SAANICH INLET OXYGEN MIMIMUM ZONE: SUP05-DRIVEN MICHAELIS-MENTEN SULFIDE OXIDATION KINETICS

Oxygen Minimum Zones (OMZs) are oxygen starved regions of the ocean operationally defined by dissolved oxygen concentrations less than 20μM. OMZs form due to depletion of oxygen in poorly ventilated waters via aerobic microbial respiration driven by organic matter exported from the sunlit surface. Currently, OMZs comprise 8% of the volume of the global ocean and have been expanding over the last 50 years, resulting in a loss of 4.5 million km²of fish supporting habitat.

Within OMZs, microbes employ alternative respiratory strategies, including nitrate reduction via denitrification, a process that removes biologically available nitrogen species (NO₃^-, NO₂^-) from the water column. OMZs can also support microbial sulfate reduction, resulting in the production of toxic hydrogen sulfide. Hydrogen sulfide accumulation, however, can be mitigated through the metabolic coupling of the nitrogen and sulfur cycles via denitrification and sulfide oxidation by the simplified reaction: H₂S + NO₃^- -> S° + N₂. Members of the g-proteobacterial SUP05 lineage, ubiquitous in OMZs globally, have been shown to mediate this process using cultivation-independent molecular methods. However, cell-specific rates of SUP05 mediated sulfide oxidation and denitrification remains to be determined.

Saanich Inlet, British Columbia is a seasonally anoxic fjord and a model ecosystem for studying microbial responses to OMZ formation. Here, we use process rate measurements to show that sulfide oxidation takes place anaerobically in Saanich Inlet waters and is likely coupled to nitrate reduction. In these waters, SUP05 comprised approximately 22% of the community, with cell abundances up to 10⁴ cells per mL. SUP05, the dominant member of the microbial community, possesses a full compliment of sulfur oxidation pathways, and is likely the primary driver of the measured sulfide oxidation rates. Michaelis-Menten models based on these rates suggest that the sulfur oxidizing community e.g. SUP05 in Saanich Inlet exhibits half-saturation constants for sulfide in the low 6-24 μM range.