Murase Jun, Noll Matthias, Frenzel Peter
Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya 464-8601, Japan.
Appl Environ Microbiol. 2006 Aug;72(8):5436-44. doi: 10.1128/AEM.00207-06.
Flooded rice fields have become a model system for the study of soil microbial ecology. In Italian rice fields, in particular, aspects from biogeochemistry to molecular ecology have been studied, but the impact of protistan grazing on the structure and function of the prokaryotic community has not been examined yet. We compared an untreated control soil with a gamma-radiation-sterilized soil that had been reinoculated with a natural bacterial assemblage. In order to verify that the observed effects were due to protistan grazing and did not result from sterilization, we set up a third set of microcosms containing sterilized soil that had been reinoculated with natural assemblage bacteria plus protists. The spatial and temporal changes in the protistan and prokaryotic communities were examined by denaturing gradient gel electrophoresis (DGGE) and terminal restriction fragment length polymorphism (T-RFLP) analysis, respectively, both based on the small-subunit gene. Sequences retrieved from DGGE bands were preferentially affiliated with Cercozoa and other bacteriovorous flagellates. Without protists, the level of total DNA increased with incubation time, indicating that the level of the microbial biomass was elevated. Betaproteobacteria were preferentially preyed upon, while low-G + C-content gram-positive bacteria became more dominant under grazing pressure. The bacterial diversity detectable by T-RFLP analysis was greater in the presence of protists. The level of extractable NH4+ was lower and the level of extractable SO4(2-) was higher without protists, indicating that nitrogen mineralization and SO4(2-) reduction were stimulated by protists. Most of these effects were more obvious in the partially oxic surface layer (0 to 3 mm), but they could also be detected in the anoxic subsurface layer (10 to 13 mm). Our observations fit well into the overall framework developed for protistan grazing, but with some modifications pertinent to the wetland situation: O2 was a major control, and O2 availability may have limited directly and indirectly the development of protists. Although detectable in the lower anoxic layer, grazing effects were much more obvious in the partially oxic surface layer.
淹水稻田已成为土壤微生物生态学研究的一个模型系统。特别是在意大利的稻田中，从生物地球化学到分子生态学等方面都已得到研究，但原生生物捕食对原核生物群落结构和功能的影响尚未得到检验。我们将未经处理的对照土壤与用天然细菌群落重新接种的经伽马射线灭菌的土壤进行了比较。为了验证观察到的效应是由于原生生物捕食而非灭菌所致，我们设置了第三组微观世界，其中包含用天然群落细菌加原生生物重新接种的灭菌土壤。分别基于小亚基基因，通过变性梯度凝胶电泳（DGGE）和末端限制性片段长度多态性（T-RFLP）分析来检测原生生物和原核生物群落的时空变化。从DGGE条带中检索到的序列主要与Cercozoa和其他食细菌鞭毛虫相关。没有原生生物时，总DNA水平随培养时间增加，表明微生物生物量水平升高。β-变形菌优先被捕食，而低G + C含量的革兰氏阳性菌在捕食压力下变得更占优势。在有原生生物存在的情况下，通过T-RFLP分析可检测到的细菌多样性更高。没有原生生物时，可提取的NH4+水平较低，可提取的SO4(2-)水平较高，表明原生生物刺激了氮矿化和SO4(2-)还原。这些效应大多在部分有氧的表层（0至3毫米）更为明显，但在缺氧的亚表层（10至13毫米）也能检测到。我们的观察结果很好地符合了为原生生物捕食所构建的总体框架，但针对湿地情况有一些相关的修正：氧气是主要的控制因素，氧气的可利用性可能直接或间接限制了原生生物的发展。尽管在较低的缺氧层也能检测到，但捕食效应在部分有氧的表层更为明显。
