Faculty of Life and Environmental Science, Shimane University, 1060 Nishikawatsu, Matsue, Shimane 690-8504, Japan.
Faculty of Education, Shimane University, 1060 Nishikawatsu, Matsue, Shimane 690-8504, Japan; Estuary Research Center, Shimane University, 1060 Nishikawatsu, Matsue, Shimane 690-8504, Japan.
Harmful Algae. 2019 Apr;84:19-26. doi: 10.1016/j.hal.2019.01.010. Epub 2019 Mar 15.
Geosmin is an earthy-muddy smelling compound produced in aquatic ecosystems by microorganisms including cyanobacteria. An increase in geosmin levels affecting the local fishery occurred in May 2007 in Lake Shinji, Japan, and geosmin-producing colonial cyanobacterium, Coelosphaerium sp. G2, was isolated from a water sample from the lake and identified. Cyanobacteria Coelosphaerium sp. is commonly found in Lake Shinji; however, prior to 2007, earthy-muddy odors were not a frequent issue. Further, there was no information regarding the geosmin biosynthetic gene in colonial cyanobacteria. Here, the geosmin biosynthetic gene (geoA) in strain G2 was identified and its nucleotide sequence was determined. It was found that geoA had 79% and 78% identity with geoA from filamentous geosmin-producing cyanobacteria Fischerella sp. PCC 9431 and geoA2 from Phormidium sp. P2r, respectively. The deduced amino acid sequence of GeoA consisted of two domains that were annotated as terpene cyclase. In 2015, geosmin non-producing Coelosphaerium sp. S3C5 was isolated from Lake Shinji and identified by morphological and genetic analyses. There was no difference in morphology or nucleotide sequences of 16S rRNA and 16S-23S internal transcribed spacer (ITS) between geosmin-producing and non-producing strains, which are therefore closely related and can exist in Lake Shinji. Distinguishing the two strains by observation under a microscope and sequencing of 16S rRNA and 16S-23S ITS have proven difficult. Inconsistency between the appearance of Coelosphaerium cells and the detection of the odor in water samples could therefore be attributed to dominance by the geosmin-producing strain or the non-producing strain. The increase in earthy smell is assumed to be caused by an increase in the geosmin-producing strain in Lake Shinji. Genetic analysis of geoA in Coelosphaerium sp. and the relative abundances of geosmin-producing and non-producing Coelosphaerium strains in Lake Shinji can be used to mitigate the economic damages caused by geosmin. Development of a molecular method to monitor the geosmin-producing strain in water ecosystems is equally important to alleviate the earthy smell caused by this particular strain.
土臭素是一种泥土味的化合物，由包括蓝藻在内的水生生态系统中的微生物产生。2007 年 5 月，日本神井水体内的土臭素水平升高，影响当地渔业，从该湖水样本中分离出并鉴定出产生土臭素的蓝藻聚球藻属（Coelosphaerium）G2 菌株。聚球藻属在神井湖中很常见，但在 2007 年之前，土腥味并不是一个常见的问题。此外，关于蓝藻聚球藻属的土臭素生物合成基因也没有相关信息。在此，鉴定了 G2 菌株的土臭素生物合成基因（geoA），并确定了其核苷酸序列。发现 geoA 与丝状产土臭素蓝藻 Fischerella sp. PCC 9431 的 geoA 和 Phormidium sp. P2r 的 geoA2 分别具有 79%和 78%的同源性。GeoA 的推导氨基酸序列由两个被注释为萜烯环化酶的结构域组成。2015 年，从神井湖中分离出不产土臭素的聚球藻属 S3C5 菌株，并通过形态和遗传分析进行鉴定。产土臭素和不产土臭素菌株的 16S rRNA 和 16S-23S 内部转录间隔区（ITS）的核苷酸序列没有差异，因此它们密切相关，可以存在于神井湖中。通过显微镜观察和 16S rRNA 和 16S-23S ITS 测序来区分这两个菌株一直很困难。因此，无法解释细胞形态与水样中土臭素检测结果之间的不一致，这可能归因于产土臭素菌株或不产土臭素菌株的优势。神井湖土腥味的增加被认为是由于产土臭素菌株的增加所致。对聚球藻属 sp.geoA 的遗传分析以及神井湖中产土臭素和不产土臭素聚球藻属菌株的相对丰度可以用来减轻土臭素造成的经济损失。开发一种监测水生态系统中产土臭素菌株的分子方法对于减轻该特定菌株引起的土腥味同样重要。
