Effects of bottom-water hypoxia on sediment bacterial community composition in a seasonally hypoxic enclosed bay (Omura Bay, West Kyushu, Japan)

Abstract

The bacterial community strongly drives carbon and other biogeochemical cycles in marine sediment. However, little is known about the impact of dissolved oxygen (DO) availability on bacterial community composition. To fill this gap, we examined diversity, richness and structure of the bacterial population for three consecutive years (2011-2013) in the uppermost (0-5 and 0-7 mm depth) and the subsurface layers (5-10 and 7-14 mm depth) of Omura Bay, Kyushu, Japan, a seasonally hypoxic enclosed bay. Automated ribosomal intergenic spacer analysis revealed a unimodal pattern of diversity indices with DO, peaking at the suboxic (11 μM O2) condition. Shifts in the bacterial communities were also evident in response to the availability of DO. Changes in the operational taxonomic units (OTUs) that were less abundant accounted for a large part of the community dissimilarity. It was further demonstrated that the relative abundance of OTUs affiliated with Gammaproteobacteria was correlated positively with DO, while that with Deltaproteobacteria was inversely correlated with DO. These results strongly suggest that DO availability of bottom water plays a fundamental role in shaping the bacterial community in sediment surfaces of shallow coastal areas.

Dynamics of Sulfate-Reducing Bacteria Community Structure in Surface Sediment of a Seasonally Hypoxic Enclosed Bay

“…One of the possible causes for depth-related differences is bioturbation mediated by burrow-building infauna. Mori et al (20) found that small polychaetes generally formed a greater number of burrows in the uppermost layer than in the subsurface sediment layer during the normoxic period.…”

“…However, as reported in the previous study, DO in the bottom water overlying the sediment surface may be used as a proxy for that within the surface sediment. This assumption may be valid at a depth layer of at least 0–5 mm in which bioturbation appears to be very active under normoxic conditions (20). Sediment below this depth must always be less oxic than that above.…”

“…During the summer months in 2012 and 2013 (Table S1), sediment sampling was conducted at central Omura Bay (32°55.390N, 129°51.350E), as described by Mori et al (20). In brief, three replicate sediment core samples were collected from the sampling station at a depth of 20±1 m with an acrylic pipe (31 cm in length and an inner diameter of 26 mm) by scuba diving, carefully avoiding areas with visible bioturbation.…”

“…The central region of Omura Bay, a seasonally hypoxic enclosed bay in Western Kyushu, is a natural laboratory in which a wide range of different DO conditions may be investigated due to consistent seasonal hypoxia, providing a suitable research environment to conduct the long-term monitoring of sediment bacterial communities (20, 22, 33, 35). During the hypoxic period (generally between July and September), the production and accumulation of H 2 S by SRB in the sediment becomes very active, contributing greatly to oxygen consumption by sediment (19), and apparently fueling the massive growth of Beggiatoa -like sulfur-oxidizing bacteria on surface sediment (37).…”

“…During the hypoxic period (generally between July and September), the production and accumulation of H 2 S by SRB in the sediment becomes very active, contributing greatly to oxygen consumption by sediment (19), and apparently fueling the massive growth of Beggiatoa -like sulfur-oxidizing bacteria on surface sediment (37). Mori et al (20) demonstrated that each of the uppermost (0–5 mm) and subsurface (5–10 mm) sediments harbored a distinctive bacterial community structure underlying hypoxic bottom water. They also showed that Deltaproteobacteria , presumably representing a large portion of the SRB community, increased in relative abundance as conditions became more hypoxic.…”