溶藻菌群对铜绿微囊藻的溶藻效应及其作用机理研究

水华暴发是一个世界性的问题，近年来在发展中国家显得尤其严重。水华暴发给环境和公众健康带来巨大灾难，一些蓝藻产生的毒素可以造成鱼类、鸟禽和家畜的死亡，而臭名昭著的微囊藻产生的微囊藻毒素更是有强烈致癌效应。因此，寻找控制水华藻类的有效方法非常迫切。在利用物理和化学方法处理不甚理想的情况下，利用溶藻细菌控藻成为一个新的研究方向。溶藻细菌一般直接从富营养化水体中分离，杀藻活力对有害蓝藻具有较强的选择性而不危害其它生物，尤其适合在水华发生初期使用，可以在短时间内达到阻止藻类增殖的效果。本研究富集分离到一个高效溶解铜绿微囊藻的溶藻菌群，对其溶藻效应和溶藻机制进行了探索研究。 1溶藻菌群的富集筛选及其溶微囊藻效果 富集筛选得到一个有明显抑藻效果的菌群，它对铜绿微囊藻有显著溶藻效果。与对照组相比，加入富集的溶藻菌后，第4 d开始出现溶藻现象，6~8 d出现明显的溶藻效果，8 d后测得叶绿素去除率在85%以上。 2 溶藻菌群的作用范围及溶藻特性 富集分离到的溶藻菌群对铜绿微囊藻和念珠藻有显著溶藻作用，对水华微囊藻和其它几株受试微囊藻没有明显溶藻效应。该溶藻菌群不仅可以在液体中溶解铜绿微囊藻，生长在固体平板上的藻苔也有一定的溶藻效应，生成溶藻空斑。保证快速溶藻的最大稀释度可以达到1/100, 000。 3 环境因子对菌群溶藻效力的影响 试验发现，不同的pH、温度、和光照条件下，溶藻菌群溶藻效力明显不同，且不同种类的氮源对其溶藻作用也有一定影响。这些条件对该菌群溶藻作用的影响，在相当的程度上可能取决于它们对藻和细菌两者的生长状况的影响综合。 4 溶藻菌群的溶藻作用机理 溶藻菌液过滤除菌和煮沸灭菌处理后溶藻液，未见明显的溶藻效果，只有原液具有很好的溶藻效果。因此可初步确定，蓝藻细胞的溶解可能是由溶藻菌直接接触藻细胞产生的作用效果。显微镜观察发现，细菌在溶藻的过程中频繁地接触藻细胞并侵入藻细胞，破坏进而裂解杀死藻细胞。这也进一步说明了此溶藻菌是通过直接方式杀藻。 5 溶藻菌群的菌群结构解析 分离有溶藻效果的纯菌的多次尝试都没有成功。结合DGGE和16S rDNA文库综合分析发现：Rubritepida菌，假单胞菌和鞘氨醇单胞菌是存在于铜绿微囊藻中的三种伴生细菌。加入富集的溶藻菌群后，菌群结构发生明显的变化，Rubritepida菌、假单胞菌消失，混合菌群则包含未培养黄杆菌，鞘氨醇单胞菌和噬氢菌，其中黄杆菌是优势菌群，并且细菌种群结构的变化与藻细胞消亡之间有显著的相关性。通过菌种的分离鉴定与DGGE和16S rDNA文库的测序结果比较，一些未培养菌可能在溶藻过程中起重要调控作用。 6 溶藻细菌控藻应用基础 (1) 扩大规模的模拟水华实验进一步确定了细菌对微囊藻的强烈溶解作用。 (2) 铜绿微囊藻(Microcystis aeruginosa 905, zc)、微囊藻(Microcystis spp., zd)和溶藻菌群共培养试验表明，zc可以抑制zd生长，而溶藻菌群可以溶zc。 本研究是第一次报道混合菌群的溶藻效应。该溶藻菌群对带有藻际细菌的铜绿微囊藻具有高效的溶藻效力，表明它对自然界中存在的带菌铜绿微囊藻和其它一些蓝藻的生消具有一定的控制作用。对进一步研究菌藻关系与生态学作用，以及对富营养化湖泊和水库水体中蓝藻暴发的防控，该菌群具有一定的应用潜力。 Cyanobacterial blooms break out frequently all over the world, especially in developing countries. Blooms create enormous disasters to public health and to the environment. Some cyanobacterial blooms produce extremely toxic substances that have killed fish, domestic animals and birds. It has been well known that microcystins, a hepatoxin produced by Microcystis, can promote tumors in humans. So it is very important to find an effective method for controlling the growth of the bloom-forming algae. Measures for controlling such kind of algae include physical, chemic and biologic means, but the former two may damage the aquatic environment and require high-energy inputs. The alternative approach for the elimination of nuisance algae involves the application of algicidal bacteria. The algicidal bacteria, which are nontoxic to other organisms and most of which are isolated from the eutrophic lake in situ, may be potential microbial algaecides. In the initial stages of the water blooms, they are able to restrain the biomass or multiplication of the bloom-forming algae in a short time. In order to use algicidal bacteria to suppress blooms of M. aeruginosa, we isolated a bacterial culture capable of lysing the noxious cyanobacteria M. aeruginosa. In this paper we described some properties of the bacterial culture and its growth-inhibiting or algicidal effects on the growth of M. aeruginosa, and investigated its algicidal mechanisms. 1 Enrichment of a microbial culture that lyses Microcystis aeruginosa A mixed bacterial culture was isolated from a hypereutrophic pond and showed significant algicidal activity against the noxious Microcystis aeruginosa. Algae lysis would be seen obviously 4 days later when the algae culture was killed and became yellow contrast to no-addition controls, and chlorophyll a (chl-a) reduction went beyond 85% 8 days later. 2 The host range and some other algicidal feature of the mixed algicidal culture. Microcystis aeruginosa, Nostoc sp., were susceptible to the mixed algicidal culture, while the lytic effects of this mixed culture on Microcystis flos-aquae and some other tested Microcystis were feeble.The algicidal culture can not only lyse M. aeruginosa in liquid media, but aslo lyse M. aeruginosa lawns on soft agar plates and form plaques. The maximun dilution of the mixed culture required for rapid Microcystis lysis is 1/100, 000. 3 Influences of environmental factors such as pH, temperature, illumination, and the nitrogen source on the lytic activity of the mixed bacterial culture on Microcystis aeruginosa. In our investigations, it was shown that the lytic activity of the mixed bacterial culture on Microcystis aeruginosa was straightly correlated with pH, temperature, illumination, as well as the nitrogen source in the medium. The impacts of these environmental factors on the algicidal activity of the mixed bacterial culture, to a certain extent, may depend on both the algal and the bacterial growth rates under the tested environmental conditions. 4 The mechanisms of algal cell lysis by the algicidal bacteria Death was detected when the mixed bacterial culture was added to the algal culture, but not when only the culture filtrate or autoclaved bacterial culture was added. This indicates that the mixed bacterial culture did not release extracellular products inhibitory to Microcystis aeruginosa. In addition, under the microscope, we observed frequent contacts btween bacteria and algae cells, and some bacteria can even penetrate into target algal cells and destroyed them. These results may suggest that the bacterium kill the alga by direct contact. 5 Molecular Characterization of the algicidal bacterial culture Attempts for isolation of pure bacterium or bacteria from the enrichment culture responsible for Microcystis lysis have so far been failed. Based on PCR-DGGE (denaturing gradient gel electrophoresis) and 16S rDNA clone library analysis, Rubritepida sp., Pseudomonas sp. and Sphingomonas sp., as accompanying bacteria, were existed in M. aeruginosa. The bacterial community in M. aeruginosa showed significant change after adding the enrichment culture, where uncultured Flavorbacterium sp., Sphingomonas sp. and Hydrogenophaga sp. were observed, and the uncultured Flavorbacterium sp. became a dominant species. The obvious correlation can be seen between change of bacterial population and extinction of M. aeruginosa. Compared identification of pure bacterium with sequencing of DGGE bands and the clone distribution of the clone libraries, it was inferred that some uncultured bacteria were probably play an important role in controlling the growth and abundance of M. aeruginosa. This report is the first example of a mixed bacterial culture with the ability to lyse M. aeruginosa. 6 Further study for algae control by applications of algicidal bacteria (1) Algae lysis would be seen obviously 6 days later when the algae culture was killed and became yellow contrast to no-addition controls, and chlorophyll a (chl-a) was reducted to a low level 20 days later in the simulated water bloom experiments. (2) The growth of Microcystis sp. (zd) was restrained by Microcystis aeruginosa 905 (zc) when they were co-cultured together, and zc was lysed by the algicidal bacterial culture. This report is the first example of a mixed bacterial culture with the ability to lyse M. aeruginosa, and its algicidal activity remained high against non-axenic tested M. aeruginosa, suggesting that bacteria in the natural environment could play a role in controlling the growth and abundance of M. aeruginosa and other cyanobacteria. Such bacteria could also potentially be used as agents to prevent the mass development of cyanobacteria in eutrophic lakes and reservoirs.