Organic Matter Cycling Research Articles

Cycling of organic matter and trace metals in nearshore coastal ecosystem sediments

Cycling of organic matter and trace metals in nearshore coastal ecosystem sediments

Scallop farming impacts on dissolved organic matter cycling in coastal waters: Regulation of the low molecular weight fraction

To elucidate the impacts of scallop farming on the biogeochemical characteristics of low molecular weight (LMW, <1 kDa) dissolved organic matter (DOM), samples collected from a bay scallop mariculture area (MA) and its surrounding areas were determined for absorption and fluorescence spectroscopy after microfiltration and centrifugal ultrafiltration. The values of absorption coefficient a350 showed a spatial variation trend of inshore area (IA) > MA > non-mariculture area (NMA) for both bulk (<0.7 μm) and LMW fractions. Four fluorescent components, namely two protein-like components (tryptophan-like C1 and tyrosine-like C2) and two humic-like components (microbial humic-like C3 and terrestrial humic-like C4), were identified. Scallop farming influenced DOM transformation by altering phytoplankton abundance and promoting microbial degradation. In July, the net contributions of phytoplankton to the spectroscopy parameters of LMW-DOM in the surface seawater were 11.0% for a350, 4.3% for C1, 0.8% for C2, 0.6% for C3 and 3.0% for C4, respectively; the corresponding values of bulk DOM in the surface seawater were 24.3% for a350, 20.1% for C1, 5.9% for C2, 2.0% for C3, 2.9% for C4, respectively. Compared with NMA, the contributions of microbial degradation to a350 in MA's surface seawater increased by 9.0% for LMW-DOM and 6.9% for bulk DOM in July; however, the effects on different fluorescent components varied. In August, compared with NMA, the contributions of microbial degradation to spectroscopy parameters in the bottom water of MA decreased by 35.7% for a350, 6.3% for C2, 1.3% for C3, and 4.4% for C4 for LMW-DOM fraction; for bulk DOM, the corresponding contribution decreased by 10.8% for C1. These variations indicate that protein-like substances from scallop aquaculture are easily degraded into LMW substances, while humic-like substances degradation diminishes over time.

Altitude, Land Use and Soil Depth Effects on Earthworm Density and its Relationship to Soil Properties in an On-Farm Study

Abstract Purpose Earthworms play a critical role in soil ecosystem functions through the cycling of organic matter and nutrients. However, some land uses or environmental conditions are more favorable habitats for them than others. Thus, the objectives of this study were to evaluate the prevalence of earthworm density and its relationship to land uses and soil properties. Methods The study was conducted in three districts (Wangdue Phodrang, Chhukha and Dagana) in Bhutan in three land uses (organic fields - OrgF, conventional fields - ConF and natural vegetation - NatV) under three altitudes (high-, mid- and low-altitudes) and three soil depths. Results Overall, earthworm density at high-altitude was significantly (P &lt; 0.001) higher than that at mid- and low-altitude sites. Further, across altitudes and soil depths, OrgF sites had a significantly (P &lt; 0.001) higher earthworm density (120 earthworms m− 2) compared to that in the NatV (56 earthworms m− 2) and ConF (43 earthworms m− 2) sites, and the density decreased significantly (P &lt; 0.001) and successively with the increase in soil depth. The coefficient of determination (R2 ≥ 0.51; P &lt; 0.001) showed a positive and moderate relationship between the earthworm density with soil organic C and total N in OrgF sites, but the relationship was weak (R2 ≤ 0.22) in the ConF or none in the NatV sites. Conclusions Substituting chemical fertilizers with organic manures could increase earthworm density by enhancing soil health through the cycling of organic materials and nutrients in the soil. The findings demonstrate empirical evidence for earthworm prevalence in different land use types across altitudinal gradients and provide valuable decision-making insights to land users and policymakers alike.

Microplastics impair extracellular enzymatic activities and organic matter cycling in oligotrophic sandy marine sediments

Microplastics (MPs) are ubiquitous and constantly accumulating in the marine environment, especially sediments. Yet, it is not well clarified if and how their carbon backbone could interact with surrounding sediments, eventually impairing key benthic processes. We assessed the effects of a ‘pulse’ contamination event of MPs on sedimentary organic matter (OM) quantity, quality and extracellular enzymatic activities (EEAs), which are well established descriptors of benthic ecosystem functioning. Marine sediments were exposed for 30 days to environmentally relevant concentrations (∼0.2 % in weight) of naturally weathered particles (size range 70–210 μm) of polyurethane, polyethylene, and a mixture of the most common polymers that are documented to accumulate in marine sediments. Despite the low concentration, contaminated sediments showed significantly different composition of OM, showing a decrease in lipid content and increase in protein. Moreover, we document a significant decrease (over 25 %) in quantity of biopolymeric C already after 15 days of exposure, compared to controls. Contaminated sediments showed lower C degradation rates (up to −40 %) and altered EEAs, with alkaline phosphatase being ∼50 % enhanced and aminopeptidase being reduced over 35 % compared to control treatments. Overall, the effects generated by the mixture of polymers were smaller than those exerted by the same amount of a single polymer. Our results provide insights on how that MPs can significantly alter marine sedimentary biogeochemistry through altered benthic processes, that could cumulatively impair whole benthic trophic webs by enhancing the accumulation and possible longer-term storage of recalcitrant organic C in the seabed.

Exploring microbial dynamics, metabolic functions and microbes–metabolites correlation in a millennium paddy soil chronosequence using metabolome and microbiome

BackgroundPaddy soil is a typical soil type affected by anthropogenic management and factors related to natural soil formation. The evolution from mudflats to typical paddy soils can significantly affect the soil microecology. Previous studies have reported the evolution of soil physicochemical properties, microbes, and related soil environmental factors in a millennium paddy soil chronosequence. However, the potential biological mechanisms of changes in metabolites and microbes–metabolites interaction are poorly understood. Therefore, a combination of high-throughput sequencing and environmental pseudotargeted metabolomics techniques was adopted to explore the effects of the millennium paddy soil chronosequence on microbial communities, metabolites, and their functions and interactions.ResultsThe soil ecology changed greatly in the first 60 years of the transition from mudflat to paddy planting. Among the microbial communities, the response of the bacteria to the chronosequence was more sensitive than that of fungi. Among them, the bacterial communities of Proteobacteria, Bacteroidetes, Acidobacteria, and Nitrospirae exhibited regular succession over the chronosequence, but the fungal communities did not show regular changes. Bacterial function prediction revealed that the beginning of the critical stage of the evolution from mudflat to paddy soil involved the organic matter cycle and energy flow. In contrast, fungi were characterized mainly by pathogenic and saprophytic functions. The results of the principal component analysis of the metabolites revealed a similar pattern of change as that of the microbes. Seventy-five characteristic metabolites exhibited three trends of change during the development of the paddy soil chronosequence. Twenty-five differentially active metabolic pathways, including glyoxylate and dicarboxylate metabolism, starch and sucrose metabolism, and galactose metabolism, were enriched. In addition, correlation analysis revealed that long-chain fatty acids, short-chain fatty acids, phenolic acids, carbohydrates, and polyalcohols significantly regulate the microbial communities in paddy soil.ConclusionsCombining metabolome and microbiome has expanded the overall understanding of the development of paddy soil under anthropogenic management. During the development of a paddy soil chronosequence, the synergistic regulation of soil physicochemical properties and metabolites in the microbial community results in increased productivity. This study provides a new perspective on microbes and metabolites interaction.Graphical

Seasonal dynamics of amino acids in the Southern Yellow Sea: Feedback on the mechanism of green tides caused by Ulva prolifera

The biogeochemical processes of amino acids in the Southern Yellow Sea (SYS) have become more dynamic under the influence of the world's largest-scale green tide. The potential relationship between amino acids and green tides has not been effectively assessed, despite its critical importance for exploring dissolved organic matter (DOM) cycling processes in marginal seas. In this study, three cruises were conducted to analyze the concentrations and compositions of total hydrolyzed amino acids (THAAs) in the SYS during the spring, summer, and autumn of 2019. The bioavailability potential of DOM was evaluated using the degradation index (DI) and THAA nitrogen normalized yield (THAA (%DON)) (DON as dissolved organic nitrogen). The variation dynamics of amino acid indicators during different stages of green tide were further explored. The results showed that the THAA concentrations and DOM bioavailability in the SYS were considerably influenced by biological processes. The THAA concentrations (0.96 ± 0.34 μmol L−1) exhibited the lowest mean values in the summer, while the DI values (0.106 ± 0.461) and mean THAA (%DON) values (18.20 ± 6.58 %) were the highest during this season. The distribution of amino acid indicators in the summer (the late-tide stage) was regulated by the green tide mechanism, and kept pace with the green tide floating region. In comparison with the waters in south of 35° N, the THAA concentrations and DI values experienced significant seasonal variations (p < 0.05) in north of 35° N, with the highest DI values (1.217) observed in the green tide aggregation area. This indicates the transformation of nutrient sources for Ulva prolifera in the late-tide stage and its impact on DOM bioavailability. Thus, as a potential feedback indicator of green tides, the study of amino acids is meaningful for understanding the occurrence of green tides and the source-sink pattern of organic nitrogen.

Different impacts of natural and anthropogenic factors on dissolved organic matter chemistry in coastal rivers: Implications for water management

The chemical composition of dissolved organic matter (DOM) exerts significant influence on aquatic energy dynamics, pollutant transportation, and carbon storage, thereby playing pivotal roles in the local water quality and regional-global biogeochemical cycling. However, the effects of natural climate change and local human activities on watershed characteristics and in-river processes have led to uncertainties regarding their contributions to DOM chemistry in coastal rivers, creating challenges for effective water management and the study of organic matter cycling. In this investigation, we employed a combination of stable isotopic analysis, optical techniques, and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) to elucidate the sources, optical properties, and molecular composition of DOM in three South China coastal rivers. Our results suggest that terrestrial DOM entering the three rivers through natural or anthropogenic pathways is gradually diluted by in situ primary production as it moves downstream, ultimately being influenced by seawater intrusion near the estuary. Additionally, terrestrial processes influenced by temperature likely govern DOC concentration, while seawater intrusion promotes the natural production of S-containing organic compounds. In contrast, human-altered landcover significantly impacts DOM molecular composition. Increased water areas lead to the enrichment of lignins with high disinfection byproduct formation potential, and agricultural residue burning appears to be the dominant source of pyrogenic DOM in these coastal rivers. Our distinct results suggest that the development of specific water management plans that consider the combined effects of temperature, seawater intrusion, landcover changes, and agricultural practices will be essential to ensure sustainable water resource.

Demersal fishery Impacts on Sedimentary Organic Matter (DISOM): a global harmonized database of studies assessing the impacts of demersal fisheries on sediment biogeochemistry

Abstract. Marine sediments are among the largest carbon reservoirs on the planet and play a key role in the global cycling of organic matter. Bottom fisheries are the most widespread anthropogenic physical disturbance to seabed habitats, prompting NGOs and governments to act on regulating mobile bottom-contacting fishing gear. However, the scientific evidence of the effects of bottom trawling on sediment biogeochemistry is highly diverse and presents contrasting results. Here we present a global harmonized dataset of 71 independent studies that assess the effects of demersal fisheries on sedimentological (i.e. grain size, porosity) and biogeochemical (i.e. organic carbon, phytopigments, nutrient fluxes) properties: the Demersal fishery Impacts on Sedimentary Organic Matter (DISOM) database (Paradis, 2023; https://doi.org/10.3929/ethz-b-000634336). We identify considerable gaps, namely in the geographical extension of the data; coverage of environmental predictors (i.e. seasons); fishing descriptors such as the availability of true controls, quantification of fishing effort, and distribution of fishing gear types; and biogeochemical variables that study the remineralization of organic matter. Future studies should address these data gaps to enhance the comprehensiveness of the dataset. With this harmonized database, we aim to allow researchers to explore the effects of demersal fisheries in variable environmental settings to disentangle the effects of this disturbance and provide efficient management strategies.

The Seasonal Effect of Coastal Current and Upwelling on the Dynamics of Dissolved Organic Matter in the Northwestern South China Sea

AbstractPrevious studies have primarily focused on the impact of human activities on coastal dissolved organic matter (DOM) cycles, while neglecting the role of dynamic processes. This limits our comprehensive understanding of the dynamic processes of coastal DOM, particularly in coastal waters affected by multiple dynamic processes. In this study, the dynamics of DOM in the northwestern South China Sea (SCS) were investigated using DOM‐related parameters and a three end‐member mixing model, which accounted for the effects of coastal currents and upwelling during the spring, summer, and winter seasons. Our results demonstrated that the net addition of DOM was predominantly observed in the nearshore during all seasons, owing to strong coastal currents that induced phytoplankton production and particle desorption. However, owing to weaker coastal currents and upwelling in spring and winter, DOM removal induced by decomposition was observed in the western offshore area. This observation can be attributed to the abundance of microorganisms in spring and increased vertical water mixing in winter. Conversely, in summer, stronger upwelling provided more bio‐refractory DOM, limiting its decomposition, whereas stronger coastal currents stimulated productivity and particle desorption, resulting in the net addition of DOM during this period. In the eastern offshore area, DOM decomposition was the primary mechanism for eliminating DOM owing to weak or absent coastal currents and upwelling. Our study revealed that DOM decomposition is widespread in the coastal regions of the northwestern SCS and that stronger coastal currents and upwelling facilitate the net addition of DOM.

Linking nematode trophic diversity to plantation identity and soil nutrient cycling

Ecological services provided by forest plantations depend on soil biodiversity, which encompasses taxonomic and functional diversity. These diversity components may respond specifically to environmental changes with consequences for soil functions. Given the large differences in plant-derived resource input between coniferous and broadleaved plantations, we investigated their impact on nematode species and trophic diversity, and on the soil nutrient cycling. The establishment of plantations (Larix gmelinii, Pinus tabuliformis, and Cercidiphyllum japonicum) 30 years ago led to a 31 %-37 % reduction in nematode species diversity, which is connected with decreased microbial biomass in soil. The reduced species diversity, however, was not linked to the cycling of organic matter, nitrogen, or phosphorus. Coniferous plantations (Larix gmelinii and Pinus tabuliformis) reduced nematode trophic diversity (34 %-55 %), indicating a strong decline in herbivorous and omnivorous-predatory groups of the food webs. This decline was primarily attributed to moisture limitations and reduced litter quality, and decreased nutrient cycling within the coniferous plantations. Conversely, the broadleaved plantation (Cercidiphyllum japonicum) maintained a complex nematode trophic structure with intensive nutrient cycling. Consequently, the trophic functional traits of nematode communities were determined by resource availability and linked to nutrient cycling in soil after plantation establishment.

Nickel's behaviour in marine sediments under aerobic to anaerobic diagenetic conditions

In the marine environment, nickel distributions are linked to the biogeochemical cycling of both organic matter and manganese. Thus, Ni and its isotopes have the potential to be used to reconstruct changes of bioproductivity and oxygenation in paleoenvironments. However, their utility relies on an understanding of the behaviour of Ni in modern marine environments. Here we investigate the distribution of Ni under a range of oxidation-reduction conditions, with organic carbon (Corg) burial rates that range from approximately 0.08 to 8.4 mmol m−2 d−1, none of the sites show deep (> 5 cm) penetration of dissolved oxygen into the sediment. We present Ni concentrations for sediments and pore fluids, as well as Ni isotope compositions from pore fluids, from the California and Mexico continental margins. The sites are sufficiently reducing that solid-phase Mn concentrations are typically <1000 ppm, but two of the stations, where oxygenated bottom water bathes the underlying sediment, have near-surface sediment Mn concentrations that reach up to 2.3%.Dissolved Ni is lower in pore fluids for stations with high solid phase Corg compared to those stations with oxygenated bottom water and high solid phase Mn concentrations. The calculated benthic fluxes of Ni at all stations are small relative to their burial rates, which implies a high Ni burial efficiency (> 85%). Pore fluid δ60Ni values range from approximately −0.39 to +2.36 ‰, with the higher δ60Ni values occurring at the Corg-rich station, and the lower values at the Mn-rich stations. At the station with the highest Corg content, the distribution of solid-phase Ni as a function of Corg content is consistent with the strong association of Ni with Corg seen at open-ocean upwelling margins globally. In contrast, the other stations investigated here clearly do not show such an association. Our data offer support for the notion that Ni accumulation within sediments is linked to organic matter accumulation in regions of high photic zone productivity and where the sediments are reducing. However, at the sites in our study where Mn oxidation-reduction reactions occur near the sediment-water boundary, any simple relationship between Ni and Corg burial is obfuscated. With a sufficiently deep oxygen penetration depth and the formation of a solid-phase Mn oxide layer, Ni burial within the sediment can be highly efficient. Importantly, Ni is well preserved in sediments deposited under the full range of conditions studied here. This observation of high Ni preservation is an important constraint if Ni is to be used as a proxy to reconstruct paleoenvironmental conditions.

Polystyrene microplastics facilitate formation of refractory dissolved organic matter and reduce CO2 emissions

Microplastics, as a type of anthropogenic pollution in aquatic ecosystems, affect the carbon cycle of organic matter. Although some studies have investigated the effects of microplastics on dissolved organic matter (DOM), the impact of alterations in the chemical properties of microplastics on refractory DOM and carbon release remains unclear. Here, we observed that microplastic treatments (e.g., polystyrene, PS) altered the composition and function of microbial community, notably increasing the abundance of microbial families involved in consuming easily degradable organic matter. During the process in which microbial community decomposed organic matter into DOM, PS underwent surface oxidation. The oxidized PS aggregated with DOM and microorganisms through electrostatic interactions and chemical bonds. Moreover, these interactions between oxidized PS and microbial community affect the utilization of organic matter, resulting in a significant decrease in CO2 emissions. Specifically, total CO2 emissions decreased by approximately 23.76 % with 0.1 mg/L PS treatment and by 44.97 % with 10 mg/L PS treatment compared to those in PS-free treatments over the entire reaction. These findings underscored the significance of the chemical properties of PS in the interactions among DOM and microorganisms, emphasizing the potential impact of PS microplastics on the carbon cycle in ecosystems.

Anthropogenically Driven Changes in the Carbon to Phosphorus Ratio of Marine Dissolved Organic Matter

AbstractMarine dissolved organic matter (DOM) cycles play a pivotal role in sustaining marine ecosystems and regulating the ocean's carbon sequestration from the atmosphere. However, the response of DOM cycles, including dissolved organic carbon (DOC) and dissolved organic phosphorus (DOP), to future climate change remains highly uncertain. Using the Community Earth System Model version 2 large ensemble simulations, we find that the C:P ratios in DOM are projected to increase by up to two‐fold in oligotrophic gyres by 2100. Increased upper ocean stratification reduces surface phosphate availability, thereby elevating phytoplankton C:P ratios and enhancing phytoplankton utilization of DOP, both acting to deprive DOM of P. Moreover, ocean stratification has a direct effect on exporting less DOC to the subsurface while accumulating more DOC at the sea surface. As a result of the strong sensitivity to ocean surface warming, the anthropogenically driven trends in upper ocean DOM concentration and its C:P ratios are estimated to emerge earlier from the simulated natural variability than upper ocean phosphate concentrations and net primary production—two key biogeochemical variables that are frequently monitored. This study suggests that changes in the C:P ratios of DOM could serve as a sensitive fingerprint of anthropogenic ocean warming, potentially exerting broad impacts on marine microbes. Our estimated 4% reduction in the globally integrated DOC export below 100 m is comparable to a 2% reduction in particulate organic carbon (POC) export by 2100, implying that global warming is likely to weaken the biological carbon pump through both DOC and POC.

Experimental Open Air Burning of Vegetation Enhances Organic Matter Chemical Heterogeneity Compared to Laboratory Burns.

Wildfires produce solid residuals that have unique chemical and physical properties compared to unburned materials, which influence their cycling and fate in the natural environment. Visual burn severity assessment is used to evaluate post-fire alterations to the landscape in field-based studies, yet muffle furnace methods are commonly used in laboratory studies to assess molecular scale alterations along a temperature continuum. Here, we examined solid and leachable organic matter characteristics from chars visually characterized as low burn severity that were created either on an open air burn table or from low-temperature muffle furnace burns. We assessed how the different combustion conditions influence solid and dissolved organic matter chemistries and explored the potential influence of these results on the environmental fate and reactivity. Notably, muffle furnace chars produced less leachable carbon and nitrogen than open air chars across land cover types. Organic matter produced from muffle furnace burns was more homogeneous than open air chars. This work highlights chemical heterogeneities that exist within a single burn severity category, potentially influencing our conceptual understanding of pyrogenic organic matter cycling in the natural environment, including transport and processing in watersheds. Therefore, we suggest that open air burn studies are needed to further advance our understanding of pyrogenic organic matter's environmental reactivity and fate.

Near-natural streams: Spatial factors are key in shaping multiple facets of zooplankton α and β diversity

In near-natural basins, zooplankton are key hubs for maintaining aquatic food webs and organic matter cycles. However, the spatial patterns and drivers of zooplankton in streams are poorly understood. This study registered 165 species of zooplankton from 147 sampling sites (Protozoa, Rotifers, Cladocera and Copepods), integrating multiple dimensions (i.e., taxonomic, functional, and phylogenetic) and components (i.e., total, turnover, and nestedness) of α and β diversity. This study aims to reveal spatial patterns, mechanisms, correlations, and relative contribution of abiotic factors (i.e., local environment, geo-climatic, land use, and spatial factors) through spatial interpolation (ordinary kriging), mantel test, and variance partitioning analysis (VPA). The study found that α diversity is concentrated in the north, while β diversity is more in the west, which may be affected by typical habitat, hydrological dynamics and underlying mechanisms. Taxonomic and phylogenetic β diversity is dominated by turnover, and metacommunity heterogeneity is the result of substitution of species and phylogeny along environmental spatial gradients. Taxonomic and phylogenetic β diversity were strongly correlated (r from 0.91 to 0.95), mainly explained by historical/spatial isolation processes, community composition, generation time, and reproductive characteristics, and this correlation provides surrogate information for freshwater conservation priorities. In addition, spatial factors affect functional and phylogenetic α diversity (26%, 28%), and environmental filtering and spatial processes combine to drive taxonomic α diversity (10%) and phylogenetic β diversity (11%). Studies suggest that spatial factors are key to controlling the community structure of zooplankton assemblages in near-natural streams, and that the relative role of local environments may depend on the dispersal capacity of species. In terms of diversity conservation, sites with high variation in uniqueness should be protected (i) with a focus on the western part of the thousand islands lake catchment and (ii) increasing effective dispersal between communities to facilitate genetic and food chain transmission.

Substrate uptake patterns shape niche separation in marine prokaryotic microbiome.

Marine heterotrophic prokaryotes primarily take up ambient substrates using transporters. The patterns of transporters targeting particular substrates shape the ecological role of heterotrophic prokaryotes in marine organic matter cycles. Here, we report a size-fractionated pattern in the expression of prokaryotic transporters throughout the oceanic water column due to taxonomic variations, revealed by a multi-"omics" approach targeting ATP-binding cassette (ABC) transporters and TonB-dependent transporters (TBDTs). Substrate specificity analyses showed that marine SAR11, Rhodobacterales, and Oceanospirillales use ABC transporters to take up organic nitrogenous compounds in the free-living fraction, while Alteromonadales, Bacteroidetes, and Sphingomonadales use TBDTs for carbon-rich organic matter and metal chelates on particles. The expression of transporter proteins also supports distinct lifestyles of deep-sea prokaryotes. Our results suggest that transporter divergency in organic matter assimilation reflects a pronounced niche separation in the prokaryote-mediated organic matter cycles.

Investigating the physiological ecology of mesopelagic zooplankton in the Scotia sea (Southern ocean) using lipid and stable isotope signatures

The mesopelagic zooplankton community plays an important role in the cycling and sequestration of carbon via the biological pump. However, little is known about the physiology and ecology of key taxa found within this region, hindering our understanding of their influence on the pathways of energy and organic matter cycling. We sampled the eight most abundant zooplankton (Calanoides acutus, Rhincalanus gigas, Paraeuchaeta spp., Chaetognatha, Euphausia triacantha, Thysanoessa spp., Themisto gaudichaudii and Salpa thompsoni) from within the mesopelagic zone in the Scotia Sea during a sinking diatom bloom and investigated their physiological ecology using lipid biomarkers and stable isotopic signatures of nitrogen. Data suggest that the large calanoid copepods, C. acutus and R. gigas, were in, or emerging from, a period of metabolic inactivity during the study period (November 15th – December 15th, 2017). Abundant, but decreasing lipid reserves in the predominantly herbivorous calanoid copepods, suggest these animals may have been metabolising previously stored lipids at the time of sampling, rather than deriving energy solely from the diatom bloom. This highlights the importance of understanding the timing of diapause of overwintering species as their feeding is likely to have an impact on the turnover of particulate organic matter (POM) in the upper mesopelagic. The δ15N signatures of POM became enriched with increasing depth, whereas all species of zooplankton except T. gaudichaudii did not. This suggests that animals were feeding on fresher, surface-derived POM, rather than reworked particles at depth, likely influencing the quantity and quality of organic matter leaving the upper mesopelagic. Our study highlights the complexity of mesopelagic food webs and suggests that the application of broad trophic functional types may lead to an incorrect understanding of ecosystem dynamics.

Phytoplankton-derived polysaccharides and microbial peptidoglycans are key nutrients for deep-sea microbes in the Mariana Trench

BackgroundThe deep sea represents the largest marine ecosystem, driving global-scale biogeochemical cycles. Microorganisms are the most abundant biological entities and play a vital role in the cycling of organic matter in such ecosystems. The primary food source for abyssal biota is the sedimentation of particulate organic polymers. However, our knowledge of the specific biopolymers available to deep-sea microbes remains largely incomplete. One crucial rate-limiting step in organic matter cycling is the depolymerization of particulate organic polymers facilitated by extracellular enzymes (EEs). Therefore, the investigation of active EEs and the microbes responsible for their production is a top priority to better understand the key nutrient sources for deep-sea microbes.ResultsIn this study, we conducted analyses of extracellular enzymatic activities (EEAs), metagenomics, and metatranscriptomics from seawater samples of 50–9305 m from the Mariana Trench. While a diverse array of microbial groups was identified throughout the water column, only a few exhibited high levels of transcriptional activities. Notably, microbial populations actively transcribing EE genes involved in biopolymer processing in the abyssopelagic (4700 m) and hadopelagic zones (9305 m) were primarily associated with the class Actinobacteria. These microbes actively transcribed genes coding for enzymes such as cutinase, laccase, and xyloglucanase which are capable of degrading phytoplankton polysaccharides as well as GH23 peptidoglycan lyases and M23 peptidases which have the capacity to break down peptidoglycan. Consequently, corresponding enzyme activities including glycosidases, esterase, and peptidases can be detected in the deep ocean. Furthermore, cell-specific EEAs increased at 9305 m compared to 4700 m, indicating extracellular enzymes play a more significant role in nutrient cycling in the deeper regions of the Mariana Trench.ConclusionsTranscriptomic analyses have shed light on the predominant microbial population actively participating in organic matter cycling in the deep-sea environment of the Mariana Trench. The categories of active EEs suggest that the complex phytoplankton polysaccharides (e.g., cutin, lignin, and hemicellulose) and microbial peptidoglycans serve as the primary nutrient sources available to deep-sea microbes. The high cell-specific EEA observed in the hadal zone underscores the robust polymer-degrading capacities of hadal microbes even in the face of the challenging conditions they encounter in this extreme environment. These findings provide valuable new insights into the sources of nutrition, the key microbes, and the EEs crucial for biopolymer degradation in the deep seawater of the Mariana Trench.6w3UcpMmGbcStWK5zJqVACVideo

Polyester microfiber impacts on coastal sediment organic matter consumption

As plastic pollution continues to accumulate at the seafloor, concerns around benthic ecosystem functionality heightens. This research demonstrates the systematic effects of polyester microfibers on seafloor organic matter consumption rates, an important benthic ecosystem function connected to multiple reactions and processes. We used a field-based assay to measure the loss of organic matter, both with and without polyester microfiber contamination. We identified sediment organic matter content, mud content, and mean grain size as the main drivers of organic matter consumption, however, polyester microfiber contamination decoupled ecosystem relationships and altered observed organic matter cycling dynamics. Organic matter consumption rates varied across horizontal and vertical spaces, highlighting that consumption and associated plastic effects are dependent on environmental heterogeneity at both small (within sites) and larger (between sites) scales. Our results emphasize the important role habitat heterogeneity plays in seafloor organic matter consumption and the associated effects of plastic pollution on ecosystem function.

The Combined Effects of Ocean Acidification and Respiration on Habitat Suitability for Marine Calcifiers Along the West Coast of North America

AbstractThe California Current Ecosystem (CCE) is a natural laboratory for studying the chemical and ecological impacts of ocean acidification. Biogeochemical variability in the region is due primarily to wind‐driven near‐shore upwelling of cold waters that are rich in re‐mineralized carbon and poor in oxygen. The coastal regions are exposed to surface waters with increasing concentrations of anthropogenic CO2 (Canth) from exchanges with the atmosphere and the shoreward transport and mixing of upwelled water. The upwelling drives intense cycling of organic matter that is created through photosynthesis in the surface ocean and degraded through biological respiration in subsurface habitats. We used an extended multiple linear‐regression approach to determine the spatial and temporal concentrations of Canth and respired carbon (Cbio) in the CCE based on cruise data from 2007, 2011, 2012, 2013, 2016, and 2021. Over the region, the Canth accumulation rate increased from 0.8 ± 0.1 μmol kg−1 yr−1 in the northern latitudes to 1.1 ± 0.1 μmol kg−1 yr−1 further south. The rates decreased to values of about ∼0.3 μmol kg−1 yr−1 at depths near 300 m. These accumulation rates at the surface correspond to total pH decreases that averaged about 0.002 yr‐1; whereas, decreases in aragonite saturation state ranged from 0.006 to 0.011 yr‐1. The impact of the Canth uptake was to decrease the amount of oxygen consumption required to cross critical biological thresholds (i.e., calcification, dissolution) for marine calcifiers and are significantly lower in the recent cruises than in the pre‐industrial period because of the addition of Canth.