Recalcitrant Dissolved Organic Matter Research Articles

Calcium regulates the interactions between dissolved organic matter and planktonic bacteria in Erhai Lake, Yunnan Province, China

In recent years, the global carbon cycle has garnered significant research attention. However, details of the intricate relationship between planktonic bacteria, hydrochemistry, and dissolved organic matter (DOM) in inland waters remain unclear, especially their effects on lake carbon sequestration. In this study, we analyzed 16S rRNA, chromophoric dissolved organic matter (CDOM), and inorganic nutrients in Erhai Lake, Yunnan Province, China. The results revealed that allochthonous DOM (C3) significantly regulated the microbial community, and that autochthonous DOM, generated via microbial mineralization (C2), was not preferred as a food source by lake bacteria, and neither was allochthonous DOM after microbial mineralization (C4). Specifically, the correlation between the fluorescence index and functional genes (FAPRPTAX) showed that the degree of utilization of DOM was a critical factor in regulating planktonic bacteria associated with the carbon cycle. Further examination of the correlation between environmental factors and planktonic bacteria revealed that Ca2+ had a regulatory influence on the community structure of planktonic bacteria, particularly those linked to the carbon cycle. Consequently, the utilization strategy of DOM by planktonic bacteria was also determined by elevated Ca2+ levels. This in turn influenced the development of specific recalcitrant autochthonous DOM within the high Ca2+ environment of Erhai Lake. These findings are significant for the exploration of the stability of DOM within karst aquatic ecosystems, offering a new perspective for the investigation of terrestrial carbon sinks.

Recalcitrant dissolved organic matter in lakes: a critical but neglected carbon sink

Lakes are hotspots for the biogeochemical processing of carbon, including dissolved organic matter (DOM). A less degradable fraction of DOM, preserved for a long time, can be categorized as recalcitrant DOM (RDOM). Lake RDOM is an important but neglected carbon sink, and its characteristics and transformation processes remain largely unknown.

Spatial distribution characteristics and interaction effects of DOM and microbial communities in kelp cultivation areas

The influence of macroalgae cultivation on aquaculture carbon sinks is significant, with microbial carbon (C) pumps contributing to a stable inert dissolved carbon pool in this context. Concurrently, dissolved organic matter (DOM) exchange at the marine sediment-water interface profoundly affects global ecosystem element cycling. However, the interactions between DOM and bacterial communities at the sediment-water interface in kelp cultivation areas, especially regarding microbial function prediction, have not been fully explored. This study analyzed the DOM characteristics, environmental factors, and bacterial community structure in the Tahewan kelp——Saccharina japonica cultivated area and compared them with those in non-cultivated areas. The results indicated significantly higher dissolved organic carbon (DOC) concentrations in the kelp culture area, particularly in surface seawater and overlying water. The dominant bacterial phyla in both regions included Pseudomonadota, Actinomycetota, and Bacteroidota in both regions, while Desulfobacterota was more prevalent in the sediment environment of the cultivated region. Parallel factor analysis (EEM-PARAFAC) was used to identify DOM components, among which component C2 (a microbial humic-like substance DOM) was highly resistant to microbial degradation. We infer that C2 has similar properties to recalcitrant dissolved organic matter (RDOM). Analysis of the predicted functional genes based on 16S rRNA gene data showed that methanol oxidation, methylotrophy, and methanotrophy were significant in the bottom seawater of the cultivation area. The carbon (C), nitrogen (N), and sulfur (S) cycle functional genes in the sediment environment of the kelp cultivation area were more active than those in other areas, especially in which sulfate reduction and denitrification were the two main processes. Furthermore, a DOM priming effect was identified in the cultivated sediment environment, where kelp-released labile dissolved organic matter (LDOM) stimulates rapid degradation of the original RDOM, potentially enhancing C sequestration.

Outwelling of reduced porewater drives the biogeochemistry of dissolved organic matter and trace metals in a major mangrove‐fringed estuary in Amazonia

AbstractMangrove‐fringed estuaries are intertidal ecosystems discharging significant amounts of dissolved organic matter (DOM) into coastal oceans. DOM in these ecosystems is derived from autochthonous production, fluvial input, and mangrove porewater outwelling; however, differentiating between these sources remains challenging. Our incomplete understanding of the biogeochemical factors controlling DOM dynamics and its relationship with nutrient and trace metal cycling still hinders the formulation of elemental budgets in coastal environments. Here, we relate the DOM composition in a mangrove‐fringed estuary in North Brazil (Amazonia) to the redox conditions at the formation sites. We combined molecular DOM analyses via ultrahigh‐resolution mass spectrometry (FT‐ICR‐MS) with parallel factor analysis of excitation–emission fluorescence matrices (EEM‐PARAFAC), nutrient and redox‐sensitive trace metal analyses. During low tide, the influx of oxygen‐depleted porewater carried terrigenous DOM, inorganic nutrients, and trace metals into the mangrove‐fringed creeks. Precipitation of metal(hydr)oxides and microbial turnover controlled nutrient and trace metal dynamics in the estuary. The highest inorganic nitrogen concentrations within the upper mangrove‐fringed estuary indicated outwelling from mangrove sediments as an essential source. Phosphate concentrations were highest within the lower mangrove‐fringed estuary, where available phosphate likely exceeded precipitation with iron(hydr)oxides. We tracked the DOM transport to the coastal ocean using a novel molecular index derived from sulfidic porewater (ISuP). Outwelling of recalcitrant DOM from mangrove habitats is relevant in the context of blue carbon storage. Therefore, applying our molecular proxy (ISuP), together with trace‐metal and optical DOM analyses, is a powerful approach for differentiating contributions of diverse DOM sources in highly complex coastal ecosystems.

Suboxic DOM is bioavailable to surface prokaryotes in a simulated overturn of an oxygen minimum zone, Devil's Hole, Bermuda.

Oxygen minimum zones (OMZs) are expanding due to increased sea surface temperatures, subsequent increased oxygen demand through respiration, reduced oxygen solubility, and thermal stratification driven in part by anthropogenic climate change. Devil's Hole, Bermuda is a model ecosystem to study OMZ microbial biogeochemistry because the formation and subsequent overturn of the suboxic zone occur annually. During thermally driven stratification, suboxic conditions develop, with organic matter and nutrients accumulating at depth. In this study, the bioavailability of the accumulated dissolved organic carbon (DOC) and the microbial community response to reoxygenation of suboxic waters was assessed using a simulated overturn experiment. The surface inoculated prokaryotic community responded to the deep (formerly suboxic) 0.2 μm filtrate with cell densities increasing 2.5-fold over 6 days while removing 5 μmol L-1 of DOC. After 12 days, the surface community began to shift, and DOC quality became less diagenetically altered along with an increase in SAR202, a Chloroflexi that can degrade recalcitrant dissolved organic matter (DOM). Labile DOC production after 12 days coincided with an increase of Nitrosopumilales, a chemoautotrophic ammonia oxidizing archaea (AOA) that converts ammonia to nitrite based on the ammonia monooxygenase (amoA) gene copy number and nutrient data. In comparison, the inoculation of the deep anaerobic prokaryotic community into surface 0.2 μm filtrate demonstrated a die-off of 25.5% of the initial inoculum community followed by a 1.5-fold increase in cell densities over 6 days. Within 2 days, the prokaryotic community shifted from a Chlorobiales dominated assemblage to a surface-like heterotrophic community devoid of Chlorobiales. The DOM quality changed to less diagenetically altered material and coincided with an increase in the ribulose-1,5-bisphosphate carboxylase/oxygenase form I (cbbL) gene number followed by an influx of labile DOM. Upon reoxygenation, the deep DOM that accumulated under suboxic conditions is bioavailable to surface prokaryotes that utilize the accumulated DOC initially before switching to a community that can both produce labile DOM via chemoautotrophy and degrade the more recalcitrant DOM.

Processes in the Surface Ocean Regulate Dissolved Organic Matter Distributions in the Deep

AbstractMarine dissolved organic matter (DOM) is a major global carbon pool, consisting of thousands of compounds with distinct lifetimes. While marine DOM persists for millennia, its molecular and isotopic composition imply that it is dynamic on shorter timescales. To determine the extent to which DOM deviates from conservative water mass mixing, we conducted a two‐endmember mixing analysis on dissolved organic carbon (DOC) concentration and DOM molecular composition in the Atlantic and Pacific. Endmembers were the deep water masses near their formation sites. For DOM composition, we considered 6118 molecular formulae (MF) identified via Fourier‐transform ion cyclotron resonance mass spectrometry in solid‐phase extracts (SPE) of 837 samples. Bulk DOC and SPE‐DOC concentrations behaved conservatively in both basins and ≥70% of the MF (14–20 μM SPE‐DOC) mixed conservatively. However, a small fraction (10%–20%) of the MF (<3 μM SPE‐DOC) were added or removed during mixing. These MF were more reduced and oxidized, respectively, than the conservative fraction. There were also MF absent from the endmembers; these accounted for ≤1 μM of SPE‐DOC and positively correlated with DOM lability. Based on their distribution across the two basins, we conclude that the conserved MF are formed in the surface subtropical ocean and modified in overturning areas. In the deep ocean, however, these MF are solely controlled by mixing. This finding contrasts with the current paradigm of slow, continuous degradation of recalcitrant DOM in the deep ocean. Our analysis illustrates the importance of the surface ocean in controlling DOM cycling in the deep.

High stability of autochthonous organic matter in inland aquatic ecosystems

Recalcitrant dissolved organic matter (RDOM) is a key component of ocean carbon sinks and it can be preserved in seawater for thousands of years. However, the fate of RDOM derived from the primary production in inland lake is unclear. In this study of Erhai lake in China, used the combination of δ13C, carbon/nitrogen (C/N) ratio, and optical spectroscopy analyses to constrain the variation of organic matter in the vertical direction (i.e., lake water → trap sediments → surface sediments → core sediments), to trace the burial process of RDOM. Two autochthonous components (C1, C2) and two allochthonous components (C3, C4) were identified in the water. High concentrations of C2 in winter and spring indicate that these two seasons provide favourable burial conditions for autochthonous dissolved organic matter. Three autochthonous components (C1, C2, C5) and three allochthonous components (C3, C4, C6) were identified in the sediments. The contributions of autochthonous organic matter were ∼49.9 ± 5.84 % (based on PARAFAC analysis) and ∼56.7 ± 5.62 % (based on the C/N ratio) in the sediment trap samples; 43.4 ± 8.84 % (based on PARAFAC analysis) and 40.8 ± 14.26 % (based on the C/N ratio) in the surface sediments; and 44.5 ± 14.4 % (based on PARAFAC analysis) and 48.4 ± 6.04 % (based on the C/N ratio) in the core sediments. Additionally, C2, which is the autochthonous component after microbial mineralisation, was preserved as a significant part of the RDOM in the water. Our results suggest that the mineralisation of autochthonous organic matter (OM) in the core sediments did not promote bacterial mineralization, and that the burial of OM on long timescales dependents primarily on its concentration rather than on its origin. Our results provide a new perspective for studying the stability of autochthonous OM and highlight a new direction for the study of the carbon sink of inland lakes.

Transformation of dissolved organic matter and related arsenic mobility at a surface water-groundwater interface in the Hetao Basin, China

Porewater arsenic mobility above the groundwater table has been recognized as a potential cause of arsenic-rich groundwater, but the processing pathways of dissolved organic matter (DOM) in that hyporheic zone and their effect on porewater arsenic release remain poorly understood. To address these issues, two porewater profiles were sampled in a surface water-groundwater interaction zone from the Hetao Basin, China, to monitor the porewater geochemistry and DOM molecular characteristics. The results show that the porewater arsenic, Fe(II), and DOC concentrations were all significantly higher than those of the intruding pond water, and were located above the conservative mixing model lines. This indicates a net release of these solutes from the sediment. By comparing the porewater with pond water DOM, we found that the carboxyl-rich alicyclic molecules (CRAM) were selectively preserved, carbohydrates and aliphatics/proteins were preferentially consumed, and low O/C-ratio compounds with high bioproduction index (I_bioprod) and terrestrial index (I_terr) were produced. The transformation of CHO to CHOS compounds also represented a pathway of recalcitrant DOM production. The produced recalcitrant organic compounds mostly contributed to the elevated porewater DOC concentrations, but their contribution decreased along the filtration path. The consumption of labile DOM compounds would be responsible for Fe(III) hydroxide reduction and arsenic release. The generated recalcitrant DOM may also be a driver of porewater arsenic mobility by acting as electron shuttles. This study highlights the importance of the hyporheic zone in shaping shallow groundwater DOM composition and the potential contribution to arsenic enrichment.

Molecular composition of dissolved organic matter across diverse ecosystems: Preliminary implications for biogeochemical cycling

Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been widely applied to characterize the molecular composition of dissolved organic matter (DOM) in different ecosystems. Most previous studies have explored the molecular composition of DOM focused on one or a few ecosystems, which prevents us from tracing the molecular composition of DOM from different sources and further exploring its biogeochemical cycling across ecosystems. In this study, a total of 67 DOM samples, including soil, lake, river, ocean, and groundwater, were analyzed by negative-ion electrospray ionization FT-ICR MS. Results show that molecular composition of DOM varies dramatically among diverse ecosystems. Specifically, the forest soil DOM exhibited the strongest terrestrial signature of molecules, while the seawater DOM showed the most abundant of biologically recalcitrant components, for example, the carboxyl-rich alicyclic molecules were abundant in the deep-sea waters. Terrigenous organic matter is gradually degraded during its transport along the river-estuary-ocean continuum. The saline lake DOM showed similar DOM characteristics with marine DOM, and sequestrated abundant recalcitrant DOM. By comparing these DOM extracts, we found that human activities likely lead to an increase in the content of S and N-containing heteroatoms in DOM, this phenomenon was commonly found in the paddy soil, polluted river, eutrophic lake, and acid mine drainage DOM samples. Overall, this study compared molecular composition of DOM extracted from various ecosystems, providing a preliminary comparison on the DOM fingerprint and an angle of view into biogeochemical cycling across different ecosystems. We thus advocate for the development of a comprehensive molecular fingerprint database of DOM using FT-ICR MS across a wider range of ecosystems. This will enable us to better understand the generalizability of the distinct features among ecosystems.

Niche differentiation of microbial community shapes vertical distribution of recalcitrant dissolved organic matter in deep-sea sediments

Sedimentary organic matter provides carbon substrates and energy sources for microorganisms, which drive benthic biogeochemical processes and in turn modify the quantity and quality of dissolved organic matter (DOM). However, the molecular composition and distribution of DOM and its interactions with microbes in deep-sea sediments remain poorly understood. Here, molecular composition of DOM and its relationship with microbes were analyzed in samples collected from two sediment cores (∼40 cm below the sea floor), at depths of 1157 and 2253 m from the South China Sea. Results show that niche differentiation was observed on a fine scale in different sediment layers, with Proteobacteria and Nitrososphaeria dominating the shallow sediments (0–6 cm) and Chloroflexi and Bathyarchaeia prevailing in deeper sediments (6–40 cm), indicating correspondence of microbial community composition with both geographical isolation and the availability of organic matter. An intimate link between the DOM composition and microbial community further indicates that, microbial mineralization of fresh organic matter in the shallow layer potentially resulted in the accumulation of recalcitrant DOM (RDOM), while relatively low abundance of RDOM was linked to anaerobic microbial utilization in deeper sediment layers. In addition, higher RDOM abundance in the overlying water, as compared to that in the surface sediment, suggests that sediment might be a source of deep-sea RDOM. These results emphasize the close relation between the distribution of sediment DOM and different microbial community, laying a foundation for understanding the complex dynamics of RDOM in deep-sea sediment and water column.

Viral lysing can alleviate microbial nutrient limitations and accumulate recalcitrant dissolved organic matter components in soil.

Viruses are critical for regulating microbial communities and biogeochemical processes affecting carbon/nutrient cycling. However, the role of soil phages in controlling microbial physiological traits and intrinsic dissolved organic matter (DOM) properties remains largely unknown. Herein, microcosm experiments with different soil phage concentrates (including no-added phages, inactive phages, and three dilutions of active phages) at two temperatures (15 °C and 25 °C) were conducted to disclose the nutrient and DOM dynamics associated with viral lysing. Results demonstrated three different phases of viral impacts on CO2 emission at both temperatures, and phages played a role in maintaining Q10 within bounds. At both temperatures, microbial nutrient limitations (especially P limitation) were alleviated by viral lysing as determined by extracellular enzyme activity (decreased Vangle with active phages). Additionally, the re-utilization of lysate-derived DOM by surviving microbes stimulated an increase of microbial metabolic efficiency and recalcitrant DOM components (e.g., SUV254, SUV260 and HIX). This research provides direct experimental evidence that the "viral shuttle" exists in soils, whereby soil phages increase recalcitrant DOM components. Our findings advance the understanding of viral controls on soil biogeochemical processes, and provide a new perspective for assessing whether soil phages provide a net "carbon sink" vs. "carbon source" in soils.

Seasonal variations of dissolved organic matter chemistry in a semi-enclosed and eutrophic coastal bay in southeastern China: Implications for carbon cycling

As the widely distributed transition zones connecting the terrestrial and marine ecosystems, coastal bays have long water retention times to reconstruct the DOM pool affecting global carbon cycling. However, we still have a limited understanding of mechanisms shaping the carbon cycling of coastal bays at regional scales. To address this issue, we combined stable carbon isotopes, ultraviolet–visible absorption, fluorescence spectroscopy, and ultrahigh-resolution mass spectrometry to investigate the spatiotemporal chemistry of dissolved organic matter (DOM) in Xiangshan Bay, a semi-enclosed, eutrophic bay in southeastern China. The DOM in Xiangshan Bay was characterized by a higher degree of humification and aromaticity during winter, whereas the inverse signature during summer. Results showed that the significant seasonal variation of DOM chemistry was mainly modulated by intense primary productivity stimulated by nutrients during summer, and other factors including sediment pore water DOM release and photochemical reactions. Additionally, based on the microbial carbon pump (MCP) hypothesis, marine microorganisms may synthesize biologically recalcitrant DOM with millennia turnover time. The efficient MCP associated with reduced nutrients probably acted as a critical factor in Xiangshan Bay, leading to the formation of additional refractory molecular formulae (carboxyl-rich alicyclic molecules) and a more recalcitrant signature of DOM during winter. Incorporated with seven other coastal bays across China, it is revealed that anthropogenic inputs substantially contributed to Chinese coastal bays. Further studies need to better constrain the biogeochemical implications of these anthropogenic inputs. More importantly, decreasing the anthropogenic input of nutrients to coastal bays may enhance their functions in long-term carbon sequestration.

Source characterization of dissolved organic matter in the eastern Beagle Channel from a spring situation

Dissolved organic matter (DOM) was analyzed by its optical and fluorescent properties in order to study its distribution, source and transformation in relation to microbial abundances, chlorophyll and physicochemical gradients along a transect in the eastern Beagle Channel (BC) during austral spring 2019. Moreover, DOM evolution was followed during three tidal cycles in two fixed stations (F1, F2) separated by the Mackinlay Strait, a hydrographic frontier between water masses under stronger continental influence (inner sector) and that of less modified Sub-Antarctic waters (outer sector). The fluorescence signals of protein- and humic-like compounds were used as a proxy for labile and non-labile material, respectively, while the concentration of nano-, pico- and virioplankton populations, chlorophyll-a (Chl-a) and dissolved organic carbon (DOC) were used as a proxy for biological activity. Moreover, several spectroscopic indices, such as the indicator of microbially produced DOM (BIX), the Freshness index (FIX), an indicator of molecular weight, the Slope Ratio (SR) and an indicator of chromophoric DOM (CDOM), were used to trace the quality and origin of DOM. Chl-a concentration was generally low in the sampling area (< 1 μg L−1) but increased towards the end of the campaign and especially during the visit of F1 along with the concentration of picoeukaryotes and the protein-like fluorophores evidencing the onset of a bloom event. Protein-like fluorophores peaked in the euphotic zone of the inner sector of the BC while humic-like material of terrestrial origin was homogeneously distributed throughout the water column and showed a decreasing gradient towards the outer sector and Atlantic waters. Furthermore, the inner sector was characterized by higher amounts of CDOM, high molecular weight material and more recalcitrant DOM. Accumulation of autochthonous humic-like material in bottom waters of the inner sector along with enhanced values of FIX and BIX preceded by colored organic material suggest an efficient functioning of the microbial carbon pump fueled by terrestrial DOM sources in these waters.

Source, distribution, and transformation of dissolved and particulate organic matters in Qinzhou Bay, Northern Beibu Gulf

Both dissolved and particulate organic matters (DOM and POM) provide a reduced carbon pool of considerable size in coastal ecosystems, and the two are closely linked. Currently, however, the integrated study of DOM and POM remains limited, precluding a more in-depth understanding of their interaction in coastal regions. In April 2021, 13 surface water samples were collected from Qinzhou Bay, in the northern Beibu Gulf. The DOM samples were characterized using dissolved organic carbon (DOC) analysis and UV-visible and fluorescence spectroscopy techniques. We determined the POM for the particulate organic carbon and nitrogen (POC and PN) and also isotopic composition (δ13C and δ15N). The weak to negligible relationships found between the DOC, colored and fluorescent DOM, salinity, and chlorophyll a together suggested that DOM’s distribution in Qinzhou Bay is concurrently shaped by various processes, namely, hydrological and in situ biological processes. A high C/N ratio of ~17, high POC/chlorophyll a ratio (253 ± 112), and depleted δ13C (−25.7 ± 1.6‰) confirmed that POM is highly degraded and originates mainly from allochthonous input, to which the terrigenous organic matter and freshwater phytoplankton each contributes 35%. The total organic carbon (TOC = DOC + POC) was positively correlated with the humic-like peak M, revealing the transformation of labile DOM and POM into recalcitrant DOM components. The in situ production efficiency of peak M in surface waters of Qinzhou Bay is one order of magnitude greater than that in inland waters or open oceans, indicating that not only temperature but also the activity of substrate is a key factor controlling the in situ production of recalcitrant DOM in Qinzhou Bay. High levels of TOC and humic-like fluorescent DOM suggest the mass coexistence of organic matter differing in its reactivity, highlighting the large potential for photochemical as well as microbial degradation in the future.

Solid phase extraction of ocean dissolved organic matter with PPL cartridges: efficiency and selectivity

Our current knowledge of the chemical composition of ocean dissolved organic matter (DOM) is limited, mainly because of its extreme molecular diversity, low concentration of individual compounds and the elevated ionic strength of ocean waters. As a result, many analytical methods require a previous extraction step. The efficiency and selectivity of the extraction method defines the representativeness of the extracted DOM fraction. Nowadays, the most widespread procedure for concentrating DOM is solid phase extraction (SPE) using styrene divinyl benzene polymer cartridges (PPL). Here, we investigate the effect of SPE-PPL on DOM elemental and optical properties to assess the efficiency and selectivity of this extraction method on water samples from the main intermediate and deep water masses of Arctic, Mediterranean and Antarctic origin present in the Cape Vert Frontal Zone (CVFZ, NW Africa). Furthermore, North and South Atlantic Central waters converge in this area and coastal DOM is injected by the giant upwelling filament of Cape Blanc. On one side, the colored fraction of DOM (CDOM) presented extraction efficiencies comparable to that of the bulk dissolved organic carbon (DOC), but decreased significantly with increasing wavelength, suggesting an affinity of PPL cartridges for low molecular weight organic compounds. While the protein-like fluorescent fraction of DOM (FDOM) was also extracted with the same efficiency than DOC, the extraction efficiency of the humic-like fraction was comparatively much higher. On the other side, dissolved organic nitrogen (DON) extraction efficiencies were about half that of DOC. These contrasting extraction efficiencies of the different DOM pools indicated that the extracts were enriched in N-poor, low molecular weight and recalcitrant DOM, therefore showing less variability than the corresponding bulk DOM. Furthermore, DOC, DON, CDOM and FDOM extracted were not homogeneous through the water column but displayed certain significant differences among water masses in both efficiency and selectivity.

Water mass age and dissolved organic matter properties drive the diversity of pelagic prokaryotes in the Western Mediterranean Sea

The deep sea is among the largest, yet still poorly known, ecosystems on Earth. This knowledge gap is particularly evident for the bathypelagic layer (between 1000 and 4000 m) of the deep Mediterranean Sea (MS), characterized by peculiar environmental conditions at meso- and bathypelagic depths, such as the thermal signature of the deep waters, showing temperatures approximately 10 °C higher than any oceanic system at comparable depths. This sustains high rates of prokaryotic activities and fast dissolved organic matter (DOM) mineralization rates, and likely selects for unique microbial assemblages. We collected seawater samples in stations representative of different areas of the Western Mediterranean Sea (Algero-Provençal Basin, Alboran Sea and Gulf of Lion), from the surface to the bathypelagic layer (down to 2680 m) and in different water masses, and described the composition of pelagic prokaryotic communities, along with measurements of the main physical-chemical variables, concentration and optical properties (absorption and fluorescence) of DOM. Remarkable differences in DOM optical properties were observed among water masses, with highest dissolved organic carbon (DOC) values in surface waters and lowest in the oldest water masses represented by the Western Mediterranean Deep Water (WMDW) and the Levantine Intermediate Water (LIW), which were characterized by the highest concentration of recalcitrant DOM. The water mass had a significant partitioning effect on microbial community composition, which showed the highest richness in LIW. DOC, the marine humic-like component of DOM (C1mh) and oxygen were the main drivers of prokaryotic community structure. Changes in quality of DOM were reflected in shifts in community composition, supporting the existence of strong relationships between DOM quality and microbial community composition in the deep MS. Our data shed light on the community composition and diversity patterns of prokaryotic plankton in the deep western MS, helping to elucidate the major microbial players in the DOM cycling, and to progress towards a better comprehension of its future trends in light of changing conditions that are modifying the oceanography of the entire MS basin.

Terrigenous dissolved organic matter input and nutrient-light-limited conditions on the winter microbial food web of the Beagle Channel

The viral shunt and microbial loop are trophic pathways in the microbial food web, recognized as major drivers of carbon and nutrient cycles in the sea. The Beagle Channel, located at the southernmost extreme of South America, is a subantarctic marine environment influenced by terrestrial runoff and glacial meltwater with high temporal and spatial variability. There is a knowledge gap on the trophic pathways and the microbial food web in the eastern part of the Channel. We herein present for the first time a comprehensive and spatially wide study of the effects of physicochemical and bio-optical parameters on the microbial food web along the Eastern Beagle Channel. The study took place under winter hydrographic conditions, featuring weak vertical stratification of the water column and low chlorophyll a concentrations (0.13 ± 0.07 mg m−3). Biogeochemical gradients were notable between the inner and outer (easternmost) sectors of the Channel, separated by a shallow sill at Mackinlay Strait. As a chemical water mass tracer, the presence of the highest concentrations in bottom waters of NO3 + NO2 in the inner channel and the difference in the T-S diagram indicate the Subantarctic Pacific origin of this water mass. Structural equation models showed that the specific UV absorbance of water (SUVA254, a proxy for the aromatic carbon content) and NO3 + NO2 were the main parameters affecting the microbial food web in surface/subsurface waters suggesting that prokaryotes biomass was partially supported by allochthonous dissolved organic matter (DOM) derived from river discharges. Our results show a major role of the viral shunt in surface/subsurface waters retaining nutrients in the basal levels of the food web. We suggest that the viral shunt in surface/subsurface waters and the water residence time west of Mackinlay Strait allows infected cells and viral lysis products to reach bottom waters propelling the production of recalcitrant and labile DOM there.

Variations in dissolved organic matter chemistry on a vertical scale in the eastern Indian Ocean

Oceans cover approximately 71% of the Earth's surface area, which is why some people refer to the Earth as a large water sphere. Marine dissolved organic matter (DOM) constitutes the main carbon pool for biogeochemical cycles and plays an important role in global carbon dynamics. Here, the molecular composition and component characteristics of surface (5 m), deep chlorophyll maximum (DCM), and deep (2000 m) layer DOM in the eastern Indian Ocean (EIO) were investigated using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and three-dimensional fluorescence spectroscopy. Thousands of individual DOM formulas (approximately 3716–6986 formulas) were detected at 100–700 Da, showing a Gaussian distribution. The elements carbon (C), hydrogen (H), oxygen (O), nitrogen (N) and sulfur (S) were detected and constituted four formula classes in solid-phase extracted marine DOM samples. Furthermore, the order of the percent intensity of the formulas was CHO > CHNO > CHOS > CHNOS. Carboxylic-rich alicyclic molecule (CRAM) compounds, as part of recalcitrant DOM (RDOM), were detected at 61.32%-78.77% (by intensity). In addition, the concept of islands of stability (IOS, approximately 3.99%-11.22%) has been proposed in this study, representing the most stable components in the marine environment. Such molecular formulas as described above probably contribute to increased RDOM content in the EIO and potentially reflect enhanced accumulation or sequestration of RDOM in the deep layer. The variation in the spectroscopic indices (FI, β/α, BIX, and HIX) and fluorescent components (C1 to C4) with depth indicates a shift from protein-like to humic-like components, leading to gradual aging of the water column. In brief, this study relies on data from marine DOM in the EIO to provide a molecular and chemical background for global models of marine DOM production, transformation and sequestration.

Linking Microbial Decomposition to Dissolved Organic Matter Composition in the Revegetation of the Red Soil Erosion Area

Studying the changes and linkages between dissolved organic matter (DOM) and microorganisms in soils during vegetation restoration will help to understand the role of vegetation restoration in soil carbon sequestration and thus improve the understanding of the global soil carbon cycle. Soil DOM molecules were characterized by Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and the results showed that the soil DOM consisted mainly of lignin/carboxylic rich alicyclic molecule (CRAM)-like structures, while the ratios of lipids and aliphatic/protein decreased in sequence with recovery time. Lipids and aliphatic/proteins with high H/C DOM (labile DOM) degrade preferentially, while lignin/CRAM-like structures and tannins with low H/C DOM (recalcitrant DOM) are recalcitrant during vegetation restoration. With the restoration of vegetation, DOM molecules tend to be diversified and complicated, and DOM compounds with low double bond equivalent (DBE), low aromatic, and low alkyl structures will be converted into persistent organic matter with high carbon numbers and high DBE. The diversity of soil microorganisms was determined by high-throughput sequencing. The results showed that the abundance and diversity of soil bacteria increased significantly after revegetation, while the abundance and diversity of soil fungi began to increase when the ecosystem became a more mature coniferous forest. The soil microbial community exhibited complex connectivity and strong interaction with DOM molecules during vegetation restoration. As most of the DOM molecules are recalcitrant, vegetation restoration facilitates C sequestration in the soil, thereby contributing to climate change mitigation.

Longitudinal patterns of dissolved organic matter quality and bacterial community degradation function along the mainstream of Three Gorges Reservoir during impoundment period

Longitudinal patterns of dissolved organic matter (DOM) influence the biogeochemical cycle in river ecosystems. Yet, linkages between DOM quality and bacterial degradation of emerging contaminants over a river continuum are unclear. In combination with DOM characterization, bioassay, and microbial analysis, this study unraveled the spatial variations of DOM composition and biodegradability as well as bacterial community structures, which determined the estrone biodegradation potential (EBP) along the mainstream in Three Gorges Reservoir during impoundment period. While urban and agricultural utilization increased the protein-like and biodegradable DOM components (BDOC%) in upstream, the in-situ biotransformation preferentially utilized tryptophan-like substances followed by tyrosine-like substances, resulting in the accumulated recalcitrant DOM with higher aromaticity, molecular weight, and humic-like components in downstream. Due to the promotion of biodegradable DOM on bacterial abundances, a significant and positive correlation was constructed between BDOC% and EBP (P < 0.001, R2 = 0.56), both of which were controlled by the ratio of fresh- to humic-like fluorescence. Nevertheless, a higher biomass-normalized EBP was found under recalcitrant conditions in downstream, demonstrating that the degradation function was also associated with the biogeographical distribution of bacterial communities. Results of microbial ecology revealed that the in-situ transformation of aromatic and humic-like DOM exhibited more correlations with bacterial taxa than protein-like DOM, leading to increased diversity and decreased niche breadths along the mainstream. The triggered deterministic selection of community assemblages in downstream favored potential estrone-degraders and up-regulated their specific metabolic capacity. These findings will provide critical insights toward understanding the crucial role of DOM in biogeochemical cycles and the self-purification of emerging contaminants along the river continuum.