Vertical distribution and phylogenetic characterization of marine Archaea in the western subarctic Pacific

Archaea represent a prevalent and abundant microbial group and are believed to play a crucial role in marine ecosystems. Archaeal distribution remains poorly resolved in the western subarctic Pacific (WSP). We investigated the relative abundance, vertical distribution, and phylogenetic composition of Archaea in WSP. The abundance of Euryarchaeota marine group II (Poseidoniales) was the highest (> 77%) in the epipelagic layer and decreased with depth, the abundance of Thaumarchaeota increased with depth, and Euryarchaeota marine group III (Pontarchaea) was absent in the epipelagic layer but was highly abundant in mesopelagic and bathypelagic layers. Based on phylogenic analyses of archaeal 16S rRNA gene partial sequences, we noted that archaeal phylotypes in the epipelagic layer were different from those in the mesopelagic and bathypelagic layers. The phylogenetic composition in the archaeal community was strongly segregated along a depth gradient. The Shannon index and the richness values revealed that the mesopelagic layer was the most diverse habitat of the archaeal community. Additionally, the trends of the vertical distribution of Archaea and Thaumarchaeota, elucidated through the qPCR assay, at station K2 were similar to those documented in other ocean regions, although their absolute quantity was low. Moreover, the unexpected bias in qPCR was noted to be caused by the commonly used Thaumarchaeota-specific primer MCGI391F, which mismatched some sequences derived from the epipelagic water.