Characterizing The Microbial Lability And Isotopic (14C, 13C) Signatures Of Marine Organic Matter With A Novel Culture Vessel System

The rate at which non-living organic matter is respired in the ocean is an unconstrained and important property of the marine carbon cycle. Studies of inherent mineralization rates are complicated by the fact that marine organic matter is a mixture of compounds that vary in reactivity and concentration. While natural radiocarbon ages (¹⁴C, half-life = 5730 yr) have served as proxies for lability, they have not been used extensively to characterize that fraction of marine organic matter that is biologically accessible. To address this problem, we developed a novel batch culture system to monitor the time-dependent production rates and isotopic signatures of CO₂ released during microbial degradation of natural organic matter. The system simulated a nepheloid layer by maintaining a slurry of decarbonated sediment and minimal media (M9) in a custom 2-liter culture vessel. The natural microbial community was allowed to develop within the sediment, and respired CO₂ was continuously sparged from the medium with helium and oxygen, quantified in real time with an infrared gas analyzer, and isolated as a series of contiguous fractions for subsequent isotopic (∆¹⁴C, d¹³C) characterization. Control experiments indicated the accumulation of just 4.5 mg of background carbon per hour of continuous gas flow, which constituted ≤ 10 % of the respired carbon mass in each fraction. Since ∆¹⁴C values are conserved during molecular transformations, this low-blank system enables the detection of subtle shifts in the “age” of organic matter respired during the course of a culture experiment. Analyses of sediments from Falmouth, MA revealed both a variable CO₂ production rate and an increase in post-bomb ∆¹⁴C values during a 10-day incubation. This suggests that the microbial lability of organic matter at this site decreased non-linearly with apparent ¹⁴C age, and that the least labile fraction observed was not more than ~50 years old. These results underscore the complex relationship between microbial communities, organic matter composition, and its ¹⁴C age distribution.