Non‐trophic effects of oribatid mites on cord‐forming basidiomycetes in soil microcosms

1. Saprotrophic cord‐forming basidiomycetes are the primary agents of decomposition in forest ecosystems. Collembola and oribatid mites affect fungal growth and foraging, and therefore decomposition, through direct mycelial grazing. 2. Grazing on the fungal species Hypholoma fasciculare, Resinicium bicolor and Phanerochaete velutina by the collembola Folsomia candida, and the oribatid mites Steganacarus magnus, Euzetes globulus and Hermannia gibba was investigated in soil microcosms. 3. Folsomia candida grazed on all fungal species: radial extent of R. bicolor, hyphal coverage of all fungal species, and fractal dimension of R. bicolor and P. velutina were all reduced. Oribatid mites did not graze the fungi but did affect mycelial morphology. Steganacarus magnus caused a reduction in the radial extent of H. fasciculare, and the hyphal coverage and fractal dimension in both H. fasciculare and R. bicolor. Euzetes globulus and H. gibba reduced the hyphal coverage of P. velutina. 4. Oribatid mites are associated with a cornucopia of chemical secretions with possible anti‐fungal properties. Chemical analysis of H. gibba opisthonotal secretions revealed four main compounds, all of which are new to the known spectrum of opisthonotal components. The most abundant was (E)‐β‐farnesene. 5. Treatment effects were species‐specific in terms of both fungal and invertebrate species. This study provides the first evidence of non‐grazing effects of oribatid mites on fungal growth and morphology. This could potentially influence the spatial organisation of mycelium in forest soils and therefore the ability of fungi to locate, colonise and decompose dead organic matter.     

Lulwoana sp., a dark septate endophyte in roots of Posidonia oceanica (L.) Delile seagrass

Posidonia oceanica is the most common, widespread and important monocotyledon seagrass in the Mediterranean Basin, and hosts a large biodiversity of species, including microorganisms with key roles in the marine environment. In this study, we ascertain the presence of a fungal endophyte in the roots of P. oceanica growing on different substrata (rock, sand and matte) in two Sicilian marine meadows. Staining techniques on root fragments and sections, in combination with microscope observations, were used to visualise the fungal presence and determine the percentage of fungal colonisation (FC) in this tissue. In root fragments, statistical analysis of the FC showed a higher mean in roots anchored on rock than on matte and sand. In root sections, an inter‐ and intracellular septate mycelium, producing intracellular microsclerotia, was detected from the rhizodermis to the vascular cylinder. Using isolation techniques, we obtained, from both sampling sites, sterile, slow‐growing fungal colonies, dark in colour, with septate mycelium, belonging to the dark septate endophytes (DSEs). DNA sequencing of the internal transcribed spacer (ITS) region identified these colonies as Lulwoana sp. To our knowledge, this is the first report of Lulwoana sp. as DSE in roots of P. oceanica. Moreover, the highest fungal colonisation, detected in P. oceanica roots growing on rock, suggests that the presence of the DSE may help the host in several ways, particularly in capturing mineral nutrients through lytic activity.     

Pythium aphanidermatum May Reduce Cucumber Growth without Affecting Leaf Photosynthesis

Plants respond to many stress factors, including infections caused by root pathogens, with reductions in photosynthesis and growth. We studied the response of mature cucumber plants after a weak inoculation of the roots with Pythium aphanidermatum. The epidemiology of the disease was recorded using an indirect ELISA. Although mycelium was detected in the roots, photosynthesis was not affected over a period of five weeks. Nevertheless, plant growth was significantly reduced by the pathogen. Possible modes of action are discussed.     

Attachment of different soil bacteria to arbuscular mycorrhizal fungal extraradical hyphae is determined by hyphal vitality and fungal species

Attachment of certain bacteria to living arbuscular mycorrhizal fungal extraradical hyphae may be an important prerequisite for interactions between these microorganisms, with implications for nutrient supply and plant health. The attachment of five different strains of gfp-tagged soil bacteria (Paenibacillus brasilensis PB177 (pnf8), Bacillus cereus VA1 (pnf8), Pseudomonas fluorescens SBW25 :: gfp/lux, Arthrobacter chlorophenolicus A6G, and Paenibacillus peoriae BD62 (pnf8)) to vital and nonvital extraradical hyphae of the arbuscular mycorrhizal fungi Glomus sp. MUCL 43205 and Glomus intraradices MUCL 43194 was examined. Arthrobacter chlorophenolicus did not attach to hyphae, whereas the other bacterial strains did to a varying degree. Only P. brasilensis showed greater attachment to vital hyphae than nonvital hyphae of both Glomus species tested. Pseudomonas fluorescens showed a higher attachment to vital compared with nonvital Glomus sp. MUCL 43205 hyphae, whereas this relationship was opposite for attachment to G. intraradices. Both B. cereus and P. peoriae showed higher attachment to nonvital hyphae. This study provides novel evidence that under laboratory conditions soil bacteria differ in their ability to colonize vital and nonvital hyphae and that this can also be influenced by the arbuscular mycorrhizal fungal species involved. The significance of bacterial attachment to mycorrhizal fungal extraradical hyphae is discussed.     

Influence of arbuscular mycorrhizal fungi and simulated acid rain on the growth and coexistence of the grasses Calamagrostis villosa and Deschampsia flexuosa

Two grass species — Calamagrostis villosa (Chaix) J.F. Gmelin and Deschampsia flexuosa (L.) Trin. — are expanding in mountain Norway spruce (Picea abies L. Karst.) forests of Central Europe damaged by anthropogenic pollution constituted particularly of acid rain. This invasion of grasses may be caused by the higher irradiance reaching the forest floor after the pollution-induced tree defoliation. The relative abundance of the two grass species is changing during the process of forest decline. Our study investigated the effects of arbuscular mycorrhizal fungi (AMF) on the growth and coexistence of both species under simulated acid rain (SAR) and two levels of irradiance. Three microcosm experiments were conducted to investigate how both grasses are influenced by the AMF when grown separately or together interacting via extraradical mycelium (ERM). A positive growth response to inoculation with Glomus mosseae BEG 25 was found for both grass species when cultivated separately and the mycorrhizal dependence and the growth benefit for D. flexuosa was greater than for C. villosa. However, when both grass species were grown together in the rhizoboxes with separated root and hyphal compartments, the growth effect of the AMF was the opposite, i.e. C. villosa benefited more. The plants did not benefit from the AMF inoculation under the SAR treatment compared with dH2O treatment. The SAR also negatively influenced root length colonised by AMF, length of the ERM, alkaline phosphatase and NADH diaphorase activities of the ERM. The role of the ERM in transporting phosphorus between these grasses was verified by applying the radioisotope ³²P. There was a greater transport of isotopic ³²P between inoculated plants C. villosa and D. flexuosa grown in separated root compartments, as compared to non-inoculated plants. The amount of transported ³²P was low: a maximum of 3% of applied ³²P was detected in the shoots of receiver plants. Mechanical disturbance of the ERM significantly decreased the ³² P transport between plants. The ³²P transport between mycorrhizal plants was higher in the D. flexuosa to C. villosa direction than in the opposite one. Neither the SAR nor the low level of irradiance influenced the amount of transported ³²P. We discuss the role of ERM links between root systems in the coexistence of both grass species. This revised version was published online in June 2006 with corrections to the Cover Date.     

The spatial distribution of soil hyphae in structured spruce-forest soils

The external mycelium forms the major part of the absorbing surface of mycorrhized tree roots. Because the macro pore space of acid forest soils is selectively depleted of mobile nutrient cations, it is ecologically important, whether soil hyphae grow into the soil aggregates or not. Seedlings of Norway Spruce (Picea abies (L.) Karst.) with defined mycorrhiza were grown in unsterilized soil cores taken from the A and B-horizon of a limed and an unlimed cambisol on triassic sandstone in the Northern Black Forest, Germany. Water-tension treatments were 10, 30, 160 and 900 hPa. On ground and polished vertical cuts stained with acridine orange, we identified and measured the location of hyphae and characterized their micropedological environment using an image analyzing system. Mean length density varied between 17 m/cm³ and 100 m/cm³ and was independent of aeration parameters. The percentage of hyphae completely embedded in the soil matrix varied between 30% and 8% and decreased significantly with increasing CO₂ concentration in the soil air. Of the hyphae in the soil matrix, 70% were located in a 50 m shell around the macro pores. Pair correlation functions show, that the majority of soil hyphae occur in clusters with diameters below 100 m. Between 60% and 80% of randomly chosen circles with 250 m diameters were completely devoid of hyphae. The inefficient opening up of the intra-aggregate space by soil hyphae is explained by the very slow oxygen diffusion between air-filled macro pores and the intra-aggregate space and mechanical restrictions for hyphae growth.