The role of sedimentary organic matter in bacterial sulfate reduction: The <i>G</i> model tested1

TLDR

Laboratory decomposition of Long Island Sound plankton in oxygenated seawater follows first‑order kinetics, yielding two reactive fractions and a non‑metabolizable component. The study added fresh or oxically degraded planktonic material to anoxic Long Island Sound sediment and measured bacterial sulfate reduction rates with a 35S radiotracer, also monitoring natural anoxic decomposition in surface sediment. Sulfate reduction rates increased proportionally with added planktonic carbon, confirming the G model and demonstrating that in situ rates are limited by organic matter, with two distinct reactive fractions and a decline over time that matches first‑order kinetics.

Abstract

Laboratory study of the bacterial decomposition of Long Island Sound plankton in oxygenated seawater over a period of 2 years shows that the organic material undergoes decomposition via first‐order kinetics and can be divided into two decomposable fractions, of considerably different reactivity, and a nonmetabolizable fraction. This planktonic material, after undergoing varying degrees of oxic degradation, was added in the laboratory to anoxic sediment taken from a depth of 1 m at the NWC site of Long Island Sound and the rate of bacterial sulfate reduction in the sediment measured by the 35 S radiotracer technique. The stimulated rate of sulfate reduction was in direct proportion to the amount of planktonic carbon added. This provides direct confirmation of the first‐order decomposition, or G model, for marine sediments and proves that the in situ rate of sulfate reduction is organic‐matter limited. Slower sulfate reduction rates resulted when oxically degraded plankton rather than fresh plankton was added, and the results confirm the presence of the same two fractions of organic matter deduced from the oxic degradation studies. Near‐surface Long Island Sound sediment, which already contains abundant readily decomposable organic matter, was also subjected to anoxic decomposition by bacterial sulfate reduction. The decrease in sulfate reduction rate with time parallels decreases in the amount of organic matter, and these results also indicate the presence of two fractions of organic carbon of distinctly different reactivity. From plots of the log of reduction rate vs. time two first‐order rate constants were obtained that agree well with those derived from the plankton addition experiment. Together, the two experiments confirm the use of a simple multi‐first‐order rate law for organic matter decomposition in marine sediments.