Evaluation of fate of organic matter in sediment through microcosm experiments

Laboratory microcosm experiments help to understand the degradation pattern of different sources derived from organic matter (OM) at the sediment–water interface and relate it to field processes. Microcosm experiments were carried out for 65 days to assess the degradation of organic materials such as diatoms, zooplankton, and mangrove leaves using source-specific fatty acid (FA) biomarkers. The viable bacterial population in the microcosm sediment was low on day 1, but on day 3, it increased substantially by two to threefold in all the treatments. An increase in the bacterial population resulted in a decrease in the FAs specific to organic materials during the initial incubation phase (10 days), suggesting the degradation of organic materials derived from different sources. The FAs specific to diatoms showed faster degradation when compared to zooplankton and mangrove-specific FAs. The degradation of FAs was influenced by the nature of the source material, unsaturation of FAs, and the sediment type. Sandy sediment facilitated higher degradation of diatom and zooplankton-specific FAs, whereas mangrove-specific FAs showed higher degradation in silty-clay sediment. Thus, bacterial community composition coupled with the habitat characteristics, the OM composition, and the grain size of the sediment, influenced the degradation of FAs. The degradation of mangrove-specific long-chain FAs was ~1.11–2.5 times higher when spiked with plankton-derived labile OM from mixed sources, which otherwise took a longer time for degradation pointing toward co-metabolism (priming effect), which could play a significant role in the cycling of terrestrial plant-derived OM in an estuarine ecosystem.