Experimental Evaluation of Factors Controlling Microbial Carbon Pump Efficiency: Roles of Temperature, Substrate Quality, Sediments, and Benthic Organisms

Abstract

Microorganisms metabolize labile organic matter in aquatic environments, generating refractory dissolved organic matter (RDOM) as a metabolic byproduct, a portion of which is detectable as humic-like fluorescent DOM (FDOM-H). This process, known as the microbial carbon pump, has garnered significant attention as a mechanism for long-term sequestration of organic carbon in aquatic systems. In this study, we examined the influences of water temperature, substrate composition, sedimentary flux, and benthic organism activity on microbial carbon pump efficiency, as approximated by FDOM-H production efficiency, through long-term biodegradation experiments using organic matter in water, as well as incubation experiments with surface sediment cores. We utilized Lake Biwa (Japan), one of the hotspots of the microbial carbon pump, as a model site. Our results revealed that elevated water temperatures and reduced concentrations of semi-labile organic matter in the substrate facilitated enhanced FDOM-H production. Additionally, we observed a significant sedimentary flux of FDOM-H into the overlying water, with a further increase following the addition of benthic organisms. These findings highlight the sensitivity of the microbial carbon pump to environmental conditions in the water column and at the sediment-water interface, suggesting that environmental changes may impact its capacity for carbon sequestration.