Molecular Evidence for a Uniform Microbial Community in Sponges from Different Oceans

Sponges (class Porifera) are evolutionarily ancient metazoans that populate the tropical oceans in great abundances but also occur in temperate regions and even in freshwater. Sponges contain large numbers of bacteria that are embedded within the animal matrix. The phylogeny of these bacteria and the evolutionary age of the interaction are virtually unknown. In order to provide insights into the species richness of the microbial community of sponges, we performed a comprehensive diversity survey based on 190 sponge-derived 16S ribosomal DNA (rDNA) sequences. The sponges Aplysina aerophoba and Theonella swinhoei were chosen for construction of the bacterial 16S rDNA library because they are taxonomically distantly related and they populate nonoverlapping geographic regions. In both sponges, a uniform microbial community was discovered whose phylogenetic signature is distinctly different from that of marine plankton or marine sediments. Altogether 14 monophyletic, sponge-specific sequence clusters were identified that belong to at least seven different bacterial divisions. By definition, the sequences of each cluster are more closely related to each other than to a sequence from nonsponge sources. These monophyletic clusters comprise 70% of all publicly available sponge-derived 16S rDNA sequences, reflecting the generality of the observed phenomenon. This shared microbial fraction represents the smallest common denominator of the sponges investigated in this study. Bacteria that are exclusively found in certain host species or that occur only transiently would have been missed. A picture emerges where sponges can be viewed as highly concentrated reservoirs of so far uncultured and elusive marine microorganisms.

海绵（多孔动物门Porifera）是演化上古老的后生动物（metazoans），它们大量栖息于热带海洋，同时也见于温带海域甚至淡水环境中。海绵体内定植有大量嵌入宿主组织基质的细菌。目前学界对这类细菌的系统发育关系以及二者互作的演化年代几乎一无所知。为深入解析海绵微生物群落的物种丰富度，我们基于190株海绵来源的16S核糖体DNA（16S rDNA）序列开展了全面的多样性调查。本研究选取筒海绵（Aplysina aerophoba）与斯温霍海绵（Theonella swinhoei）构建细菌16S rDNA文库，原因在于二者在分类学上亲缘关系较远，且栖息的地理区域互不重叠。在这两种海绵中，均发现了统一的微生物群落，其系统发育特征与海洋浮游生物或海洋沉积物显著不同。研究共鉴定出14个单系的海绵特异性序列簇，它们隶属于至少7个不同的细菌门类。根据定义，每个簇内的序列之间的亲缘关系均近于其与非海绵来源序列的亲缘关系。这类单系序列簇涵盖了目前所有公开可用的海绵来源16S rDNA序列的70%，印证了该现象的普遍性。该共享微生物组分代表了本研究所调查海绵的最小共同微生物组。那些仅存在于特定宿主物种或仅短暂定植的细菌则未被本研究纳入。综上可见，海绵可被视为高度富集迄今尚未培养且难以获取的海洋微生物的储库。