Microbial community respiration kinetics and their dynamics in coastal seawater

Oxygen (O₂) concentrations in coastal seawater have been declining for decades and models predict continued deoxygenation into the future. As O₂ declines, metabolic energy use is progressively channelled from higher trophic levels into microbial community respiration, which in turn influences coastal ecology and biogeochemistry. Despite its critical role in deoxygenation and ecosystem functioning, the kinetics of microbial respiration at low O₂ concentrations in coastal seawater remain uncertain and are mostly modeled based on parameters derived from laboratory cultures and a limited number of environmental observations. To explore microbial responses to declining O₂, we measured respiration kinetics in coastal microbial communities in Hong Kong over the course of an entire year. We found the mean maximum respiration rate (V_max) ranged between 560 ± 280 and 5930 ± 800 nmol O₂ L^-1 h^-1, with apparent half-saturation constants (K_m) for O₂ uptake of between 50 ± 40 and 310 ± 260 nmol O₂ L^-1. These kinetic parameters vary seasonally in association with shifts in microbial community composition that were linked to nutrient availability, temperature, and biological productivity. In general, coastal communities in Hong Kong exhibited low affinities for O₂, yet communities in the dry season had higher affinities, which may play a key role in shaping the relationship between community size, biomass, and O₂ consumption rates through respiration. Overall, parameters derived from these experiments can be employed in models to predict the expansion of deoxygenated waters and associated effects on coastal ecology and biogeochemistry.