石人山古墳装飾石棺表面に形成した着生生物群集の構造解析

摘要

福岡県広川町に所在する国指定史跡·石人山古墳は，5世紀前半に築造された前方後円墳であり，後円部中心に横口式家形石棺と前方部と後円部の境には武装石人が置かれた古墳である。阿蘇溶結凝灰岩で作られた石棺の棺蓋表面や横口部には，直弧文や同心円文などの彫刻文様が浮彫されている。石棺には，鉄筋コンクリート製の保護覆屋が設けられており，西側に鉄格子の扉と東側の側壁上部に小窓を有している。石棺は常時公開されており，西側から鉄格子の扉を通して石棺を自由に見学することができるが，西側が大きく開口しているために，ここから日射や風雨の影響が石棺に及んでいると考えられる。朽津ら(2015)は，石棺の西側で顕著な風化が認められ，浮彫の凹凸が一部で不明瞭になっていること，加えて石棺西側面の広範囲にわたって緑色の着生生物が分布しており，この部分の文様の認識が最良でない状態になっていることを保存と活用上の課題として指摘している(図1)。%Sekijinsan Tumulus in Hirokawa, Fukuoka prefecture is a keyhole-shaped burial mound with a decorated stone sarcophagus that was built in the first half of the 5~(th) century. Recently, green epiphytic organisms have spread widely on the western side of the sarcophagus surface, preventing preservation and exhibition to the public. The aim of the present study was to analyze the epiphytic microbial community by using molecular techniques to determine the algal, fungal and bacterial community structures on the sarcophagus surface. The algal gene sequences closely related to those of nitrogen-fixing Tolypothrix distorta and salt-tolerant Halospirulina were detected. The environmental factors of limited nitrogen source, solar radiation, and dry and salty conditions were considered to have contributed to the ability of these algae to grow on the sarcophagus surface. Moreover, algal gene sequences closely related to that of Cyanidium were detected. The genus Cyanidium is a red algal extremophile which typically grows in acidic hot springs. It is known that organisms related to such an evolutionarily old red alga are able to grow on the stone sarcophagus surface. In fungal community structure analysis, sequences closely related to Mariannaea elegans and Trichoderma virens were commonly and frequently observed. In bacterial community structure analysis, Arthrobacter russicus, Sphingomonas leidyi and Brevundimonas vesicularis were commonly detected. In the present study, by using a molecular biology approach, the community structures of microorganisms on the stone sarcophagus surface were clarified. However, these community structures are expected to change even under natural conditions, depending on the surrounding environment that is influenced by annual and seasonal fluctuations or the growth of trees. With these changes, it is important to conduct ongoing surveys while continuing studies on biological control methods.