Abstract:[Background] The seagrass meadow was a “Blue carbon” ecosystem contributing greatly to global carbon sequestration in coastal sediments. The vertical profile of seagrass sediments exhibits a strong redox gradient, where the surface layer was oxic and rich in labile organic matters and the deeper layer was reduced and dominated by recalcitrant organic matters. [Objective] We hypothesized that bacterial and archaeal communities varied greatly in abundance and community structure along the vertical gradient in the seagrass sediments. [Methods] Quantitative real-time PCR and high-throughput sequencing were applied to characterize the prokaryotic communities at different sediment depths (5, 10, 15, 20, 25 and 30 cm) in the Zostera marina dominated meadow. [Results] The 16S rRNA gene copy numbers of bacteria and archaea decreased with the increasing sediment depth, and the bacterial copy numbers in the 5-cm layer was significantly higher than those in the 20-cm and 30-cm layers (ANOVA test, P<0.05). Depth had no significant effect on bacterial and archaeal α diversity indices (P>0.05). The most dominant bacterial phylum was Proteobacteria, followed by Chloroflexi, Bacteroidetes, and Planctomycetes. The relative abundances of δ-Proteobacteria and Planctomycetes were significantly increased with increasing depth (P<0.05). Bathyarchaeota was the most dominant archaeal phylum, accounting for 70% in the 25-cm layer. Other abundant archaeal phyla were Woesearchaeota, Lokiarchaeota, Euryarchaeota and Thaumarchaeota. The relative abundance of Thaumarchaeota decreased significantly with increasing depth (P<0.05). [Conclusion] The benthic archaeal and bacterial communities in the seagrass meadow sediments exhibited obvious vertical characteristic, which could be driven by organic matter composition and sediment redox status.
