Heterotrophic microbes directly consume organic carbon within particles with an estimated contribution of 70-92% of POC remineralization 25. Organisms alter the POC flux through a variety of processes. Even the size of particles and thus their sinking speed is strongly influenced by the rate of degradation by microbes and consumption by zooplankton 31, 32, 33. For example, lability as represented by these models is inherently related to microbial activity (as we demonstrate below). This is problematic as many of the aforementioned physical and chemical mechanisms shown to impact the vertical flux of POC are controlled by microbial and grazer dynamics 22, 23, 24, 25, 30. However, these large-scale models have not explicitly included microbial ecological dynamics. Previous carbon cycle modeling studies aimed at understanding basin-scale fluxes out of the upper ocean have focused primarily on physical and chemical processes 5, 7, 8, 26 and have shown that differences in particle size spectra 5, 9, 16, 17, lability 6, and temperature 8, 27 play important roles in determining the efficiency of POC transfer, consistent with observations 4, 28, 29. Ultimately, the vertical flux of POC is determined by the rate of POC production in the surface ocean where de novo particle production takes place, the sinking speed of particles, and the rate of POC consumption in the subsurface ocean. The vertical flux of particulate organic carbon (POC) in the ocean has been the subject of numerous field campaigns, laboratory experiments, and modeling studies over the past four decades e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25. Accurately capturing particle-microbe interactions is essential for predicting variability in large-scale carbon cycling. These dynamics are controlled by the interaction between multiple biotic and abiotic factors. We show that the enhanced transfer of carbon to depth can result from populations struggling to establish colonies on sinking particles due to diffusive nutrient loss, cell detachment, and mortality. Our model provides mechanistic insight into the microbial contribution to the particulate organic carbon flux profile. Here we scale-up essential features of particle-associated microbial dynamics to understand the large-scale vertical carbon flux in the ocean. Microbes play a crucial role in particulate organic carbon degradation, but the impact of depth-dependent microbial dynamics on ocean-scale particulate carbon fluxes is poorly understood. Predicting the particulate carbon flux is therefore critical for understanding both global carbon cycling and the future climate. Sinking particulate organic carbon out of the surface ocean sequesters carbon on decadal to millennial timescales.
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