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The stable hydrogen isotope composition (δD) of major plant biochemicals is variable. We present δD values for cellulose, hemicelluloses and lignin of six plant species. The δD value for lignin is consistently lower than that of bulk tissue (by ∼50‰) and cellulose (by ∼100‰). We show that these differences can be used to assess the extent of degradation of organic matter from a single source. A decrease in the δDbulk of decomposing Spartina alterniflora roots and rhizomes from −72‰ to −87‰ was observed over 18 months, reflecting a relative enrichment of lignin content due to the preferential removal of polysaccharides from the detrital material. Similar changes in δ13C were observed previously during the degradation of these plant tissues. These findings indicate that the extent of organic matter degradation should be considered when using stable isotope approaches to assess possible sources of organic matter in soils and sediments. We show that the change in δDbulk of plant detritus is best described by an exponential equation, which is simpler than the multiple exponential decay (multi-G) model which best describes the change in δ13Cbulk of plant detritus. Therefore correcting for isotopic shifts caused by decomposition may be more easily accomplished using δD.

Bulk nitrogen (N) isotope signatures have long been used to investigate organic N source and food web structure in aquatic ecosystems. This paper explores the use of compound-specific δ15N patterns of amino acids (δ15N-AA) as a new tool to examine source and processing history in non-living marine organic matter. We measured δ15N-AA distributions in plankton tows, sinking particulate organic matter (POM), and ultrafiltered dissolved organic matter (UDOM) in the central Pacific Ocean. δ15N-AA patterns in eukaryotic algae and mixed plankton tows closely resemble those previously reported in culture. δ15N differences between individual amino acids (AA) strongly suggest that the sharply divergent δ15N enrichment for different AA with trophic transfer, as first reported by [McClelland, J.W. and Montoya, J.P. (2002) Trophic relationships and the nitrogen isotopic composition of amino acids. Ecology 83, 2173–2180], is a general phenomenon. In addition, differences in δ15N of individual AA indicative of trophic transfers are clearly preserved in sinking POM, along with additional changes that may indicate subsequent microbial reworking after incorporation into particles. We propose two internally normalized δ15N proxies that track heterotrophic processes in detrital organic matter. Both are based on isotopic signatures in multiple AA, chosen to minimize potential problems associated with any single compound in degraded materials. A trophic level indicator (ΔTr) is derived from the δ15N difference between selected groups of AA based on their relative enrichment with trophic transfer. We propose that a corresponding measure of the variance within a sub-group of AA (designated ΣV) may indicate total AA resynthesis, and be strongly tied to heterotrophic microbial reworking in detrital materials. Together, we hypothesize that ΔTr and ΣV define a two dimensional trophic “space”, which may simultaneously express relative extent of eukaryotic and bacterial heterotrophic processing. In the equatorial Pacific, ΔTr indicates an average of 1.5–2 trophic transfers between phytoplankton and sinking POM at all depths and locations. The ΣV parameter suggests that substantial variation may exist in bacterial heterotrophic processing between differing regions and time periods. In dissolved material δ15N-AA patterns appear unrelated to those in POM. In contrast to POM, δ15N-AA signatures in UDOM show no clear changes with depth, and suggest that dissolved AA preserved throughout the oceanic water column have undergone few, if any, trophic transfers. Together these data suggest a sharp divide between processing histories, and possibly sources, of particulate vs. dissolved AA.

The molecular sources, dynamics and analytical characterizations of the phosphorus (P) containing components of marine dissolved and particulate organic matter (OM) are reviewed. Using a combination of 13 C and 31 P nuclear magnetic resonance spectroscopy on samples collected from a depth profile (20–4000 m) at Station Aloha in the North Pacific subtropical gyre, the biomolecular associations of P functional groups in marine OM are identified. Compositional differences between ultrafiltered dissolved organic matter (UDOM; 1–100 nm size fraction) and ultrafiltered particulate organic matter (UPOM; 0.1–60 lm size fraction) as reflected by NMR and elemental analyses indicate that UDOM does not originate from simple solubilization of UPOM, and the aggregation of UDOM is not the primary source of UPOM. Regression analyses indicated a large fraction of the P in UDOM is associated with carbohydrates and amino acids, but not with lipids. Similar analyses for UPOM indicated that P is associated with carbohydrates, amino acids and lipids. The P functional groups also appear to be balanced in their distribution among molecular classes, because they remain in relatively constant proportion throughout the ocean.

16 pages, 4 figures, 6 tables., Water samples were collected from rivers and estuarine environments within the Florida Coastal Everglades (FCE) ecosystem, USA, and ultrafiltered dissolved organic matter (UDOM; 1 kDa) was isolated for characterization of its source, bioavailability and diagenetic state. A combination of techniques, including 15N cross-polarization magic angle spinning nuclear magnetic resonance (15N CPMAS NMR) and X-ray photoelectron spectroscopy (XPS), were used to analyze the N components of UDOM. The concentrations and compositions of total hydrolysable amino acids (HAAs) were analyzed to estimate UDOM bioavailability and diagenetic state. Optical properties (UV–visible and fluorescence) and the stable isotope ratios of C and N were measured to assess the source and dynamics of UDOM. Spectroscopic analyses consistently showed that the major N species of UDOM are in amide form, but significant contributions of aromatic-N were also observed. XPS showed a very high pyridinic-N concentration in the FCE–UDOM (21.7 ± 2.7%) compared with those in other environments. The sources of this aromatic-N are unclear, but could include soot and charred materials from wild fires. Relatively high total HAA concentrations (4 ± 2% UDOC or 27 ± 4% UDON) are indicative of bioavailable components, and HAA compositions suggest FCE–UDOM has not undergone extensive diagenetic processing. These observations can be attributed to the low microbial activity and a continuous supply of fresh UDOM in this oligotrophic ecosystem. Marsh plants appear to be the dominant source of UDOM in freshwater regions of the FCE, whereas seagrasses and algae are the dominant sources of UDOM in Florida Bay. This study demonstrates the utility of a multi-technique and multi-proxy approach to advance our understanding of DON biogeochemistry., This study was funded by the National Oceanographic and Atmospheric Administration (COP R.J.) and the National Science Foundation as part of the FCE-LTER program (DEB-9910514).

Refractory carboxyl-rich alicyclic molecules (CRAM) are characterized in marine dissolved organic matter (DOM) using nuclear magnetic resonance spectroscopy and ultrahigh resolution mass spectrometry. CRAM are distributed throughout the water column and are the most abundant components of deep ocean DOM ever characterized. CRAM are comprised of a complex mixture of carboxylated and fused alicyclic structures with a carboxyl-C:aliphatic-C ratio of 1:2 to 1:7. CRAM are expected to constitute a strong ligand for metal binding, and multiple coordination across cations could promote aggregation and marine gel formation thereby affecting CRAM reactivity and the bioavailability of nutrients and trace metals. It appears CRAM are ultimately derived from biomolecules with structural similarities to sterols and hopanoids. The occurrence of CRAM in freshwater and terrestrial environments seems likely, considering the global distribution of biomolecules and the similarities of biogeochemical processes among environments.

Microbial contributions to the detritus of two vascular plant tissues, smooth cordgrass (Spartina alterniflora) and black mangrove leaves (Avicennia germinans), were estimated over a 4-year decomposition period under subaqueous marine conditions. During this period, 93–97% of the initial plant tissues was decomposed. Bulk elemental and isotopic compositions of the detritus were measured along with hydrolyzable amino sugars (AS) and amino acids (AA), including the bacterial biomarkers muramic acid and the d -enantiomers of AA. A major enrichment in N relative to C occurred during decomposition. Net increases of AS, AA, and bacterial biomarkers in decaying detritus were observed. Three independent approaches indicated that on average 60–75% of the N and 20–40% of the C in highly decomposed detritus were not from the original plant tissues but were mostly from heterotrophic bacteria. During decomposition hydrolyzable AS + AA yields (∼54% of total N) were strongly correlated with total N in both types of detritus. The uncharacterized N appeared to have the same origin and dynamics as AA, suggesting the contribution of other bacterial biomolecules not measured here. There was little indication of humification or abiotic processes. Instead, N-immobilization appeared primarily bacterially mediated. Although varying dynamics were observed among individual molecules, bacterial detritus exhibited an average reactivity similar to plant detritus. Only a minor fraction of the bacterial detritus escaped rapid biodegradation and the relationship between bacterial activity and N-immobilization is consistent with an enzymatically mediated preservation mechanism. Bacteria and their remains are ubiquitous in all ecosystems and thus could comprise a major fraction of the preserved and uncharacterized organic matter in the environment.

Molecular-level condensed tannin analyses were conducted on a series of mangrove ( Rhizophora mangle) leaves at various stages of decomposition in a tropical estuary. Total molecular tannin yields ranged from 0.5% ash-free dry weight (AFDW) in the most highly degraded black leaves (6 -7 weeks in the water) up to .7% AFDW in fresh leaves (,1 week in the water). Total tannin exhibits an intermediate lability in these leaves relative to other measured biochemicals. Leaching is an important mechanism in tannin removal from leaves as indicated by the 30% loss of measurable tannin during a leaching experiment. Condensed tannin was .80% procyanidin (PC) with the remainder being prodelphinidin (PD). PD tannin, with its higher degree of hydroxylation, proved to be more labile than PC tannin. Average chain length of condensed tannin (degree of polymerization) exhibited an initial increase in response to leaching, but later decreased in the subsequent shift toward abiotic or microbially mediated chemical reactions. Several trends point toward a possible condensation reaction in which tannin plays a role in nitrogen immobilization. These include an apparent inverse correlation between molecular tannin and nitrogen, a positive correlation between molecular tannin and percent basic amino acids, 13 C-NMR data indicating transformation of tannin as opposed to remineralization, and 13 C-NMR data showing loss of condensed tannin B-ring phenolic carbons coupled with preservation of A-ring phenolic carbon. In addition to condensed tannin, the molecular method used also yielded several triterpenoids. Triterpenoids accounted for up to 3.5% AFDW of the leaf material and exhibited a threefold increase between yellow senescent leaves entering the estuary and black leaves. This trend is likely due to the weakening of protective cuticular membranes during leaf decomposition, which leads to increased yields in the acidic conditions used for tannin analyses. Copyright © 2001 Elsevier Science Ltd

Bacterial and photochemical mineralization of dissolved organic matter were investigated in the Amazon River system. Dissolved oxygen, dissolved organic carbon (DOC), and bacterial growth were measured during incubations conducted under natural sunlight and in the dark. Substrate addition experiments indicated that the relatively low rates of bacterial activity in Amazon River water were caused by C limitation. Experiments to determine the photoreactivity of this biologically refractory DOC revealed unusually high rates of photochemical consumption of DOC (~4.0 μM C h−1) and dissolved oxygen (~3.6 μM O2 h−1) in Rio Negro surface waters. In additional experiments we observed that bacterial growth and respiration were not significantly stimulated or inhibited during periods of sunlight exposure. The molar ratio of DOC to O2 consumed during photochemical processes was close to one (1.11–1.14) in all photooxidation experiments. Sunlight exposure over 27 h showed that at least 15% of Rio Negro DOM was photoreactive. The rate of photochemical consumption of DOC was approximately sevenfold greater than bacterial DOC utilization in Rio Negro surface waters; however, integrated over the entire water column microbial remineralization was the dominant process for oxygen and DOC consumption. Photomineralization of biologically refractory riverine DOM appears to be more important than previously believed and could be a major removal mechanism for terrestrially-derived DOM in the coastal ocean.

Long-term subaqueous decomposition patterns of five different vascular plant tissues including mangrove leaves and wood ( Avicennia germinans ), cypress needles and wood ( Taxodium distichum ) and smooth cordgrass ( Spartina alternifora ) were followed for a period of 4.0 years, representing the longest litter bag decomposition study to date. All tissues decomposed under identical conditions and final mass losses were 97, 68, 86, 39, and 93%, respectively. Analysis of the lignin component of herbaceous tissues using alkaline CuO oxidation was complicated by the presence of a substantial ester-bound phenol component composed primarily of cinnamyl phenols. To overcome this problem, we introduce a new parameter to represent lignin, Λ 6 . Λ 6 is comprised only of the six syringyl and vanillyl phenols and was found to be much less sensitive to diagenetic variation than the commonly used parameter Λ, which includes the cinnamyl phenols. Patterns of change in lignin content were strongly dependent on tissue type, ranging from 77% enrichment in smooth cordgrass to 6% depletion in cypress needles. In contrast, depletion of cutin was extensive (65–99%) in all herbaceous tissues. Despite these differences in the overall reactivity of lignin and cutin, both macromolecules were extensively degraded during the decomposition period. The long-term decomposition series also provided very useful information about the compositional parameters which are derived from the specific oxidation products of both lignin and cutin. The relative lability of ester-bound cinnamyl phenols compromised their use in parameters to distinguish woody from herbaceous plant debris. The dimer to monomer ratios of lignin-derived phenols indicated that most intermonomeric linkages in lignin degraded at similar rates. Acid to aldehyde ratios of vanillyl and syringyl phenols became elevated, particularly during the latter stages of decomposition supporting the use of these parameters as indicators of diagenetic alteration. Given the observation that cutin-derived source indicator parameters were generally more sensitive to diagenetic alteration than those of lignin, we suggest the distributional patterns of cutin-derived acids and their associated positional isomers may be most useful for tissue-specific distinctions complementing the general categorical information obtained from lignin phenol analysis alone.

Leaves of the red mangrove, Rhizophora mangle, were collected from trees and submerged sediments in a tropical mangrove swamp in the Bahamas. Weight-to-area measurements indicated that the most highly degraded black-colored leaves had undergone a 36% loss in ash-free dry weight (AFDW). Green leaves were characterized by an abundance of cyclitols (8.5% AFDW), glucose-rich aldose mixtures (20% AFDW) and relatively low yields of lignin-derived phenols (2% AFDW). Cyclitols were rapidly lost from senescing and submerged decomposing leaves, whereas the overall yields of aldoses and lignin-derived phenols remained fairly constant throughout the various stages of leaf decay. Decomposition patterns were therefore unusual in that lignin was lost at about the same rate as polysaccharides. Variable loss rates were observed within the individual aldoses, indicating the following stability series (most to least stable) for submerged leaves: hemicelluloses > cellulose > pectins and gums. Compositional patterns of lignin-derived phenols remained fairly constant throughout decomposition and were characterized by unusually high acid-to-aldehyde ratios (0.4) in all leaf samples. A laboratory leaching experiment was conducted with senescent leaf material to estimate the contribution of leaching to the observed compositional patterns. Assuming the amounts and patterns of materials leached in the laboratory experiment were similar to those for field degraded leaves, we estimated that leaching could account for 58% of the mass loss, 97% of the cyclitol loss, 46% of the aldose polysaccharide loss and 74% of the lignin loss. Microbial degradation appeared to be responsible for about half of the overall mass and polysaccharide loss. Vanillyl phenols were leached in preference to syringyl phenols and the vanillic acid-to-vanillin ratio for the leachate was very high (0.7). These results indicate that high acid-to-aldehyde ratios are not necessarily indicative of microbial oxidation and should be interpreted with caution. During early diagenesis of leaves, leaching produces relatively large quantities of dissolved organic matter (DOM) and the lignin component, in particular, may provide a convenient tracer for studying the distribution and fate of mangrove-derived DOM in estuarine and coastal waters.

Changes in the chemical composition of mangrove (Rhizophora mangle) leaves during decomposition in tropical estuarine waters were characterized using solid-state 13C nuclear magnetic resonance (NMR) and elemental (CHNO) analysis. Carbohydrates were the most abundant components of the leaves accounting for about 50 wt% of senescent tissues. Tannins were estimated to account for about 20 wt% of leaf tissues, and lipid components, cutin, and possibly other aliphatic biopolymers in leaf cuticles accounted for about 15 wt%. Carbohydrates were generally less resistant to decomposition than the other constituents and decreased in relative concentration during decomposition. Tannins were of intermediate resistance to decomposition and remained in fairly constant proportion during decomposition. Paraffinic components were very resistant to decomposition and increased in relative concentration as decomposition progressed. Lignin was a minor component of all leaf tissues. Standard methods for the colorimetric determination of tannins (Folin-Dennis reagent) and the gravimetric determination of lignin (Klason lignin) were highly inaccurate when applied to mangrove leaves. The N content of the leaves was particularly dynamic with values ranging from 1.27 wt% in green leaves to 0.65 wt% in senescent yellow leaves attached to trees. During decomposition in the water the N content initially decreased to 0.51 wt% due to leaching, but values steadily increased thereafter to 1.07 wt% in the most degraded leaf samples. The absolute mass of N in the leaves increased during decomposition indicating that N immobilization was occurring as decomposition progressed.