Microbial Respiration in Contrasting Ocean Provinces via High-Frequency Optical Assays

ABSTRACT Microbial respiration plays a pivotal role in the marine carbon cycle, influencing the fraction of fixed carbon that undergoes remineralization versus export to depth. Despite its importance, methodological constraints have led to an inadequate understanding of this process, especially in low-activity oligotrophic and mesopelagic regions. Here, we quantify respiration rates as low as 0.2 µ mol O2 L -1 d -1 in contrasting ocean productivity provinces using optical oxygen sensors to identify size-fractionated respiration trends. At the low productivity North Pacific Ocean Station Papa, surface microbial respiration was relatively stable at 1.2 µ mol O2 L -1 d -1 . Below the surface there was a decoupling of respiration and bacterial production potentially driven by phytodetritus remineralization. Size-fractionated rates showed cells <5 µ m contributed the most to Pacific respiration. At the North Atlantic Porcupine Abyssal Plain, the optode measurement frequency was drastically increased. Surface microbial respiration was higher (1.7 µ mol O2 L -1 d -1 ) and decreased by 3-fold below the euphotic zone. The Atlantic filtered fraction contributions to total respiration shifted with the phytoplankton bloom evolution. The high resolution optode method used in the Atlantic is consistent with coinciding in vivo 2-para-(iodophenyl)-3(nitrophenyl)-5(phenyl) tetrazolium chloride respiratory stain measurements and historical site estimates. We estimate that 58% of gross primary production was respired at the Pacific site and 34% at the Atlantic site, demonstrating that the Atlantic had a higher carbon export potential. This study highlights the dynamic nature of respiration across vertical, temporal, and size-fractionated factors, emphasizing the need for sensitive, high-throughput techniques to better understand ocean ecosystem metabolism.