Diagenesis

The overall objective was to elucidate the effect of iron (Fe) on nitrogen (N) diagenesis in Lake Okeechobee. Somewhat counterintuitively, sediment ammonium (NH+4) inventories decreased during algal growth as dissolved organic nitrogen (DON) inventories increased. Whole core incubations were staged for denitrification experiments using isotopic N tracer. Core incubations showed the percentage of sediment N removal increase between summer (25 ± 21 %) and winter (39 ± 13 %). The amendment of Fe2+ enhanced this seasonal effect likely via dissimilatory nitrate reduction to ammonium (DNRA). The isotopic signature of N2 flux also suggested an additional, sedimentary, N2 source via Fe coupled anaerobic oxidation of ammonium (feammox). Sediment slurry incubations supported the occurrence of both DNRA and feammox, showing first that nitrate (NO3−) was converted to NH4+ via DNRA, which contributed 23-26% of overall NO3− reduction.
Fe amendment in slurries similarly stimulated the feammox process. However, aged Fe minerals accumulated linearly with N bound to Fe (Fe-N) in a subseasonal sediment time series, suggesting Fe-organic matter aggregation may lower the sediment NH4+ equilibrium concentration and benthic flux.

Aged unispecific cultures and environmental samples have been analyzed using HPLC-PDA in order to reveal chlorophyll-a degradation processes. Unispecific aging studies showed that alteration of chlorophyll occurred during senescence, and revealed that chlorophyll-a breakdown differs amongst various groups (i.e. species specificity). Sediment trap and sediment studies reveal that a variety of precursor chlorophyll-a derivatives are present, and that senescence and geochemistry, per se, overlap strongly. Many of the reactions in the Treibs' scheme occur before and during deposition. Intracellular senescence and diagenetic pathways have been examined in this work. Comparison of aged unispecific cultures and environmental samples strongly suggests major in puts from the senescence pathway. The results further supports the concept "pyro" reaction (loss of the carbomethoxy group) fates chlorophyll diagenesis towards true DPEP compounds.