土壌菌核粒子の形成と生物的意図

Abstract

Sclerotium grains, the resting structure of an ectomycorrhizal fungus, Cenococcum graniforme (currently, Cenococcum geophilum), are black and spherical, 1-2 mm in diameter have a characteristic hollow structure, and can be easily found in forest soils. The maximum content of sclerotium grains in surface soils of Japanese Andosols is 3.40 g kg-1, which suggests that the contribution of sclerotium grains to forest soils cannot be ignored as one of the soil organic components. This paper introduces the interests of the biological strategies of Cenococcum geophilum and sclerotium grains by reviewing the findings of advanced studies on micromorphological features, chemical composition, and microbial communities of sclerotium grains.Instruments such as scanning electron microscope (SEM) and energy dispersion X-ray fluorescence micro-analyzer (EDX), electron probe micro-analyzer of wave dispersion type (EPMA) and 27Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) were used to obtain the characteristics of sclerotium grains. A basic finding was that C is the major element in sclerotium grains associated with a relatively large concentration of octahedral Al, which suggests an Al-humus complex. The mean concentrations of major elements in sclerotium grains are quantitatively defined as C (47.6%), O (30.2%), H (3.32%), Al (1.4%), N (0.78%). Carbon in sclerotium grains took the form of large amounts of 0-alkyl C and was also associated with aromatic C and methyl C, which strongly showed a characteristic biological origin and completely different spectral feature to humic acids from an allophonic Andosol. AMS 14C age values obtained for sclerotium grains in surface soils of Japanese forest soils were from modern to 1800 yr BP, which proves its stability and long life as an Al-humus complex. Transmission Electron Microscope (TEM) and EDX techniques revealed the micromorphological features of the Al-oxyhydroxide polymorphs found in sclerotium grains. It is assumed that a biochemical process responsible for host fungi induced Al saturation and precipitation under acidic conditions to form Boemite inside the grain. Studies on microbial communities elucidated the predominance of specific species such as Sphingomonas sp.From a further investigation on the distributions of sclerotium grains in profiles of German Podsols, Braunfaherde and Brown Podsols, it is concluded that development of sclerotium grains was not always regulated by low pH but by the content of exchangeable Al and the status of Al in the soil, regardless of soil type. The sclerotium grain was likely to be formed in soils with high ratios (>0.6) of organic bonding Al (Alp) to amorphous Al (Alo), and with high contents of exchangeable Al (Al3+) (> 0.54 g kg-1).The Al accumulation in sclerotium grains and the close relationship between sclerotium grain density and the status of active Al in soils suggest the symbiotic function of Cenococcum geophilum with root by reducing the toxicity of Al under low pH in rhizosphere. Sclerotium grain may also serve as a shelter for microorganisms and preserve their diversity under severe environmental conditions. Although the findings we have are still very limited, the goal of the study is to understand further implications of sclerotium formation in a soil ecosystem, which may help us to learn what strategy is necessary for biota to survive a catastrophe.