OTU analysis of surface seawater
To do that, total DNAs from the above four samples were extracted by the CTAB/SDS method  and were diluted to 1 ng/L with sterile water and used for PCR template. 16S rRNA genes of distinct regions (16S V3/V4) were amplified using specific primers (341F: 5-CCTAYGGGRBGCASCAG and 806R: 5- GGACTACNNGGGTATCTAAT) with the barcode. The PCR products were purified with a Qiagen Gel Extraction Kit (Qiagen, Germany) and prepared to construct libraries. Sequencing libraries were generated using TruSeq DNA PCR-Free Sample Preparation Kit (Illumina, USA) following the manufacturers instructions. The library quality was assessed on the Qubit@ 2.0 Fluorometer (Thermo Scientific) and Agilent Bioanalyzer 2100 system. And then the library was sequenced on an Illumina NovaSeq platform and 250 bp paired-end reads were generated. Paired-end reads were merged using FLASH (V1.2.7, http://ccb.jhu.edu/software/FLASH/) . Quality filtering on the raw tags was performed under specific filtering conditions to obtain the high-quality clean tags [42] according to the QIIME (V1.9.1, http://qiime.org/scripts/split_libraries_fastq.html) quality controlled process.