Bacteria, protists and fungi may hold clues of seamount impact on diversity and connectivity of deep-sea pelagic communities

The interacting features of topography and hydrography around deep seamounts have strong influence on plankton biogeography. In addition, the intrinsic properties of various biological taxa inherently shape their distribution. Therefore, bacterial, protist and fungal diversity was investigated across the water column above and below the summit of a flat-topped seamount in the Western Pacific Ocean. We determined the distribution and connectivity patterns of bacteria, protists and fungi around the seamount, and explored the processes driving the observed distribution patterns. We found that the seamount enhanced the vertical connectivity of bacterial and protist communities, but significantly reduced the protist community connectivity along the horizontal dimension. Such effects provide ecological opportunities for eukaryotic adaption and diversification, resulting in a greater diversity on seamounts than in surrounding deep seas.

Methods
Thirty-eight water samples from five sampling stations were collected at the Kocebu Guyot in the Magellan Seamount chain located in the oligotrophic tropical Western Pacific Ocean. A total of 20 L seawater was collected from each water layer at all sampling stations with Niskin Bottles attached to a rosette equipped with a Seabird CTD probe. Samples were pre-filtered first through a 200 µm sieve, and subsequently filtered through a 0.22 µm polycarbonate filter. Filters were then flash frozen in liquid nitrogen and stored at −80°C until further processing. 

Environmental DNA was extracted from seawater samples using the All Prep DNA/RNA Mini Kit (Qiagen, Germany) following the manufacturer’s instructions. Specific primers and PCR protocols were utilized to amplify different target sequences. In order to amplify 16S gene sequences in bacteria, forward primer U341F  and reverse primer R685  were set. The hypervariable V4 region of protist 18S rRNA was amplified using a set of eukaryote-specific primers Reuk454FWD1 and TAReukREV3 . For the detection of fungi, the ITS2 (Internal transcribed spacer 2) gene was amplified using specific primers ITS3 and ITS4 . Each sample was analyzed in three PCR replicates for each primer pair to minimize PCR bias.

深海海山周边地形与海洋水文的交互特征对浮游生物生物地理学具有显著调控作用。此外，不同生物类群的固有属性亦从本质上塑造了其分布格局。本研究以西太平洋一座平顶海山的峰顶上下水体柱为研究对象，调查其中细菌、原生生物与真菌的多样性；明确了该海山周边细菌、原生生物及真菌的分布模式与连通特征，并探究了驱动上述分布格局形成的生态学过程。研究结果显示，该海山可提升细菌与原生生物群落的垂直连通性，但显著降低了原生生物群落的水平连通性。此类效应为真核生物的适应与分化提供了生态机遇，最终使得海山区域的生物多样性高于周边深海海域。

## 材料与方法
本研究于西太平洋贫营养热带海域的麦哲伦海山链科塞布平顶海山（Kocebu Guyot）开展，共设置5个采样站位，采集38份海水样本。使用搭载海鸟（Seabird）公司温盐深（CTD）探头的采水架配套的尼斯金采水器（Niskin Bottles），于各采样站位的各水层采集20升海水样本。样本首先经200微米筛网预过滤，随后通过0.22微米聚碳酸酯滤膜过滤。滤膜经液氮快速冷冻后，保存于-80℃环境以待后续处理。

采用All Prep DNA/RNA Mini Kit（凯杰（Qiagen），德国），按照制造商操作说明从海水样本中提取环境DNA（Environmental DNA）。使用特异性引物与聚合酶链式反应（PCR）扩增不同靶标序列：针对细菌16S基因序列，采用上游引物U341F与下游引物R685进行扩增；针对原生生物18S核糖体RNA（18S rRNA）的高变V4区，使用真核生物特异性引物对Reuk454FWD1与TAReukREV3进行扩增；针对真菌检测，使用特异性引物对ITS3与ITS4扩增内部转录间隔区2（ITS2，Internal transcribed spacer 2）基因。为降低PCR偏好性，每个引物对应样本均设置3次PCR重复实验。