18S rDNA of marine sediments
The V4 region fragments of 18S rRNA genes were amplified using universal primer pair V4F (CCA GCA SCY GCG GTA ATW CC)-V4R (ACT TTC GTT CTT GAT YRA). The PCR reaction system contained 15 L of Phusion High-Fidelity PCR Master Mix (New England Biolabs), 2 M of forward and reverse primers, and about 10 ng template DNA. Thermal cycling was under the following conditions: 1 min at 98  for initial denaturation; 10 s at 98  for denaturation, 30 cycles; 30 s at 50  for annealing, 30 s at 72  for elongation, and 5 min at 72  for finally extension. PCR products were detected using 2% agarose gel electrophoresis with loading buffer and SYB green dye. Mixed in equidensity ratios, PCR products were then purified by using Qiagen Gel Extraction Kit (Qiagen, Germany).  Sequencing libraries were generated using TruSeq DNA PCR-Free Sample Preparation Kit (Illumina, USA) following manufacturer's recommendations and index codes were added.