Sedimentary hiatus causes abrupt decline and shifts in marine subsurface sediment microbial communities: a study from IODP Exp. 378 Site U1553 offshore southern New Zealand drilling
Hiatus within marine sediment are discontinuities in stratigraphic layers, forming adjacent layers of different ages and potentially abrupt change in microbial abundances and communities. To investigate the effects of hiatus on the microbial distribution in the marine subsurface, we examined microbial abundance and community in marine subsurface sediments across age gaps caused by hiatus at International Ocean Discovery Program (IODP) Expedition 378 Site U1553 located south of New Zealand. The microbial abundance was quantified using a cell separation technique followed by DNA-staining and epifluorescence microscopy observations. The microbial community was assessed using a metabarcoding approach.  Cell abundances decrease logarithmically with depth overall, but there was abrupt decline at ~4mbsf due to a sediment hiatus. The regression slope of the cell concentration against sediment depth at Site U1553 was steeper or comparable to  relatively nearby IODP drilling site in ultra-oligotrophic South Pacific Gyre (Expedition 329). A microbial community shift at the hiatus, suggesting disruptions to sediment deposition has significant effect of microbial distribution in marine subsurface. Aerophobia was the predominant microbe identified, showing up to 84.1% of relative abundance. A shift in interstitial water geochemistry of manganese and ammonium coincided with microbial abundance but not with the community, indicating that the porewater geochemistry changes by diffusion and does not necessarily match the microbial community. These findings indicate that sedimentary hiatus cause major shifts in microbial communities and abrupt decreases of microbial abundance in marine subsurface sediments. Further investigations into additional sedimentary hiatuses will enhance our understanding of their ecological impacts on subseafloor microbial communities.