| Proteins and their constituent amino acids are the most abundant nitrogen-containing substances in phytoplankton, and therefore are potentially important sources of organic nitrogen in aquatic systems. By studying the decay of four diverse species of phytoplankton in flow-through laboratory incubations, and through analysis of the organic-rich sediments of Mangrove Lake (Bermuda), the fate of these substances was addressed as well as the importance of environmental conditions on preservation. While most proteins are degraded during diagenesis, amino acids associated with protein or its alteration products remained a significant fraction of the total nitrogen (up to ∼60%), with the availability of oxygen having a significant impact on rates of loss. Decay constants were 3--7 times slower for incubations under anoxic conditions. Losses of nonprotein amino acids beta-alanine and gamma-aminobutyric acid were similar to those of the protein amino acids under all environmental conditions examined, suggesting that biological processing is not responsible for their enhanced relative abundances in ancient pelagic clay sediments. Rather, abiotic processes such as adsorption may be more important controls. Two-dimensional electrophoresis and size-exclusion chromatography suggest that the nitrogen retained in the phytodetritus and sediments is preserved as small amounts of discrete proteins and most as acidic proteins which are modified. A shift to higher molecular weight proteinaceous, material (Mr ≥ 1,500,000) was attributed to macromolecular aggregations. However, minimal reactivity with a specific sugar-cleaving reagent suggests that the Maillard reaction is not the predominant pathway. The aggregates appear to be formed very early in the diagenetic sequence and are subject to dissociation with chaotropic agents and to proteolytic cleavage upon extraction, indicating strong hydrophobic associations of protein as an important mechanism in long-term preservation, and the preservation of peptide linkages in sediments as old as 4000 years. The importance of organic matter associations for the protection of generally labile substances is also supported by the observation that proteins can be preserved in detritus derived from algae with outer cell walls composed of refractory macromolecules termed algaenans. Retarded rates of decay imposed by cellular "encapsulation" may allow proteins to undergo geochemical modifications (i.e., hydrophobic aggregation and covalent cross-linkings) which make them refractory. |
