Early diagenesis of protein: A seasonal study

The diagenetic behavior of proteins, defined as large polypeptides, was examined over a 2-yr period at a mudflat site subject to intense bioturbation. Protein concentrations in the surface sediment correlated well with deposition of algal detritus from the water column. Cellular biomass made up a significant, though likely not a major, component of protein in the sediments; likewise, changes in biomass could not account for the bulk of protein losses downcore. These protein losses made up most of the total acid-hydrolyzable amino acid losses and about a quarter of the total N losses. Loss rates of protein are proportional within cores to previously reported estimates of extracellular proteolytic activity at this site, though with variation among sampling times. The combination of high organic matter input and rapid bioadvection allows detection of changing kinetics of decomposition during very early stages of diagenesis, analogous to those observed in algal decay experiments. Specific decay rates, similar to first-order decay constants, ranged from 0.2 to 8 yr-’ for protein, total organic C and N, and pheophytin in the labile fraction of organic matter found in the top few centimeters. This labile fraction, which is dominated by nonprotein forms of N, is lost with progressively lower specific decay rates downcore. Various lines of evidence-such as ammonification experiments and low C : N ratios of the remineralized materialsuggest that these changing kinetics are at least partially due to replacement of algal detritus by more slowly decaying products of microbial resynthesis rather than simply to varying lability of fractions of the original algal detritus.