A unique subseafloor microbiosphere in the Mariana Trench driven by episodic sedimentation.

Hadal trenches are characterized by enhanced and infrequent high-rate episodic sedimentation events that likely introduce not only labile organic carbon and key nutrients but also new microbes that significantly alter the subseafloor microbiosphere. Currently, the role of high-rate episodic sedimentation in controlling the composition of the hadal subseafloor microbiosphere is unknown. Here, analyses of carbon isotope composition in a ~ 750 cm long sediment core from the Challenger Deep revealed noncontinuous deposition, with anomalous 14C ages likely caused by seismically driven mass transport and the funneling effect of trench geomorphology. Microbial community composition and diverse enzyme activities in the upper ~ 27 cm differed from those at lower depths, probably due to sudden sediment deposition and differences in redox condition and organic matter availability. At lower depths, microbial population numbers, and composition remained relatively constant, except at some discrete depths with altered enzyme activity and microbial phyla abundance, possibly due to additional sudden sedimentation events of different magnitude. Evidence is provided of a unique role for high-rate episodic sedimentation events in controlling the subsurface microbiosphere in Earth's deepest ocean floor and highlight the need to perform thorough analysis over a large depth range to characterize hadal benthic populations. Such depositional processes are likely crucial in shaping deep-water geochemical environments and thereby the deep subseafloor biosphere. Supplementary Information: The online version contains supplementary material available at 10.1007/s42995-023-00212-y.

Sources and high burial efficiency of fossil organic carbon in small bays and implication for coastal carbon cycle.

Coastal seas receive and store large amounts of organic carbon (OC) from land and ocean, thereby playing a crucial role in the global carbon cycle. Understanding factors that influencing OC sources and burial efficiencies in coastal areas have been challenging. We selected the Jiaozhou Bay (JZB) and its surrounding rivers heavily affected by human activities as a case study small bay. We presented bulk parameters of grain size, sediment surface area (SSA), TOC content and carbon isotopes (δ13Corg and Δ14Corg), terrestrial biomarkers (∑C27 + C29 + C31n-alkanes) and marine biomarkers (brassicasterol and dinosterol) in surface sediments and suspended particulates. Our results showed low TOC and biomarker contents in the Dagu River Estuary from the west of the JZB associated with coarse sediments and lower SSA. To estimate the OC proportions, we applied a three-end member mixing model based on TOC δ13Corg and biomarker ratios and obtained the OC contribution from phytoplankton (average 52 %), soil (average 34 %) and wetlands (average 14 %). A transect from east to west of the JZB was selected to further assess the OC age composition based on radiocarbon isotopic (14C) measurements for a new perspective. The lower Δ14Corg values in the east revealed fossil OC contributions from human activities, such as petroleum pollutant inputs from sewage outlets. Based on a dual­carbon isotope (δ13Corg and Δ14Corg) mass balance mixing model, the OC contributions were 40 %, 34 %, 14 %, 12 % from fossil carbon, phytoplankton, wetlands and soil, respectively. The very high burial efficiency of fossil OC in JZB (111 ± 19 %) indicated that small bays such as the JZB could be an important sedimentary carbon sink.

Molecular 14 C evidence for contrasting turnover and temperature sensitivity of soil organic matter components.

Climate projection requires an accurate understanding for soil organic carbon (SOC) decomposition and its response to warming. An emergent view considers that environmental constraints rather than chemical structure alone control SOC turnover and its temperature sensitivity (i.e., Q10 ), but direct long-term evidence is lacking. Here, using compound-specific radiocarbon analysis of soil profiles along a 3300-km grassland transect, we provide direct evidence for the rapid turnover of lignin-derived phenols compared with slower-cycling molecular components of SOC (i.e., long-chain lipids and black carbon). Furthermore, in contrast to the slow-cycling components whose turnover is strongly modulated by mineral association and exhibits low Q10 , lignin turnover is mainly regulated by temperature and has a high Q10 . Such contrasts resemble those between fast-cycling (i.e., light) and mineral-associated slow-cycling fractions from globally distributed soils. Collectively, our results suggest that warming may greatly accelerate the decomposition of lignin, especially in soils with relatively weak mineral associations.

A driving factor for harmful algal blooms in the East China Sea coastal marine ecosystems - Implications of Kuroshio subsurface water invasion.

Quantitative calculation of the supply ratio of nutrients by different water masses suggests that the intrusion of Kuroshio subsurface water (KSSW) may indeed be an important factor in the formation of harmful algal blooms (HABs) in the Min-Zhe coastal area. This region in China has a complex hydrographic structure as well as a high incidence of HABs. The results of the calculations of the dissolved neodymium and its isotopes show that KSSW is an important provider of dissolved inorganic nitrogen (30-90 %) and phosphates (80-100 %) compared to the cases of the Taiwan Warm Current and the Changjiang Diluted Water. Sensitivity analysis results indicate that the percent contribution of KSSW in volume increased to 50-60 % in June, supplying favorable conditions for the growth of harmful algae. Our study implies that the invasion of the Kuroshio is likely to be a driving factor of HABs and, accordingly, a key factor in predicting HABs.

Responses of Marine Diatom-Dinoflagellate Competition to Multiple Environmental Drivers: Abundance, Elemental, and Biochemical Aspects.

Ocean-related global change has strongly affected the competition between key marine phytoplankton groups, such as diatoms and dinoflagellates, especially with the deleterious consequency of the increasing occurrence of harmful algal blooms. The dominance of diatoms generally shifts toward that of dinoflagellates in response to increasing temperature and reduced nutrient availability; however, contradictory findings have also been observed in certain sea areas. A key challenge in ecology and biogeochemistry is to quantitatively determine the effects of multiple environmental factors on the diatom-dinoflagellate community and the related changes in elemental and biochemical composition. Here, we test the interplay between temperature, nutrient concentrations and their ratios on marine diatom-dinoflagellate competition and chemical composition using bi-algal competition experiments. The ubiquitous diatom Phaeodactylum tricornutum and dinoflagellate Prorocentrum minimum were cultivated semi-continuously, provided with different N and P concentrations (three different levels) and ratios (10:1, 24:1, and 63:1 molar ratios) under three temperatures (12, 18, and 24°C). The responses of diatom-dinoflagellate competition were analyzed by a Lotka-Volterra model and quantified by generalized linear mixed models (GLMMs) and generalized additive models (GAMs). The changes in nutrient concentrations significantly affected diatom-dinoflagellate competition, causing a competitive superiority of the diatoms at high nutrient concentrations, independent of temperature and N:P supply ratios. Interestingly, the effect amplitude of nutrient concentrations varied with different temperatures, showing a switch back toward a competitive superiority of the dinoflagellates at the highest temperature and at very high nutrient concentrations. The ratios of particulate organic nitrogen to phosphorus showed significant negative correlations with increasing diatoms/dinoflagellates ratios, while lipid biomarkers (fatty acids and sterols) correlated positively with increasing diatoms/dinoflagellates ratios over the entire ranges of temperature, N and P concentrations and N:P ratios. Our results indicate that the integration of phytoplankton community structure and chemical composition provides an important step forward to quantitatively understand and predict how phytoplankton community changes affect ecosystem functions and biogeochemical cycles in the ocean.

Degradation and Aging of Terrestrial Organic Carbon within Estuaries: Biogeochemical and Environmental Implications.

Estuaries are action zones for organic carbon (OC) degradation and aging. These processes influence the nature of terrestrial OC (OCterr) export and the magnitude of OCterr burial in marginal seas, with important environmental implications such as CO2 release and hypoxia. In this study, we determined the contents and carbon isotopic compositions (13C and 14C) of bulk OC and fatty acids (FAs) as well as the sedimentological characteristics of suspended particulate matter (SPM) samples collected from two sites over four seasons and of surface sediment samples from three sites in the Pearl River estuary (PRE) to evaluate processes controlling OCterr degradation and aging along an estuarine gradient. We found that the abundance-weighted average C24-32FA 14C ages increased by an average of ∼1170 years for SPM and by an average of ∼3440 years in PR/PRE sediments, along the ∼60 km PRE transect. These increases in the FA age coincided with an 86% decrease in the corresponding mineral surface area-normalized FA loading along the sediment transport pathway, implying that selective degradation of labile and younger OC resulted in apparent OC aging. These measurements reveal an important shift in the nature of OC, with implications for biogeochemical cycling within estuaries and for regional environmental changes.

Climate control on terrestrial biospheric carbon turnover.

Terrestrial vegetation and soils hold three times more carbon than the atmosphere. Much debate concerns how anthropogenic activity will perturb these surface reservoirs, potentially exacerbating ongoing changes to the climate system. Uncertainties specifically persist in extrapolating point-source observations to ecosystem-scale budgets and fluxes, which require consideration of vertical and lateral processes on multiple temporal and spatial scales. To explore controls on organic carbon (OC) turnover at the river basin scale, we present radiocarbon (14C) ages on two groups of molecular tracers of plant-derived carbon-leaf-wax lipids and lignin phenols-from a globally distributed suite of rivers. We find significant negative relationships between the 14C age of these biomarkers and mean annual temperature and precipitation. Moreover, riverine biospheric-carbon ages scale proportionally with basin-wide soil carbon turnover times and soil 14C ages, implicating OC cycling within soils as a primary control on exported biomarker ages and revealing a broad distribution of soil OC reactivities. The ubiquitous occurrence of a long-lived soil OC pool suggests soil OC is globally vulnerable to perturbations by future temperature and precipitation increase. Scaling of riverine biospheric-carbon ages with soil OC turnover shows the former can constrain the sensitivity of carbon dynamics to environmental controls on broad spatial scales. Extracting this information from fluvially dominated sedimentary sequences may inform past variations in soil OC turnover in response to anthropogenic and/or climate perturbations. In turn, monitoring riverine OC composition may help detect future climate-change-induced perturbations of soil OC turnover and stocks.

Proliferation of hydrocarbon-degrading microbes at the bottom of the Mariana Trench.

BACKGROUND: The Mariana Trench is the deepest known site in the Earth's oceans, reaching a depth of ~ 11,000 m at the Challenger Deep. Recent studies reveal that hadal waters harbor distinctive microbial planktonic communities. However, the genetic potential of microbial communities within the hadal zone is poorly understood. RESULTS: Here, implementing both culture-dependent and culture-independent methods, we perform extensive analysis of microbial populations and their genetic potential at different depths in the Mariana Trench. Unexpectedly, we observed an abrupt increase in the abundance of hydrocarbon-degrading bacteria at depths > 10,400 m in the Challenger Deep. Indeed, the proportion of hydrocarbon-degrading bacteria at > 10,400 m is the highest observed in any natural environment on Earth. These bacteria were mainly Oleibacter, Thalassolituus, and Alcanivorax genera, all of which include species known to consume aliphatic hydrocarbons. This community shift towards hydrocarbon degraders was accompanied by increased abundance and transcription of genes involved in alkane degradation. Correspondingly, three Alcanivorax species that were isolated from 10,400 m water supplemented with hexadecane were able to efficiently degrade n-alkanes under conditions simulating the deep sea, as did a reference Oleibacter strain cultured at atmospheric pressure. Abundant n-alkanes were observed in sinking particles at 2000, 4000, and 6000 m (averaged 23.5 µg/gdw) and hadal surface sediments at depths of 10,908, 10,909, and 10,911 m (averaged 2.3 µg/gdw). The δ2H values of n-C16/18 alkanes that dominated surface sediments at near 11,000-m depths ranged from - 79 to - 93‰, suggesting that these sedimentary alkanes may have been derived from an unknown heterotrophic source. CONCLUSIONS: These results reveal that hydrocarbon-degrading microorganisms are present in great abundance in the deepest seawater on Earth and shed a new light on potential biological processes in this extreme environment.

Ladderane records over the last century in the East China sea: Proxies for anammox and eutrophication changes.

Anaerobic ammonium oxidation (anammox), an important process for converting fixed nitrogen to N2, plays an important role in the present-day marine nitrogen cycle. However, little is known about anammox activities in the past, especially in regions that were strongly affected by human activities, evidenced by eutrophication and hypoxia, which promote anammox bacteria growth. In this study, ladderanes have been measured in a sediment core and suspended particulate matter (SPM) in the East China Sea (ECS), to reconstruct the anammox record and to evaluate its responses to eutrophication and hypoxia. The detection and distribution of different ladderane lipids in SPM provide additional evidence that ladderanes were mostly produced in the water column and could reflect anammox activities. Summed ladderane content from the core varied between 11 and 300 ng/g dry weight (dw) sediment, with C20-[5]-ladderane fatty acid methyl esters (FAME) as the predominant compound (5-150 ng/g dw), followed by C20-[3]-ladderane FAME (1-110 ng/g dw), C18-[3]-ladderane FAME (1-32 ng/g dw) and C18 -[5]-ladderane FAME (3-11 ng/g dw). The detection of ladderanes over the last century indicate the existence of anammox in the past. The rapidly increasing trend of ladderanes since the 1960s correlates with an increase in phytoplankton biomarkers (Σ(B + D + A), brassicasterol (B), dinosterol (D) and C37 alkenones (A)), indicating that eutrophication exacerbated anammox growth. The co-variation between our ladderane record and published records of low-oxygen tolerant foraminiferal microfossils and hypoxia events over the past 60 years suggested that sediment ladderanes are a useful indicator for past changes of oxygen depletion or hypoxia in the ECS.

Impacts of Natural and Human-Induced Hydrological Variability on Particulate Organic Carbon Dynamics in the Yellow River.

Natural and human-induced hydrological changes can influence organic carbon (OC) composition in fluvial systems, with biogeochemical consequences in both terrestrial and marine environments. Here, we use bulk and molecular carbon isotopes (13C and 14C) to examine spatiotemporal variations in particulate OC (POC) composition and age from two locations along the course of the Yellow River during 2015 and 2016. Dual carbon isotopes enable deconvolution of modern, pre-aged (millennial age) soil, and fossil inputs, revealing heterogeneous OC sources at both sites. Pre-aged OC predominated at the upstream site (Huayuankou) throughout the study period, mostly reflecting the upper riverine OC. Strong downstream (Kenli) intra-annual variations in modern and pre-aged OC were caused by increased contributions from modern aquatic OC production under the drier and less turbid conditions during this El Niño year. The month of July, which included the human-induced water and sediment regulation (WSR) event at Kenli, accounted for 82% of annual POC flux, with lower modern OC contribution compared with periods of natural seasonal variability. Both natural and human-induced hydrological events clearly exert strong influence on both fluxes and composition of Yellow River POC which, in turn, affect the balance between OC remineralization and burial for this major fluvial system.

Distribution patterns of ammonia-oxidizing archaea and bacteria in sediments of the eastern China marginal seas.

Ammonia-oxidizing archaea (AOA) and bacteria (AOB) vary in their contribution to nitrification in different environments. The eastern China marginal seas (ECMS) are featured by complex river runoffs and ocean currents, forming different sediment patches. Here, via quantitative PCR and clone library analysis of the amoA genes, we showed that AOB were more abundant than AOA in ECMS sediments. The abundance, diversity and richness of AOA, but not AOB, were higher in the East China Sea (ECS) than in the Yellow Sea (YS) and Bohai Sea (BS). Nitrosopumilus (AOA) and Nitrosospira (AOB) were predominant lineages, but their abundances varied significantly between ECS, and BS and YS. This was mainly attributed to salinity and dissolved oxygen of the bottom water. The discovery of a high abundance of Nitrosophaera at estuarine sites suggested strong terrigenous influence exerted on the AOA community. In contrast, variations in ocean conditions played more important roles in structuring the AOB community, which was separated by bottom water dissolved oxygen into two groups: the south YS, and the north YS and BS. This study provides a comprehensive insight into the spatial distribution pattern of ammonia-oxidizing prokaryotes in ECMS sediments, laying a foundation for understanding their relative roles in nitrification.

Sediment Depth-Dependent Spatial Variations of Bacterial Communities in Mud Deposits of the Eastern China Marginal Seas.

The mud sediments of the eastern China marginal seas (ECMS) are deposited under different hydrodynamic conditions with different organic matter sources. These events have been demonstrated to exert significant influences on microbial communities and biogeochemical processes in surface sediments. However, the extent to which such effects occur in subsurface microbial communities remains unclear. In this study, both horizontal and vertical (five sites, each for eight layers) distributions of bacterial abundance and community composition in mud deposits of the South Yellow Sea (SYS) and East China Sea (ECS) were investigated by quantitative PCR and Illumina sequencing of the 16S rRNA gene. Both bacterial abundance and diversity were higher in the ECS than in the SYS, and tended to be higher in up than in deep layers. Proteobacteria (JTB255 marine benthic group), Acidobacteria and Bacteroidetes were dominant in the upper layers, whereas Lactococcus, Pseudomonas, and Dehalococcoidia were enriched in the deep layers. The bacterial communities in surface and subsurface sediments showed different inter-taxa relationships, indicating contrasting co-occurrence patterns. The bacterial communities in the upper layer samples clustered in accordance with mud zones, whereas those in the deep layer samples of all sites tended to cluster together. TOC δ13C and TON δ15N significantly affected the bacterial community composition, suggesting that the abundance and composition of organic matter played critical roles in shaping of sedimentary bacterial communities. This study provides novel insights into the distribution of subsurface bacterial communities in mud deposits of the ECMS, and provides clues for understanding the biogeochemical cycles in this area.

Sources and radiocarbon ages of aerosol organic carbon along the east coast of China and implications for atmospheric fossil carbon contributions to China marginal seas.

Aerosol deposition is an important mechanism for the delivery of terrestrial organic carbon (OC) to marginal seas, but OC age characteristics of aerosols are not well constrained and their contributions to sediment OC burial have not been quantified. Total suspended particle samples were collected along the east coast of China at Changdao (CD), Qingdao (QD) and Huaniao Island (HNI), and were analyzed for total organic carbon (TOC) isotopes (13C and 14C) in order to bridge this information gap. TOC δ13C and Δ14C values ranged from -23.6 to -30.5‰, and -153 to -687‰, respectively, with the latter corresponding to 14C ages ranging from 1280 to 9260yr. Estimated contributions of fossil carbon to TOC based on 14C mass balance approach ranged from 26 to 73%, with strong seasonal variations in fossil carbon observed at CD. Fossil carbon at CD showed the highest proportion (73%) in winter, reflecting anthropogenic emissions and the lowest proportion (26%) in summer, caused by biomass contribution (annual ave., 52%±17%). In contrast, the fossil carbon at both QD (57-64%) and HNI (57-67%) dominated throughout the year, reflecting local anthropogenic influences and long-range transport. Mass balance estimates indicate that atmospheric deposition and riverine export accounted for 31% and 69% of fossil carbon inputs to the China marginal seas (CMS) respectively, with fossil carbon burial efficiencies approaching 100% in the CMS. On a global scale, an atmospheric fossil carbon deposition flux of 17.2TgCyr-1 was estimated, equivalent to 40% of the estimated fluvial flux to the ocean, and potentially accounting for 24-41% of fossil OC burial in marine sediments. Therefore, the atmospheric deposition constitutes an important source of fossil carbon to marine sediments, and could play a key role in regional and global scale OC budgets and biogeochemical cycles.

Paleoecological evidence for decadal increase in phytoplankton biomass off northwestern Australia in response to climate change.

Ocean warming can modify the phytoplankton biomass on decadal scales. Significant increases in sea surface temperature (SST) and rainfall in the northwest of Australia over recent decades are attributed to climate change. Here, we used four biomarker proxies (TEX86 index, long-chain n-alkanes, brassicasterol, and dinosterol) to reconstruct approximately 60-year variations of SST, terrestrial input, and diatom and dinoflagellate biomass in the coastal waters of the remote Kimberley region. The results showed that the most significant increases in SST and terrestrial input occurred since 1997, accompanied by an abrupt increase in diatom and dinoflagellate biomasses. Compared with the results before 1997, the average TEX86H temperature during 1997-2011 increased approximately 1°C, rainfall increased 248.2 mm, brassicasterol and dinosterol contents increased 8.5 and 1.7 times. Principal component analysis indicated that the warming SST played a more important role in the phytoplankton increase than increased rainfall and river discharge.

Pollution levels of DDTs and their spatiotemporal trend from sediment records in the Southern Yellow Sea, China.

In order to study the pollution levels and spatiotemporal trend of Dichlorodiphenyltrichloroethanes (DDTs) in the Southern Yellow Sea (SYS), thirty-two surface sediment samples and a sediment core have been analyzed, and our results have been compared with previous reports. DDTs contents in our samples ranged from below detection limit to 5.1ng/gdry weight (d.w.), which presented lower ecological risks in the SYS. Surface sediment results show a seaward increasing trend with high values in the northern region of the central basin of the SYS. Our reconstructed core record and historical data from previous reports reveal an increasing trend from 1905 to 1955 but a decline trend since 1985 for DDTs, which is consistent of the production, usage and banning of DDTs in China. The source identification, based on (DDE+DDD)/DDTs, suggested that aged DDTs were the major contributor, though there were some inputs of fresh DDTs from the usage of 1,1-bis(p-Chlorophenyl)-2,2,2-trichloroethanol (dicofol).

Diversity and Abundance of the Denitrifying Microbiota in the Sediment of Eastern China Marginal Seas and the Impact of Environmental Factors.

Investigating the environmental influence on the community composition and abundance of denitrifiers in marine sediment ecosystem is essential for understanding of the ecosystem-level controls on the biogeochemical process of denitrification. In the present study, nirK-harboring denitrifying communities in different mud deposit zones of eastern China marginal seas (ECMS) were investigated via clone library analysis. The abundance of three functional genes affiliated with denitrification (narG, nirK, nosZ) was assessed by fluorescent quantitative PCR. The nirK-harboring microbiota were dominated by a few operational taxonomic units (OTUs), which were widely distributed in different sites with each site harboring their unique phylotypes. The mean abundance of nirK was significantly higher than that of narG and nosZ genes, and the abundance of narG was higher than that of nosZ. The inconsistent abundance profile of different functional genes along the process of denitrification might indicate that nitrite reduction occurred independently of denitrification in the mud deposit zones of ECMS, and sedimentary denitrification was accomplished by cooperation of different denitrifying species rather than a single species. Such important information would be missed when targeting only a single denitrifying functional gene. Analysis of correlation between abundance ratios and environmental factors revealed that the response of denitrifiers to environmental factors was not invariable in different mud deposit zones. Our results suggested that a comprehensive analysis of different denitrifying functional genes may gain more information about the dynamics of denitrifying microbiota in marine sediments.

Diversity, Abundance, and Niche Differentiation of Ammonia-Oxidizing Prokaryotes in Mud Deposits of the Eastern China Marginal Seas.

The eastern China marginal seas (ECMS) are prominent examples of river-dominated ocean margins, whose most characteristic feature is the existence of isolated mud patches on sandy sediments. Ammonia-oxidizing prokaryotes play a crucial role in the nitrogen cycles of many marine environments, including marginal seas. However, few studies have attempted to address the distribution patterns of ammonia-oxidizing prokaryotes in mud deposits of these seas. The horizontal and vertical community composition and abundance of ammonia-oxidizing archaea (AOA) and bacteria (AOB) were investigated in mud deposits of the South Yellow Sea (SYS) and the East China Sea (ECS) by using amoA clone libraries and quantitative PCR. The diversity of AOB was comparable or higher in the mud zone of SYS and lower in ECS when compared with AOA. Vertically, surface sediments had generally higher diversity of AOA and AOB than middle and bottom layers. Diversity of AOA and AOB showed significant correlation with latitude. Nitrosopumilus and Nitrosospira lineages dominated AOA and AOB communities, respectively. Both AOA and AOB assemblages exhibited greater variations across different sites than those among various depths at one site. The abundance of bacterial amoA was generally higher than that of archaeal amoA, and both of them decreased with depth. Niche differentiation, which was affected by dissolved oxygen, salinity, ammonia, and silicate (SiO[Formula: see text]), was observed between AOA and AOB and among different groups of them. The spatial distribution of AOA and AOB was significantly correlated with δ(15)NTN and SiO[Formula: see text], and nitrate and δ(13)C, respectively. Both archaeal and bacterial amoA abundance correlated strongly with SiO[Formula: see text]. This study improves our understanding of spatial distribution of AOA and AOB in ecosystems featuring oceanic mud deposits.

Eutrophication-Driven Hypoxia in the East China Sea off the Changjiang Estuary.

Coastal hypoxia is an increasingly recognized environmental issue of global concern to both the scientific community and the general public. We assessed the relative contributions from marine and terrestrially sourced organic matter that were responsible for oxygen consumption in a well-studied seasonal coastal hypoxic zone, the East China Sea off the Changjiang Estuary. Our fieldwork was conducted in August 2011 during reinstatement of a subsurface hypoxia, when we observed a continuous decline of dissolved oxygen along with production of dissolved inorganic carbon resulting from organic carbon remineralization. On the basis of a three end-member mixing model and determinations of the stable isotopic compositions of dissolved inorganic carbon (δ(13)CDIC), the end product of particulate organic carbon (POC) degradation, we quantified the δ(13)C value of the remineralized organic carbon (δ(13)COCx), which was -18.5 ± 1.0‰. This isotopic composition was very similar to the δ(13)C of marine sourced POC produced in situ (-18.5 ± 0.3‰) rather than that of the terrestrially sourced POC (-24.4 ± 0.2‰). We concluded that marine-sourced organic matter, formed by eutrophication-induced marine primary production, was the dominant oxygen consumer in the subsurface hypoxic zone in the East China Sea off the Changjiang Estuary.

Phylogenetic shifts of bacterioplankton community composition along the Pearl Estuary: the potential impact of hypoxia and nutrients.

The significance of salinity in shaping bacterial communities dwelling in estuarine areas has been well documented. However, the influences of other environmental factors such as dissolved oxygen and nutrients in determining distribution patterns of both individual taxa and bacterial communities inhabited local estuarine regions remain elusive. Here, bacterioplankton community structures of surface and bottom waters from eight sites along the Pearl Estuary were characterized with 16S rRNA gene pyrosequencing. The results showed significant differences of bacterioplankton community between freshwater and saltwater sites, and further between surface and bottom waters of saltwater sites. Synechococcus dominated the surface water of saltwater sites while Oceanospirillales, SAR11 and SAR406 were prevalent in the bottom water. Betaproteobacteria was abundant in freshwater sites, with no significant difference between water layers. Occurrence of phylogenetic shifts in taxa affiliated to the same clade was also detected. Dissolved oxygen explained most of the bacterial community variation in the redundancy analysis targeting only freshwater sites, whereas nutrients and salinity explained most of the variation across all samples in the Pearl Estuary. Methylophilales (mainly PE2 clade) was positively correlated to dissolved oxygen, whereas Rhodocyclales (mainly R.12up clade) was negatively correlated. Moreover, high nutrient inputs to the freshwater area of the Pearl Estuary have shifted the bacterial communities toward copiotrophic groups, such as Sphingomonadales. The present study demonstrated that the overall nutrients and freshwater hypoxia play important roles in determining bacterioplankton compositions and provided insights into the potential ecological roles of specific taxa in estuarine environments.