evolution of complex organisms. In this transition, single autonomously reproducing cells became parts of larger reproducing entities that eventually constituted a new unit of selection. The first step in the evolutionary transition to multicellularity likely was the evolution of simple, undifferentiated cell clusters. However, what the selective advantage of such cell clusters may have been remains unclear. Here, we argue that in populations of unicellular organisms with cooperative behavior, clustering may be beneficial by reducing interactions with noncooperative individuals. In support of this hypothesis, we present a set of computer simulations showing that clustering can evolve as a biological, heritable trait for cells that cooperate in the use of external energy resources. Following the evolution of simple cell clusters, further benefits could have arisen from the exchange of resources between cells of a cluster.

Functional consequences of nutrient translocation in mycelial fungi

Fungi are of fundamental importance for plant and microbial nutrition with primary roles in decomposition and nutrient recycling. They also have great potential for use in areas of biotechnology such as bioremediation of organic and inorganic pollutants and biocontrol of plant pathogens. In all these contexts, environmental heterogeneity has a strong influence on growth and function. A large class of fungi overcome the difficulties encountered in such environments by the mechanism of translocation which results in the internal redistribution of nutrients within the fungal mycelium. In this paper, we use a combination of experimental techniques and mathematical modelling to examine fungal growth in general, and in particular, translocation in the common soil saprophytic fungus Rhizoctonia solani. A detailed mathematical model is presented where translocation is considered to have both diffusive and metabolically-driven components. A calibration experiment provided the necessary parameter values. Growth experiments were compared with model solutions and thus we provide strong evidence that diffusion is the dominant mechanism for translocation in homogeneous environments. In heterogeneous environments, we conclude that diffusion is still vital for exploration, i.e. the expansion of the fungal network into the surrounding area. However, we also conclude that localized resources may be utilized faster if energy is invested, i.e. when exploitation of the fungal microenvironment is enhanced by metabolically driven translocation.

Translocation induced outgrowth of fungi in nutrient-free environments

The study of alternatives to chemical methods of nematode control in agriculture has received significant recent interest. One such method is biological control using nematode trapping fungi such as Arthrobotrys superba. To understand how these fungi can be implemented as effective nematicides, it is essential to study their outgrowth into soil from localized nutrient resources. In this paper, we use a mathematical model to investigate the outgrowth of fungi into an environment essentially without available nutrients capable of supporting net growth. By comparing model solutions with experimental results, we show that in such circumstances, continual mycelial expansion can only be obtained if internal metabolites are actively translocated through the mycelium. Predictions are made concerning the maximal extension possible from a given quantity of nutrients and a testable functional relationship between the two is derived. Using this modelling technique we are able to map not only biomass extent but also biomass distribution at each stage. The type of environmental heterogeneity investigated here is encountered by many species of fungi in nature and is of relevance for the introduction of specific fungi into soil for biological control or bioremediation purposes.