Total Hydrolysable Amino Acids Research Articles

Burial of Organic Carbon in Swedish Fjord Sediments: Highlighting the Importance of Sediment Accumulation Rate in Relation to Fjord Redox Conditions

AbstractFjords are net carbon sinks with high organic carbon (OC) burial rates; however, the key drivers of OC burial in these systems are not well constrained. To study the role of water column redox condition and OC composition on OC preservation in fjord sediments, we determined OC accumulation rates (OCAR), OC source, and OC degradation in three Swedish fjords with variable redox conditions (long‐term oxic, seasonally hypoxic, and long‐term anoxic). Average OCARs were variable between and within the fjords studied (2–122 g OC m−2 yr−1), but highest rates were at the mouth for each fjord. Based on a δ13C mixing model, Swedish fjords bury predominantly marine‐derived OC (∼83% of the total OC burial) likely because of relatively gentle slopes, low riverine discharge, and high marine inflow. Using a multi‐biomarker approach (lignin, photosynthetic pigments, and total hydrolyzable amino acids) we found, terrestrially‐ and marine‐derived OC were moderately degraded under the various redox conditions sampled, suggesting water column redox and OC source are not primary drivers of OC burial in these fjords. Rather, high sediment accumulation rates, common to fjords globally, lead to low oxygen exposure times, thus promoting efficient burial of OC regardless of its chemical composition.

Influence of the Keystone Grazer, Sesarma reticulatum, on the Hydrology and Organic Matter Cycling in Salt Marshes of the Southeastern USA

AbstractIn salt marshes of the Southeastern USA, purple marsh crabs (Sesarma reticulatum), hereafter Sesarma, aggregate in grazing and burrowing fronts at the heads of tidal creeks, accelerating creek incision into marsh platforms. We explored the effects of this keystone grazer and sediment engineer on salt marsh sediment accumulation, hydrology, and carbon (C) and nitrogen (N) turnover using radionuclides (210Pb and 7Be), total hydrolyzable amino acids (THAA), and C and N stable isotopes (δ13C and δ15N) in sediment from paired Sesarma-grazed and un-grazed creeks. Sesarma-grazed-creek sediments exhibited greater bioturbation and tidal inundation compared to sediments in un-grazed creeks, as indicated by larger 210Pb and 7Be inventories. Total organic carbon (TOC) to total nitrogen (TN) weight ratios (C:N) were higher and δ15N values were lower in grazed-creek sediments than in un-grazed-creek sediments, suggesting Sesarma remove and assimilate N in their tissues, and excrete N with lower δ15N values into sediments. In support of this inference, the percent total carbon (TC) and percent TOC declined by nearly half, percent TN decreased by ~ 80%, and the C:N ratio exhibited a ~ threefold increase between Sesarma fore-gut and hind-gut contents. An estimated 91% of Sesarma’s diet was derived from Spartina alterniflora, the region’s dominant salt marsh plant. We found that, as Sesarma grazing fronts progress across marsh landscapes, they enhance the decay of Spartina-derived organic matter and prolong marsh tidal inundation. These findings highlight the need to better account for the effects of keystone grazers and sediment engineers, like Sesarma, in estimates of the stability and size of blue C stores in coastal wetlands.

Variation of amino acid in the sinking particulates in the northeastern Arabian Sea and the northern Bay of Bengal

Sediment traps are widely used to get insight into how particulate matter forms and settles in the water column. Earlier studies in the Northern Indian Ocean have shown that the sinking flux depends mainly on surface primary production and varies seasonally and inter-annually. In the case of the Bay of Bengal, river runoff plays a significant role in the sinking flux. As a part of the SIBER-INDIA program, a mooring consisting of sediment traps was deployed at three depths (495 m, 1205 m, and 2915 m) in the Arabian Sea (Northeastern Arabian Sea sediment trap, NEAST;16.87°N, 67.77°E) and at 580 m, 1110 m, and 1635 m in the Bay of Bengal (Bay of Bengal sediment trap, BoBST; 18.21°N, 89.63°E). We present here data on total fluxes, concentrations of particulate carbon and nitrogen, biogenic opal, and amino acid composition for the period 2017–2018 in both basins. Particle flux in the Arabian Sea was influenced by monsoon-related processes with higher flux during the northeast monsoon (NEM) and southwest monsoon (SWM) seasons, whereas the flux was higher in BoB during SWM. Particulate nitrogen (PN), particulate organic carbon (POC), and total hydrolysable amino acid (THAA) mostly decreased with depth in all traps due to the decomposition of labile organic constituents in the sinking flux. Changes in the decomposition of organic matter with depth were further reflected in the POC/PN molar ratios, which increased with depth in the Arabian Sea. A low POC/PN molar ratio associated with the higher THAA concentration in most of the shallow traps indicates the presence of fresher organic matter. Among the THAA, Glycine (Gly) was the dominant amino acid in all the samples followed by Aspartic acid (Asp), Glutamic acid (Glu), Serine (Ser), and Alanine (Ala). The amino acid-based degradation index (DI) reveals the degradation of labile organic matter with depth in the NEAST. However, the middle trap in the BoB showed a different trend with higher total and lithogenic fluxes, POC, PN, and DI values compared to the shallow trap indicating lateral advection in the bay. Monsoonal changes and mesoscale eddies make these basins distinct from one another and influence their sinking fluxes and biogeochemical settings.

Identification and evaluation of antioxidant peptides from highland barley distiller's grains protein hydrolysate assisted by molecular docking

The aim of this study was to identify antioxidant peptides from highland barley distiller's grains and evaluate their antioxidant activity in vitro. The results showed that the enzymatic hydrolysate of highland barley distiller's grains prepared by ultrasonic assisted alkaline protease had antioxidant properties, aromatic amino acids accounted for 61.48% of the total free amino acids and acidic/basic amino acids accounted for 40.82% of the total hydrolyzed amino acids in enzymatic hydrolysate. Ultrafiltration component F1 (Mw < 1 kDa) had the highest DPPH, ABTS and hydroxyl radical scavenging activity and ferrous ion chelating activity, which were 93.92%, 69.59%, 50.27% and 0.71, respectively. Four peptides were identified and screened by LC-MS/MS and the P1 (SWDNFFR) and P4 (WDWVGGR) showed high scavenging ability of DPPH free radical (70.23%-62.84%) and ABTS free radical (30.87%-60.54%). Molecular docking showed that P1 and P4 formed multiple hydrogen bonds with central residues of MPO.

Impact of wild fish faeces to deep sea dissolved organic carbon as revealed by laboratory incubations

Wild fish faeces play an important role in mediating marine organic carbon cycle, but the faeces released dissolved organic matter (DOM) and its potentials in contribution to refractory DOM is unclear. The mixed wild fish (including mesopelagic fish) faecal samples from the northern South China Sea were collected and incubated up to 75 days using the waters collected at 800 m depth of the same site, to reveal the stability of the faeces released DOM. Our result showed that the faeces had a good DOM-release potential and dissolved organic carbon (DOC) was up to 150 μM in the first day and reached its maximum (172 μM) on day 3, which was in comparison with the control group (DOC <70 μM). In our incubations, the faecal samples contributed net DOC of 110 μM, in which 42 μM persisted till the end of the 75 days dark incubation. This meant the faeces-released DOC had a refractory fraction of 38% over the incubation. Total hydrolysable amino acids concentration significantly dropped, and amino acids molecular indicator (DI, AA C yield and D/L ratio) changes over the incubation indicated the fast transformation of DOM from labile to degraded stage. Network analysis was applied to the amino acids and our result showed that D-form amino acids were almost positively coupled with L-form amino acids. Glycine (in mol%) showed consistent accumulation along with DOM degradation. Also, glycine was at the key node position of the amino acids network, showing negative relation with all other amino acids enantiomers in both faeces group and control group. Our results proved that glycine had the potential in tracing the refractory fraction over the DOM degradation.

Microbial and Protease Fermentation of Mao-Tai Lees Alters Nutritional Composition and Promotes In Vitro Intestinal Proteolysis

Mao-tai lees (ML) is a by-product produced in the process of Mao-tai liquor production and contains high levels of crude protein, starch and fiber, and large yield. Thus, the ML has the potential to become feedstuffs alternatives in livestock production. The present study evaluated the nutritional values of ML and fermented ML (FML), including the first stage (FML I; microbial fermentation), the second stage (FMTL II; microbial fermentation), and the final stage (FFML; microbial fermentation with proteases), and explored their effects on in vitro intestinal fermentation. The results showed that the FFML had higher contents of acid detergent fiber, acid detergent lignin, crude fiber, crude protein, neutral detergent fiber, starch, Vitamin B2, B6, and B12, whereas the FML II presented higher contents of calcium, copper, iron, manganese, magnesium, and Vitamin B1 compared with the other groups. Compared with the ML, the total free amino acids (FAAs) and total bioamine contents were higher in the FML II and FFML and had lower total hydrolyzed amino acids and total other free organic acids contents, among which the FFML had higher total FAAs and total bioamine contents. The FMLs had lower n-6:n-3 PUFA ratio compared with the ML; however, the FFML had lower n-6:n-3 PUFA ratio than the other groups. Furthermore, the FFML had higher concentrations of 1,7-diaminoheptane, isobutyrate, isovalerate, putrescine, and spermidine in vitro fermentation, suggesting that the FFML had greater proteolysis than the other groups. Collectively, these findings suggest that microbial fermentation with proteases could alter the nutritional composition and promote in vitro intestinal proteolysis of ML, which may be an effective way for promoting the protein utilization of ML. The study provides an effective potential strategy to develop novel feedstuff alternatives.

Controls on the Distribution, Enrichment, and Degradation of Dissolved Organic Matter in the Sea Surface Microlayer of the Yellow Sea and East China Sea

AbstractThe concentrations of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), total dissolved carbohydrates (TDCHO), and total hydrolyzable amino acids (THAA) in the sea surface microlayer (SML) and subsurface water (SSW) were measured in the Yellow Sea and East China Sea. Concentrations of DOC and TDCHO in the SML were correlated with their respective concentrations in the SSW (Pearson's correlations: DOC, r2 = 0.17, n = 45, p &lt; 0.01; TDCHO, r2 = 0.32, n = 45, p &lt; 0.01). In contrast, DON and THAA concentrations in the SML and SSW were not correlated, suggesting that processes other than exchange with SSW dominated their loss or gain in the SML. Both THAA and TDCHO were significantly enriched in the SML, with mean enrichment factors (EFs) of 2.24 ± 1.17 and 1.98 ± 1.08, respectively. The EFs of dissolved organic matter (DOM) were not correlated with wind speed under the influence of complex processes (e.g., bubbling and turbulence). THAA‐C% and TDCHO‐C% in the SML were 3.38 ± 1.71% and 8.49 ± 4.81%, respectively, which were significantly higher than those in the SSW (Mann‐Whitney U test, p &lt; 0.05), suggesting that DOM in the SML was fresher than that in the SSW. While the high bioavailability of DOM in the SML should be conducive to neuston proliferation in this unique environment, no significant correlations were observed between degradation indicators and neuston abundances. In contrast, the changes of temperature and pH may have an important impact on the degradation of DOM.

Amino acids as indicators of seasonal variations in organic matter degradation in surface sediments from a shallow lake

Degradation of organic matter (OM) in sediments is a key link in nutrient cycling and sedimentation processes in lakes. The aim of this study was to explore the degradation of OM in surface sediments of a shallow lake (Baiyangdian Lake, China) under seasonal temperature variations. For this, we used the amino acid-based degradation index (DI) and the spatiotemporal distribution characteristics and sources of OM. Sediment OM in the lake mainly originated from freshwater aquatic plants and terrestrial C4 plants. The sediment at some sampling sites was affected by surrounding crops. The organic carbon and total nitrogen contents, and the total hydrolyzed amino acid concentrations in the sediments were highest in summer and lowest in winter. The lowest DI occurred in spring, which indicated that the OM in the surface sediment at this time was highly degraded and relatively stable, and the highest DI occurred in winter, which showed that the sediment was fresh. The water temperature was positively correlated with the organic carbon content (p < 0.01) and total hydrolyzed amino acids concentration (p < 0.05). Seasonal variations in the overlying water temperature had a large effect on OM degradation in the lake sediments. Our results will facilitate the management and restoration of lake sediments that suffer from endogenous release of OM in a warming climate.

The bio- and thermal lability of dissolved organic matter as revealed by high-resolution mass spectrometry and thermal chemical analyses

It is critical to understand the chemical structures and lability of dissolved organic matter (DOM) in aquatic environments, as DOM represents one of the largest reduced carbon pools on earth. To achieve a comprehensive view of its composition and reactivity, DOM from both riverine and coastal waters were concurrently extracted via both ultrafiltration and modified styrene divinyl benzene polymer (PPL) based solid-phase-extraction (SPE), and further compared through multi-dimensional angles, including elemental analysis, total hydrolyzable amino acid composition, high-resolution liquid chromatography mass spectrometry (under both electrospray ionization modes), and thermochemical analysis. With a very low percentage (< 5%) of shared molecules, ultrafiltered-DOM (UDOM) and SPE-extracted DOM (SPEDOM) showed distinct differences in their bio- and thermal labilities. SPEDOM not only contained more “degraded” biomarkers, such as glycine, β-alanine, and γ-aminobutyric acid, but was also dominated by a higher abundance of more bio-recalcitrant lignin-like structures. Furthermore, thermogravimetric analysis revealed a higher activation energy of SPEDOM (138.8 ± 0.5 kJ·mol−1) than that of UDOM (125.4 ± 2.8 kJ·mol−1), suggesting that SPEDOM is more thermally recalcitrant than UDOM. Consistently, thermal slicing ramped pyrolysis showed a lower percentage of recalcitrant pyrolyzates (e.g., aromatic and phenol structures), but a higher contribution of labile compound classes (e.g., aliphatic structures) in UDOM, as well as a higher diversity of pyrolyzates from UDOM than SPEDOM. Taken together, our results demonstrated a highly heterogeneous nature in DOM composition and distinctly different bio- and thermal labilities between UDOM and SPEDOM, indicating that characterization of different fractions of DOM is needed for a comprehensive evaluation of natural DOM.

Origins and characteristics of dissolved organic matter fueling harmful dinoflagellate blooms revealed by δ13C and d/l-Amino acid compositions

We measured the concentrations of dissolved inorganic and organic nutrients, dissolved organic carbon (DOC), total hydrolyzable amino acids (THAA), fluorescent dissolved organic matter (FDOM), phytoplankton pigments, and δ13C-DOC during the summer of 2019 in the harmful dinoflagellate bloom regions of the southern coast of Korea. In the harmful dinoflagellate bloom region, the concentrations of inorganic nitrogen were depleted, inhibiting the growth of diatoms, while the concentrations of dissolved organic components (nutrients, DOC, FDOM, and amino acids) which fuel dinoflagellates were unusually high. Thus, we attempted to investigate the origins and characteristics of DOM which fuels the harmful dinoflagellate blooms. The δ13C-DOC values (− 22.2‰ to − 18.2‰) indicate that the elevated DOC concentrations result from in-situ biological production rather than terrestrial inputs. The enantiomeric (D/L) ratios of THAA indicate that dissolved organic nitrogen was more labile in the early stage of harmful dinoflagellate bloom and became more refractory in the final stage. Our results suggest that the marine production of bioavailable DOM plays an important role in initiating and sustaining harmful dinoflagellate blooms.

Sources and degradation of organic matter and its relation to trophic status in the core sediments of a tropical mangrove ecosystem along the Southwest coast of India

Present study has been designed to reveal the organic matter sources (algal vs. terrestrial), trophic status, the role of diagenesis and the applicability of biogeochemical indicators in a highly urbanized tropical mangrove forest. This has been achieved through a multiproxy approach by investigating the downcore distribution of bulk elemental properties (Total organic carbon (TOC%), total nitrogen (TN%)), biochemical composition, phytopigments and total hydrolysable amino acids (THAA). Both cores have a strong increase in algal biomass in the upper more fine grained part while core C2 has higher land plant fraction in its lower part compared to C1. The THAA profile decreased downcore ranging from 6.07 to 602.93 μmol g−1, contributing 7–53% THAA-N and 6–23% THAA-C to the sediments. Amino acids namely, serine (Ser) and tyrosine (Tyr) suggesting the contribution of fresh plankton organic matter likely by bacterial activity, while valine (Val), phenylalanine (Phe), isoleucine (Iso) and leucine (Lue) indicated more land plant derived organic matter. The surface dominance of proteins (PRT) and phaeophytin (Phaeo) pigment supporting the change from a more land plant derived organic matter to a more aquatic/marine driven productivity in the upper core sediments. Biochemical ratio (PRT/CHO >1), biopolymeric carbon (BPC), algal contribution to BPC (AL%-BPC) and degradation index (DI) indicated eutrophication related with the increase in algal organic matter. Based on mol%, amino acids viz., glycine (Gly) and alanine (Ala) were significantly enriched in downcore diagenetically reworked sediments indicating the growth of bacterial colonies and the selective preservation of bacterial biomass. Organic matter transformation from labile form into refractory character and vice versa through bacterial reworking, plays a major role in the development from oligotrophy to hypertrophy in the Cochin lagoon. The increasing urbanization have caused anthropogenic induced change in sediment composition, and recent eutrophication changed a more land plant dominated aquatic system to enhanced algal productivity.

Distribution and bioavailability of dissolved and particulate organic matter in different water masses of the Southern Yellow Sea and East China Sea

As the largest marginal seas, the Southern Yellow Sea (SYS) and the East China Sea (ECS) are important areas to study the biogeochemical cycles of organic matter from different sources. Seawater samples were collected from the SYS and ECS to determine the composition of organic carbon and major biochemicals (amino acids and carbohydrates) making up the dissolved (DOM) and particulate organic matter (POM). Concentrations of dissolved organic carbon (DOC) in the surface waters exhibited a latitudinal gradient, with high values appearing in the Yellow Sea Coastal Current Water (YSCCW). As the labile fraction of DOM, the concentrations of total hydrolysable amino acids (THAA) and total dissolved carbohydrates (TDCHO) were variable and tended to decrease from inshore to offshore sites. The DOC, THAA, and TDCHO concentrations were negatively correlated with salinity, suggesting that physical mixing and lateral transport played an important role in the distribution of DOM. In contrast, phytoplankton production directly affected the particulate amino acid (PAA) concentrations. High concentrations of PAA in the bottom water along a transect were probably due to resuspension by strong currents. DOC-normalized yields of THAA and TDCHO (THAA-C% and TDCHO-C%) were used to characterize the bioavailability of DOM in different water masses. Elevated THAA-C% values occurred in regions with high phytoplankton biomass, while high TDCHO-C% values were widespread in the Yellow Sea Warm Current Water (YSWCW) and the YSCCW. THAA-C% and TDCHO-C% had similar depth distributions, higher in the surface and middle layers than in the bottom layer, suggesting that organic matter undergoes diagenetic alterations as it sinks. The degradation index (DI) of DOM was lower than that of POM, which could be explained by their different sources and degradation states. The sporadic occurrence of hot spots likely represented important sites of biological activity. • Concentration, chemical composition, and bioavailability of OM were studied. • Physical mixing and lateral transport affect the distribution of DOM. • Amino acid and carbohydrate yields decreased with water depth. • Hot spots of bioavailable DOM occur sporadically in the surface water.

Source, distribution and degradation of sedimentary organic matter in the South Yellow Sea and East China Sea

Sedimentary organic matter (SOM) plays an important role in marine carbon and nitrogen cycling, but the sources, degradation states and fates of SOM are not well understood. In the present study, we determined the total organic carbon (TOC), total nitrogen (TN), stable carbon isotope (δ13C), and total hydrolysable amino acids (THAA) to elucidate the sources and degradation states of SOM in the South Yellow Sea (SYS) and the East China Sea (ECS). THAA concentrations ranged from 4.90 to 29.13 μmol/g, with a mean of 11.03 ± 5.06 μmol/g. The distribution pattern of THAA was similar to that of TOC (r = 0.782, P < 0.01) and TN (r = 0.865, P < 0.01), with higher THAA concentrations in the coastal areas. Based on the two end-member mixing model, the fraction of terrestrial organic matter (Ft) ranged from 12.11% to 51.34% (average: 30.17 ± 9.61%), with much higher variability at the nearshore stations compared to the offshore stations. The C/N ratios and the δ13C values showed the different distribution trend, and there was a negative correlation between them (r = −0.479, P < 0.05). Based on the C/N ratios and δ13C values, the SOM in the study area was a mixture of marine organic matter (OM) and soil-derived OM. In addition, our study showed that SOM was dominated by diatomaceous OM compared to carbonate-derived OM and that terrestrial inputs and primary production were the dominant factors controlling the distribution of OM in the study area. Carbon and nitrogen-normalized yields of THAA (THAA-C% and THAA-N%) showed that relatively fresh OM tended to occur in the coastal areas off the Shandong Peninsula. However, the generally low degradation index (DI) values revealed that all SOM in the study area had undergone significant degradation. Glycine increased in relative abundance as the SOM was degraded, while aromatic amino acids (Tyr and Phe) and neutral amino acids (Leu, Ile and Val) had consistently decreasing trends following SOM degradation.

Amino acid and chlorin based degradation indicators in freshwater systems

Lakes cover a global area that is about 35 times smaller than the oceans, but carbon burial in lakes and oceans are on the same order of magnitude. Hence, understanding the processes for such high organic carbon burial in lacustrine systems is essential. We applied proxies typically used for marine environments including amino acid (AA) content and their nitrogen stable isotope composition to the water columns and sediments of three lakes that differ in their trophic states and depositions rates of sedimentary terrestrial organic matter. Additionally, we used carbon isotope fingerprinting of AAs to characterise their sources and fates. We show that this set of proxies tracks sources and degradation processes in eutrophic lakes with high sedimentary total organic carbon and nitrogen content. Those lakes also have a high total hydrolysable amino acid (THAA) content as well as higher pigment concentrations. While the Chlorin degradation Index (CI) showed increasing values with depth, the patterns were less consistent for the Degradation Index (DI). In general, all parameters failed to describe degradation in the most oligotrophic lake due to the very low organic carbon and nitrogen content in the sediment. We show that AAs are mostly of autochthonous origin and that AA contribute 5–45% to the organic material in plankton, POM, and sediment. Proxies based on AA for bacterial reworking (ΣV) or trophic level (TL) showed increasing values in the water column but relatively stable values in the sediments. Furthermore, we show that methanotrophic bacteria led to increased values for the bacterial reworking proxy (ΣV) and extraordinarily low δ13C AA values (−30 to −60‰).

Organic biomarkers and Meiofauna diversity reflect distinct carbon sources to sediments transecting the Mackenzie continental shelf

During the Arctic spring freshet, the Mackenzie River delivers large amounts of dissolved and particulate terrestrial organic carbon to the Canadian Beaufort Sea shelf. This pulse of material and nutrients fuels phytoplankton blooms in late spring and summer and together with under ice primary production contributes to the organic carbon reservoir of the coastal shelf sediments. As a component of the Marine Arctic Ecosystem Study (MARES), samples transecting the Canadian Mackenzie Trough were collected in fall before ice cover to examine the balance of marine and terrestrial carbon in sediments and its relationship with sediment meiobenthic diversity and abundance. Organic biomarker analysis of lipid proxies together with total hydrolyzable amino acids (THAA) defined organic matter sources and potential lability. Across the shelf transect, consistent values observed in sedimentary total organic carbon (1.09%–1.62%) belied a shift in the balance of terrestrial and marine carbon sources. Predominately terrestrial inputs at the shallow site near the delta outflow appeared to constrain meiofaunal populations while the deepest location off shelf reflected lower labile marine organic carbon reaching the sediments that limited benthic production. Surface sediments at the mid-shelf, however, showed a near 3-fold increase (to 1.16 mg C OC-1) in contributions by diatoms and dinoflagellates markers plus a peak in total amino acids (9.53 mg g OC-1) which corresponded with higher meiofauna abundance. The combination of detailed organic analysis together with meiofaunal community measures suggest that mid-shelf sediments receive higher contributions of labile marine carbon which drive increased meiofauna diversity and abundance despite the large background of terrestrial organic material exiting the Mackenzie River outflow.

Temporal and spatial distribution of dissolved amino acids in the surface microlayer and subsurface water of the Bohai Sea and the Yellow Sea

Concentrations of total hydrolyzed amino acids (THAA) and chlorophyll a (Chl-a) in the Bohai Sea (BS) and Yellow Sea (YS) were determined from August to September 2015 (summer) and from January to February 2016 (winter). The average concentrations of THAA in the surface microlayer and subsurface water in the study area were 2.92 (1.14 to 7.31) and 1.64 (0.49 to 3.32) μmol/L in summer, respectively, and 1.01 (0.69 to 1.70) μmol/L in the subsurface water during winter. The spatial distributions of THAA were obviously influenced by phytoplankton biomass and terrestrial input, while vertical distributions of THAA were mainly affected by circulation and phytoplankton. The main constituents of THAA were aspartic acid (Asp), glycine (Gly), serine (Ser), glutamic acid (Glu) and alanine (Ala) during summer and winter. THAA and Chl-a were more abundant in the surface microlayer than in the subsurface water, with average enrichment factors (EFs) of 2.01 and 2.06, respectively. Enrichment of THAA and Chl-a in the surface microlayer is the result of a combination effect, such as the influence of phytoplankton, wind action and mixing processes. This research may have implications for the distribution, enrichment and source of THAA in the BS and YS.

Organic matter degradation state affects dissimilatory nitrate reduction processes in Knysna estuarine sediment, South Africa

The quality of organic matter influencing sediment nitrate (NO3−) reduction processes in estuarine zones is not well understood. This study aimed to assess the denitrification (DNF), anaerobic ammonium oxidation (ANA), and dissimilatory reduction of nitrate to ammonium (DNRA) in estuarine zones of South Africa, and to understand the effects of organic matter fractions and degradation states on anaerobic NO3− reduction processes. We measured the anaerobic NO3− reduction process rates using 15N isotope-tracing techniques in Knysna Estuary, South Africa. Total hydrolyzable amino acids and fractions and geochemical parameters were also measured. The correlation analysis and structural equation model were used to evaluate the key environmental factors driving NO3− reduction processes. Potential DNF, ANA, and DNRA rates in Knysna Estuary varied from 3.59 to 16.62, 0.28 to 1.16, and 1.52 to 8.38 nmol g−1 h−1, respectively, with a large spatial variation. The variations in NO3− reduction process rates can largely be explained by sediment water content, dissolved organic carbon, and amino acid–based degradation index, while the total organic carbon and inorganic nitrogen contents were not related to the NO3− reduction processes. The DNF process contributed 47.28–79.34% total NO3− reduction, as compared to 17.59–47.58% for DNRA and 2.53–5.76% for ANA. The retention of reactive nitrogen (N) attributed to the DNRA process was approximately 42 t N km−2 year−1. This study reported the first simultaneous investigation of the anaerobic NO3− reduction processes in estuarine areas of South Africa, implying that the qualities of substrate were more important in regulating NO3− reduction processes than substrate quantities and highlighting that DNRA played an important role in reactive N retention.

Seasonal Variation, Degradation, and Bioavailability of Dissolved Organic Matter in the Changjiang Estuary and its Adjacent East China Sea

AbstractSystematic surveys to examine seasonal variation, degradation, and bioavailability of dissolved organic matter (DOM) in the Changjiang Estuary and the adjacent East China Sea were conducted in July 2016 and February 2017. Concentrations of dissolved organic carbon (DOC) and total hydrolyzable amino acids (THAA) were higher in July than in February. THAA and chlorophyll a (Chl‐a) were positively correlated in July, but not in February. The carbon normalized yields of THAA (THAA‐C%) in surface waters in July and February were not significantly different. However, degradation index (DI) values in the surface water were higher in February than in July. Compared with outer estuary, the inner estuary had lower THAA‐C% and DI values in both surveys. Solar radiation experiments showed that THAA and THAA‐C% values increased with time at station A6‐11 in the oceanic water but declined at C3 in the freshwater, possibly due to the different origins, chemical compositions, or initial degradation states of the DOM at the time of collection. Microbial incubation experiments showed that accumulated DOC and DON in surface waters were bioavailable to the microbial community of the surface layer, but recalcitrant to the microbial fauna from the bottom layer. Leucine (Leu) was selectively consumed, while glycine (Gly), threonine, and alanine appeared to be recalcitrant in summer (July) microbial incubations; and histidine, Gly, and methionine were preferentially consumed, while aspartic acid, serine, phenylalanine, and Leu were recalcitrant in winter (February) incubations.

Variation and Reactivity of Organic Matter in the Surface Sediments of the Changjiang Estuary and Its Adjacent East China Sea

AbstractThe Changjiang Estuary and its adjacent sea make up a highly dynamic ecosystem where the processes influencing the sources and degradation of organic matter (OM) in sediments are still not well understood. In this study, we analyzed the total organic carbon (TOC), total nitrogen (TN), stable carbon isotope (δ13C), and total hydrolysable amino acids (THAA) to investigate the distribution and degradation of sedimentary organic matter (SOM). TOC and TN contents ranged from 0.13% to 0.93% and 0.03% to 0.15%, respectively, with elevated values occurring in the muddy areas near the estuary. The δ13C of SOM became increasingly enriched from inshore (−23.5‰) to offshore (−20.5‰), and the TOC/TN molar ratios (C/N ratios) decreased from 8.16 to 5.84, indicating increasing contributions of marine OM. As the labile fraction of bulk OM, THAA concentrations ranged from 1.36 to 11.76 μmol/g and the distribution of THAA was influenced by terrestrial inputs along with the complex hydrography. The δ13C values and C/N ratios indicated that the SOM in the study area was mainly from marine and terrestrial origins (soil derived). The relatively low degradation index (DI) of SOM (mean: −0.57 ± 0.36) indicated that the OM had already undergone extensive degradation. Consistent with previous findings, our results indicated that the degraded OM was adsorbed by fine‐grained particles and selectively transported southward along the coast. The fresh SOM in the sandy area was mainly from in situ production. Additionally, diatoms appeared to be the primary source of the fresh OM buried in the sediments of the study area.

Organic Matter Composition at Ocean Station Papa Affects Its Bioavailability, Bacterioplankton Growth Efficiency and the Responding Taxa.

The bioavailability of organic matter (OM) to marine heterotrophic bacterioplankton is determined by both the chemical composition of OM and the microbial community composition. In the current study, changes in OM bioavailability were identified at Ocean Station Papa as part of the 2018 Export Processes in the Ocean from Remote Sensing (EXPORTS) field study. Removal rates of carbon (C) in controlled experiments were significantly correlated with the initial composition of total hydrolyzable amino acids, and C removal rates were high when the amino acid degradation index suggested a more labile composition. Carbon remineralization rates averaged 0.19 ± 0.08 μmol C L−1 d−1 over 6–10 days while bacterial growth efficiencies averaged 31 ± 7%. Amino acid composition and tandem mass spectrometry analysis of compound classes also revealed transformations to a more degraded OM composition during experiments. There was a log2-fold increase in the relative abundances of 16S rDNA-resolved bacterioplankton taxa in most experiments by members of the Methylophilaceae family (OM43 genus) and KI89A order. Additionally, when OM was more bioavailable, relative abundances increased by at least threefold for the classes Bacteroidetes (Flavobacteriaceae NS2b genus), Alphaproteobacteria (Rhodobacteraceae Sulfitobacter genus), and Gammaproteobacteria (Alteromonadales and Ectothiorhodospiraceae orders). Our data suggest that a diverse group of bacterioplankton was responsible for removing organic carbon and altering the OM composition to a more degraded state. Elevated community diversity, as inferred from the Shannon-Wiener H index, may have contributed to relatively high growth efficiencies by the bacterioplankton. The data presented here shed light on the interconnections between OM bioavailability and key bacterioplankton taxa for the degradation of marine OM.