Association of Marine Archaea with the Digestive Tracts of Two Marine Fish Species

Recent studies have shown that archaea which were always thought to live under strict anoxic or extreme environmental conditions are also present in cold, oxygenated seawater, soils, the digestive tract of a holothurian deep-sea-deposit feeder, and a marine sponge. In this study, we show, by using PCR-mediated screening in other marine eukaryotes, that marine archaea are also present in the digestive tracts of flounder and grey mullet, two fish species common in the North Sea, in fecal samples of flounder, and in suspended particulate matter of the North Sea water column. No marine archaea could be detected in the digestive tracts of mussels or the fecal pellets of a copepod species. The archaeal 16S ribosomal DNA clone libraries of feces of flounder and the contents of the digestive tracts of grey mullet and flounder were dominated by group II marine archaea. The marine archaeal clones derived from flounder and grey mullet digestive tracts and feces formed a distinct cluster within the group II marine archaea, with 76.7 to 89.8% similarity to previously described group II clones. Fingerprinting of the archaeal community of flounder digestive tract contents and feces by terminal restriction fragment length polymorphism of archaeal 16S rRNA genes after restriction with HhaI showed a dominant fragment at 249 bp, which is likely to be derived from group II marine archaea. Clones of marine archaea that were closely related to the fish-associated marine archaea clones were obtained from suspended particulate matter of the water column at two stations in the North Sea. Terminal restriction fragment length polymorphism fingerprinting of the archaeal community present in suspended particulate matter showed the same fragment pattern as was found for the archaeal community of the flounder digestive tract contents and feces. These data demonstrate that marine archaea are present in the digestive tracts and feces of very common marine fish. It is possible that the marine archaea associated with the digestive tracts of marine fish are liberated into the water column through the feces and subsequently contribute to the marine archaeal community of suspended particulate matter.

过往研究表明，原本被认为仅能存活于严格厌氧或极端环境中的古菌（Archaea），亦可在低温含氧海水、土壤、深海沉积食性海参的消化道以及海洋海绵中被发现。本研究通过对其他海洋真核生物开展聚合酶链式反应（Polymerase Chain Reaction，PCR）介导的筛选实验，证实海洋古菌还存在于北海常见的两种鱼类——比目鱼与鲻鱼的消化道、比目鱼的粪便样本，以及北海水体中的悬浮颗粒物（Suspended Particulate Matter）内。但在贻贝的消化道以及某桡足类物种的粪团中，未检测到海洋古菌的存在。比目鱼粪便、鲻鱼与比目鱼消化道内容物的古菌16S核糖体DNA（16S rDNA）克隆文库中，海洋古菌II类群占据主导地位。从比目鱼、鲻鱼消化道及粪便中获得的海洋古菌克隆子，在海洋古菌II类群中形成了一个独立的进化簇，与已报道的II类群克隆子的相似性仅为76.7%~89.8%。通过对古菌16S rRNA基因采用HhaI限制性内切酶酶切后进行末端限制性片段长度多态性（Terminal Restriction Fragment Length Polymorphism, T-RFLP）指纹图谱分析，发现比目鱼消化道内容物与粪便的古菌群落中，存在一条丰度最高的249 bp片段，该片段大概率来源于海洋古菌II类群。从北海两个采样站的水体悬浮颗粒物中，获得了与鱼类相关海洋古菌克隆子亲缘关系密切的海洋古菌克隆子。对悬浮颗粒物中古菌群落的T-RFLP指纹图谱分析显示，其片段模式与比目鱼消化道内容物及粪便中的古菌群落完全一致。上述实验数据证实，海洋古菌广泛存在于极为常见的海洋鱼类的消化道与粪便中。由此推测，附着于海洋鱼类消化道的海洋古菌可通过粪便释放至水体中，进而参与悬浮颗粒物中海洋古菌群落的构成。