CO2 Respiration/O2 Consumption in Response to Supplemental Organic Carbon: Implications for Natural DOC Composition in San Pedro Basin, CA

Microbial community respiration plays an integral role in oxygen and carbon cycling in the world’s oceans, but it is poorly constrained in most marine settings, especially in the mesopelagic. We set out to define respiration rates in San Pedro Basin, within the Southern California Bight, with specific focus on the 100 m depth zone in order to determine both how community respiration is influenced by carbon supply perturbations, and the impact of DOC utilization on oxygen and CO₂. Evoking Michaelis-Menten kinetics, it is apparent that observed community respiration rates are always well-below their maximum potential, V_max.  Furthermore, the assumption that respiration proceeds with a respiratory quotient (RQ) of 1, i.e., 1 molecule of TCO₂ (aka DIC) produced per molecule of O₂ consumed, is not correct for this setting. A carbon addition experiment with ¹³C labeled glucose suggests that RQ-values for respiration at 100 m in San Pedro Basin are close to 2, indicating that the substrates used for respiration are likely highly-oxidized carbon compounds. Thus, any potential increases in biological O₂ uptake may lead to increases in CO₂ production of even greater magnitude, resulting in changes in ocean chemistry through a pathway that has not been fully considered. We propose that as POC produced in the surface ocean sinks and is oxidized, small organic acids are progressively produced that, by 100 m, constitute a significant portion of the DOC pool.