Browsing Marine Biology by Subject "Geochemistry"
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Decomposition and adsorption of peptides in Alaskan coastal marine sedimentsIn organic-rich coastal sediments, hydrolyzable amino acids make up a substantial fraction of the sedimentary content of organic nitrogen. How this organic nitrogen resists decomposition and is preserved in sediments is poorly understood. In order to investigate the factors controlling mineralization and preservation of hydrolyzable amino acids, decomposition and adsorption of peptides were studied in suboxic and anoxic pore water and sediments from Resurrection Bay (RB) and Skan Bay (SB), Alaska. Five tritium-labeled peptides, basic di-lysine, acidic di-glutamic acid, and neutral di-alanine, tri-alanine and hexa-alanine, were used as tracers. In filtered pore water, the hydrolysis rates were usually low. The exception was that the initial enzymatic hydrolysis of di-alanine and di-glutamic acid was rapid in SB pore water. The hydrolysis rates of both peptides increased with concentration. In sediments, hydrolysis was found to be the rate-limiting step of peptide decomposition. Alanyl and glutamyl peptides were hydrolyzed faster than lysyl peptide, and the hydrolysis rates among alanyl peptides decreased with increasing molecular weight. Peptide hydrolysis was affected more by molecular structure than by oxic or anoxic conditions. Adsorption of lysyl peptide to sediments was greater than that of other peptides. Basicity enhanced peptide adsorption more than increased molecular weight. Sedimentary organic matter was mainly responsible for peptide adsorption. The different patterns of peptide adsorption in RB and SB sediments were related to the greater total organic carbon concentration in SB sediment. Some of the peptide adsorption was irreversible. Adsorbed peptides were more resistant to biological decomposition than dissolved peptides. Adsorption may be an important step in the process of peptide preservation in sediments, and thus the preservation of sediment organic matter during early diagenesis.
Organic matter accumulation and preservation in Alaskan continental margin sedimentsContinental margin sediments provide a historical record of the sources and fate of organic matter (OM) originating both from the continents and from primary productivity in the overlying water column. However, since this record can be altered by microbial decomposition within the sediment, the history cannot be interpreted without understanding how decomposition can affect OM composition. Also, the margins accumulate much of the OM buried in ocean sediments; hence, knowledge of processes influencing preservation of OM in these sediments is essential to understanding the global carbon cycle. OM preservation was examined using two approaches. First, I studied sediments in the northeastern Gulf of Alaska to determine sources of OM and temporal changes in carbon accumulation. A large amount of OM, 45--70 x 104 tons/yr, accumulated in this region, about 50% from terrestrial sources. Most of the sediment cores showed little evidence of change in TOC, TN, or C and N stable isotope compositions due to decomposition within the sediment. Second, I investigated the processes that control OM preservation, focusing on the role of the OM adsorption to mineral surfaces. Because proteins are major constituents of sedimentary OM, I examined factors controlling their adsorption, decomposition, and preservation. Three hydrophilic proteins were strongly adsorbed by two clay minerals, an iron oxide, sub-oxic sediments from Resurrection Bay (RB), Alaska, and anoxic sediments from Skan Bay (SB), Alaska. The partition coefficients were large enough to lead to their preservation provided that the proteins did not decompose while adsorbed. Generally, adsorption of proteins to solid phases decreased decomposition rates, suggesting that adsorption is important in protecting these compounds from microbial attack. Greater protein decomposition rates were found in SB than in RB sediments, indicating that anoxia did not inhibit protein biodegradation. Naturally-occurring adsorbed proteins were extracted from SB and RB sediments using a detergent solution. Most of these adsorbed proteins were small (<12 kDa), indicating that only the proteins adsorbed within the micropores of particle surfaces are preserved long-term.