Fungi in Ecosystem Processes

John Dighton

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9781032098036
• eBook ISBN: 9781315360164
• Edition: 2
• Topic: Biological Sciences
• Subtopic: Ecology
• Index: Biological Sciences

This new edition of Fungi in Ecosystem Processes continues the unique approach of examining the roles of fungi from the perspective of ecosystem functions. It explores how fungi have adapted to survive within particular constraints, how they help to maintain homeostasis in ecosystems, how they facilitate resistance to perturbations, and how they influence the communities of other organisms.

Updated and revised, the second edition

• Expands the section on plant pathogens, invasive species, and insect–fungal interactions
• Provides more extensive coverage on insect–fungal interactions, including entomopathogens, the links between entomopathogens and endophytes, and symbiotic and mutualistic interactions
• Adds a new section on fungi in the built environment
• Presents new material on below-ground to above-ground interactions mediated through fungi, such as mycorrhizal signaling systems for herbivory defense

The book also includes expanded coverage of the role of fungi in suppressive soils, aquatic and marine fungi, modern methods of following food chains in fungal–invertebrate trophic interactions, and the physiology of nutrient uptake by mycorrhizae.

A necessary update and expansion to previous material, this book provides an essential reference on the current understanding of fungal roles in ecosystem processes. It also identifies directions for future study, including an emphasis on the need for further research on fungi in built environments.

CHAPTER 1

Introduction

1.1 WHY FUNGI?

Fungi are a group of organisms that cannot fix energy and nutrients directly (but see Tugay et al., 2006; Dadachova et al., 2007; Dadachova and Casadevall, 2008), so they use the energy stored in plant and animal biomass to create their own growth. Fungi are a key group of organisms that interact with other organisms and the abiotic environment to regulate ecosystem processes. In his introductory chapter to The Fungal Community: Its Organization and Role in the Ecosystem (Carroll and Wicklow, 1992), Alan Rayner speaks of the importance of fungi in ecosystems in the following four major functions: (1) As decomposers, fungi drive the global carbon cycle. (2) As mycorrhizal symbionts, they form absorptive accessories to roots, linking the activities of separate plants and underpinning primary production in forests, heathlands, and grasslands. (3) In lichens, they clothe what might otherwise be bare parts of the planet. (4) As parasites, they regulate population dynamics of their hosts.

The pervasiveness of fungi in, for example, woodlands, suggests that “rather than being on the margins of forest life, fungi are…central to it, interconnecting and influencing the lives and deaths of plants and animals in countless, diverse and often surprising ways. To disregard them is to misunderstand the system” (Rayner, 1992). In his presidential address to the British Mycology Society, he showed how fungal linkages are inextricable and fundamental to the processes occurring in the ecosystem and to the sustainability of the system (Rayner, 1998). Here, fungi are able to share resources and span a range of spatial and temporal scales to mediate flows of energy and materials (Morris and Robertson, 2005). These interactions are described by Rayner (1998), wherein he regards the forest as a single organism held together by the functions and structure of fungi, rather than a collection of individuals. Much of these ideas are also expressed in the work of Trappe and Luoma (1992), who suggest that fungi are the “ties that bind” components of the ecosystem together in a literal and functional sense.

There are many textbooks describing fungi and their ecology (e.g., Cooke and Rayner, 1984; Dix and Webster, 1995; Webster and Webber, 2007). However, many of these books take a fungal view of ecology by describing the habitat in which certain species are found or the general physiological processes effected by selected species. It is the intent of this book to take a broader view of ecosystems, highlighting the major processes that occur within them and focusing on the importance of fungi in these processes. Indeed, their structure and life form make them ideal “ecosystem engineers” (sensu Lawton and Jones, 1995; Lavelle, 1997), a term usually reserved for animals, which have major physical effects on the environment. In this sense, we may consider fungi as forming the plumbing of the ecosystem as they are capable of regulating spatial and temporal flows of nutrient and energy through their extensive and, often, long-lived mycelial and rhizomorphic networks extending over tens or hundreds of meters (Smith et al., 1992). Although thought of as microorganisms, fungi may form extensive mycelia. Far from being ephemeral, many of these long-lived fungi are capable of continuing their function almost indefinitely and, by virtue of their continued growth, extend these functions into new areas. Their activity affects the local environment around their hyphae, at the scale of micrometers, yet their macroscale impact on the processes in the ecosystem are large (Anderson, 1995; Morris and Robertson, 2005). The complex and diverse modes of spore dispersal, together with the perennial nature of their mycelial networks, make fungi ubiquitous in all ecosystems.

So, why should we focus on fungi, rather than the more charismatic megafauna? Why should we be interested in some seemingly ephemeral mushroom that appears as a nuisance on a neatly cut lawn where a tree used to be, only to die and reappear a few months later? Why should we be interested in that mold on a piece of discarded fruit peel or in that spot on the leaf of our apple tree? It is exactly for the reasons that fungi are not very visible components of the ecosystem that they are overlooked as significant organisms in the environment. It is because the effects of fungi may be highly significant, that we need to explore the role of fungi in ecosystem processes.

Fungi are ubiquitous; they seem to be able to survive in almost all habitats. They occur in terrestrial, aquatic (Suberkropp and Chauvet, 1995), and marine (Hyde et al., 1998; Raghukumar, 2012) ecosystems, and in many extreme environments. Fungi can be active in Arctic and Antarctic conditions where the production of specific sugars allows their function to be maintained at below freezing temperatures (Robinson, 2001; Ludley and Robinson, 2008). Oligotrophic fungi (organisms that can survive in very nutrient poor environments) have been isolated and cultured from glass, where they survive by scavenging nutrients and carbon from the air (Wainwright et al., 1997; Bergero et al., 1999) and can cause significant damage to glass (Rodrigues et al., 2014). Chemotrophic fungi (organisms surviving on chemical nutrients alone) have been found in aircraft fuel lines (Hendey, 1964), deep oceans (Nagano and Nagahara, 2012; Xu et al., 2014) and hypersaline environments (Cantrell et al., 2006). Some fungal species, which are regarded as extremophiles (organisms found in extreme environmental conditions), have been isolated from the walls of the reactor room at Chernobyl, after the nuclear explosion of 1986 (Zhdanova et al., 2000).

1.2 WHAT ARE FUNGI?

We will not dwell on the taxonomy and structure of fungi as these topics are adequately discussed in other textbooks (Alexopoulus and Mims, 1979; Webster and Weber, 2007). However, we will review some of the key features of the fungal body and its physiology that allow fungi to make an important contribution to ecosystem processes. The taxonomy of fungi is constantly under debate, and there are continual changes in the nomenclature of the hierarchical category under which species should be organized (Hibbett and Taylor, 2013; Money, 2013).

The mycelial portion of the fungus consists of hyphae. These hyphae, which are absent or rudimentary in the Chytridomycetes and yeasts, are a filamentous assemblage of tubular cells in which continuity is maintained between adjacent cells by the absence of cross-cell walls (septa) or a septum perforated by a pore. Thus, the hyphae develop a continual cytoplasmic connectivity between adjacent cells. Hyphae average 5–6 μm in diameter and grow by wall extension at the tip (Chiu and Moore, 1996; Moore, 1999; Moore and Novak Frazer, 2002). Because they have a narrow diameter and long length, fungal hyphae present a large surface area, relative to volume, to the environment around them. This property allows fungi to optimize the absorption of degradation products of simple carbohydrates and mineral nutrients that are derived from the action of extracellular enzymes produced by the fungi.

Fungal hyphae may grow independently or coalesce to form larger and structured assemblages called rhizomorphs or strands. These linear structures are larger and more robust than individual hyphae, and have been developed for long-distance transport of water and nutrients (Duddridge et al., 1980; Cairney, 1992; Lindahl and Olsson, 2004). Nutrient and carbon translocation has important implications for maintenance of functional continuity in a heterogeneous environment (Boddy, 1999), where resources can be reallocated within the fungal mycelium from areas of storage or excess to actively growing or functioning regions. As a result of the coenocytic arrangement of the hyphal network, there is the possibility of movement of resources within the hyphae from areas of high resource availability (sources) to areas of low availability or sites of resource demand (sinks). The movement of resources between sources and sinks is known as translocation and has been described by Jennings (1976, 1982) and reviewed by Cairney (1992), Boddy (1999), and Lamour et al. (2007). This attribute of the fungal mycelium allows for the movement of resources over short (millimeters) to long (meters) distances within the fungal mycelium, thereby reducing heterogeneity within ecosystems and connecting parts of the ecosystem in both space and time.

The temporal component, immobilization, of this activity is as important as the spatial component. As mycelia grow, there is incorporation of new carbon and mineral nutrients into the biomass of the advancing hyphal front and to more proximal biomass by translocation and immobilization. While the fungal mycelium is alive and active, much of this material remains bound to structural components or in the cytoplasm. However, upon the death of more proximal parts of the mycelium, materials incorporated into biomass may either be retranslocated from dying to living components or released into the environment via decomposition and mineralization processes. The duration of incorporation of material into biomass is regarded as an immobilization phase, where the material is unavailable for use by other organisms. The duration of this immobilization phase is dependent on the turnover time of the organism. In comparison with bacteria, which have turnover times of hours or days, some higher fungi may have hyphal turnover times of weeks or years. Hence, fungi may be important long-term accumulat...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• List of Figures
• Preface
• Author
• Chapter 1 Introduction
• Chapter 2 Making Nutrients Available for Primary Production
• Chapter 3 Role of Fungi in Promoting Primary Production
• Chapter 4 Role of Fungi in Reducing Primary Production
• Chapter 5 Fungi and Secondary Productivity
• Chapter 6 Fungi as Animal Pathogens
• Chapter 7 Fungal Interactions with Pollutants and Climate Change
• Chapter 8 Fungi in the Built Environment
• Index