Sandra A. Moody

Selective consumption of decomposing wheat straw by earthworms

Three species of earthworm, Lumbricus terrestris L., Aporrectodea longa (Ude) and Allolobophora chlorotica (Savigny), were offered a choice of mixtures of soil and small wheat straw fragments which had been inoculated individually with six saprotrophic fungi. All earthworm species showed preferences between the six fungal species offered. Early straw decomposers, capable of utilizing water-soluble sugars and cellulose, were preferred in most cases to the lignin-decomposing fungi characteristic of the later stages of decomposition. The removal of fungal-inoculated straw pieces from the soil surface by L. terrestris followed the same pattern. The palatability of two wheat pathogens to L. terrestris was found to be similar to that of the preferred saprotroph. The implications of these findings for fungal abundance and dispersal in wheat fields are discussed.

Variation in the responses of litter and phylloplane fungi to UV-B radiation (290–315 nm)

The development of 12 litter and seven phylloplane fungal species was examined from spore germination to colony sporulation across a series of environmentally relevant UV-B doses. For the litter fungi all aspects of fungal development and morphology studied were affected. On the basis of the responses of mycelial extension rate and spore germination to increasing UV-B, the 12 litter fungi were divided into two groups. Group A (Aspergillus fumigatus, Penicillium hordei, P. janczewskii, P. spinulosum and P. purpurogenum) were sensitive to UV-B, with the predicted effects of a 15% ozone depletion resulting in 22–44% reductions in spore germination. Mycelial extension rate on the agar surface was similarly affected, with reductions ranging from 15 to 25%. In contrast group B (Mucor hiemalis, Cladosporium cladosporioides, Leptosphaeria coniothyrium, Trichoderma viride, Ulocladium consortiale, the Verticillium state of Nectria inventa and Marasmius androsaceus) were relatively insensitive to UV-B, with significant, but small, reductions in mycelial extension rate (< 5%) and spore germination (0–22%). Spore production in response to UV-B in the litter species was very variable, reductions ranging from 5% to complete inhibition. Only P. hordei showed a significant increase in spore production in response to UV-B dose. In contrast, in all seven phylloplane species, spore germination was unaffected by increasing dose. Mycelial extension rate was slightly (2–12%), but significantly, inhibited by UV-B for the four phylloplane fungi tested. The contrasting responses of phylloplane and litter fungi to UV-B are discussed along with the implications for resource capture by competing fungal species and the possible effects of UV-B on decomposition processes.

The role of interactions between trophic levels in determining the effects of UV-B on terrestrial ecosystems

Understanding the potential impact of ozone depletion on terrestrial ecosystems is constrained by lack of information on the effects of environmentally realistic UV-B doses on terrestrial organisms other than higher plants. Increasing UV-B may alter interactions between plants and consumers through direct effects on consumer organisms (herbivores, phytopathogens, decomposers, etc.). The effects of increasing UV-B on arthropods are not known. Significant UV-B effects on fungi have been reported, and may be either negative (inhibition of spore germination and mycelial growth) or positive (increased growth, induction of reproductive development and sporulation). However, in many cases consumers are unlikely to be directly exposed to UV-B in the field. In addition, UV action spectra for fungi suggest that this major group may be less sensitive to the effects of ozone depletion than higher plants. Host mediated effects of UV-B on consumers may include alterations in plant chemistry. While secondary metabolites such as phenolics may increase under increased UV-B, this is not invariably the case and evidence that such changes have significant effects on consumers is limited. In particular, there is no evidence that increased UV-B increases resistance of higher plants to fungal pathogens. Indeed, increased UV-B prior to inoculation results in no significant effect or increased disease. Such responses may be attributable to UV-B effects on host surface properties or on compounds other than phenolics. However, such changes are poorly known, and their potential effects on phytopathogens, herbivores or decomposers cannot be assessed. Understanding the effects of UV-B on terrestrial ecosystems is further limited since virtually nothing is known of possible impacts on higher trophic levels, i.e. predators, parasites or pathogens.

Fate of some fungal spores associated with wheat straw decomposition on passage through the guts of Lumbricus terrestris and Aporrectodea longa.

The effect of passage through the earthworm gut on the viability of spores of saprotrophic fungi was found to vary depending on fungal and earthworm species. Of 5 fungal species fed to Lumbricus terrestris L., the spores of two (Fusarium lateritium Nees, and Agrocybe temulenta (Fries)) failed to germinate after gut passage, while germination of Trichoderma sp. and Mucor hiemalis Wehmer was significantly reduced. A similar fate was recorded for F. lateritium and M. hiemalis spores on passage through the gut of Aporrectodea longa (Ude), however in the case of Chaetomium globosum Kunze there was a significant increase in spore germination after transit through A. longa. The germination of spores of M. hiemalis increased after abrasion by soil particles in a peristaltic pump simulating the mechanical action of the earthworm gut. In contrast germination of this species was significantly reduced when spores were exposed to intestinal fluids from L. terrestris.