Abstract
The dynamics of potential oxygen consumption at the sediment surface in a seasonally hypoxic bay were monitored monthly by applying a tetrazolium dye (2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyl-2H-tetrazolium chloride [INT]) reduction assay to intact sediment core samples for two consecutive years (2012–2013). Based on the empirically determined correlation between INT reduction (INT-formazan formation) and actual oxygen consumption of sediment samples, we inferred the relative contribution of biological and non-biological (chemical) processes to the potential whole oxygen consumption in the collected sediment samples. It was demonstrated that both potentials consistently increased and reached a maximum during summer hypoxia in each year. For samples collected in 2012, amplicon sequence variants (ASVs) of the bacterial 16S rRNA genes derived from the sediment surface revealed a sharp increase in the relative abundance of sulfate reducing bacteria toward hypoxia. In addition, a notable shift in other bacterial compositions was observed before and after the INT assay incubation. It was Arcobacter (Arcobacteraceae, Campylobacteria), a putative sulfur-oxidizing bacterial genus, that increased markedly during the assay period in the summer samples. These findings have implications not only for members of Delta- and Gammaproteobacteria that are consistently responsible for the consumption of dissolved oxygen (DO) year-round in the sediment, but also for those that might grow rapidly in response to episodic DO supply on the sediment surface during midst of seasonal hypoxia.
Highlights
Surface sediment plays an important role in the oxygen cycle of benthic ecosystems due to the large number of microbes and oxygen reactive reductants it contains
We further demonstrated that the potential chemical oxygen consumption (COC) from formalin-fixed sediment samples comprised on average 72% of the potential whole oxygen consumption (Mori et al, 2015)
We quantitively demonstrated a significant relationship between INT reduction and potential sediment oxygen consumption (SOC) in Omura Bay
Summary
Surface sediment plays an important role in the oxygen cycle of benthic ecosystems due to the large number of microbes (bacteria) and oxygen reactive reductants it contains. Measuring SOC in situ has been widely conducted to quantify the extent of sediment metabolism and has contributed to expanding our understanding of aquatic bottom environments (Zimmerman & Benner, 1994; Rowe, Kaegi & Morse, 2002). The direct measurement of SOC may underrepresent the biogeochemical processes that contribute to oxygen dynamics in a hypoxic region, as it typically declines under hypoxic conditions due to the lower availability of oxygen in the surface sediment. A buildup of the reduced compounds leads to increased SOC whenever the bottom DO levels recover. It is, important to quantitatively evaluate the impacts of both aerobic and anaerobic respiratory processes in the sediment over a wide range of DO conditions from normoxia to anoxia
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