Light Structures Phototroph, Bacterial and Fungal Communities at the Soil Surface

The upper few millimeters of soil harbour photosynthetic microbial communities that are structurally distinct from those of underlying bulk soil due to the presence of light. Previous studies in arid zones have demonstrated functional importance of these communities in reducing soil erosion, and enhancing carbon and nitrogen fixation. Despite being widely distributed, comparative understanding of the biodiversity of the soil surface and underlying soil is lacking, particularly in temperate zones. We investigated the establishment of soil surface communities on pasture soil in microcosms exposed to light or dark conditions, focusing on changes in phototroph, bacterial and fungal communities at the soil surface (0–3 mm) and bulk soil (3–12 mm) using ribosomal marker gene analyses. Microbial community structure changed with time and structurally similar phototrophic communities were found at the soil surface and in bulk soil in the light exposed microcosms suggesting that light can influence phototroph community structure even in the underlying bulk soil. 454 pyrosequencing showed a significant selection for diazotrophic cyanobacteria such as Nostoc punctiforme and Anabaena spp., in addition to the green alga Scenedesmus obliquus. The soil surface also harboured distinct heterotrophic bacterial and fungal communities in the presence of light, in particular, the selection for the phylum Firmicutes. However, these light driven changes in bacterial community structure did not extend to the underlying soil suggesting a discrete zone of influence, analogous to the rhizosphere.

土壤表层数毫米范围内栖息着光合微生物群落，由于光照的存在，其群落结构与下层原状土壤（bulk soil）中的群落存在显著差异。此前在干旱地区开展的研究已证实，此类群落具有降低土壤侵蚀、促进碳固持与氮固持的重要功能。尽管此类群落分布广泛，但目前对土壤表层与下层原状土壤的生物多样性缺乏比较性认知，在温带地区尤为突出。本研究以牧场土壤为实验对象，设置光照与黑暗两种条件的微宇宙培养体系，探究土壤表层群落的定殖过程；并通过核糖体标记基因（ribosomal marker gene）分析，重点监测土壤表层（0–3 mm）与原状土壤（3–12 mm）中光合营养型微生物、细菌及真菌群落的动态变化。研究发现，微生物群落结构随培养时间发生改变；在光照培养体系中，土壤表层与原状土壤内的光合微生物群落结构趋于相似，这表明光照可对下层原状土壤内的光合营养型微生物群落结构产生影响。454焦磷酸测序（454 pyrosequencing）结果显示，除斜生栅藻（Scenedesmus obliquus）外，固氮蓝细菌如点形念珠藻（Nostoc punctiforme）与鱼腥藻属（Anabaena spp.）也被显著选择性富集。在光照条件下，土壤表层同样栖息着具有独特群落结构的异养细菌与真菌，其中厚壁菌门（Firmicutes）被显著富集。然而，光照驱动的细菌群落结构变化并未波及下层原状土壤，这表明光照对微生物群落的影响存在一个离散的作用区域，其特征与根际（rhizosphere）类似。