RESUMEN

Our study explored the lateral export of macroalgae and seagrass to the deep sea of the Northern South China Sea (NSCS). Particulate organic carbon (POC) collected from a depth of 500 m off southwestern Taiwan (station T) and Dongsha Atoll (station K) underwent environmental DNA (eDNA) and stable isotope assays. Metabarcoding using 18S V9 rDNA revealed lateral export of macrophyte detritus in NSCS. At station K, seagrass detritus predominated, while at station T, macroalgae-derived detritus was dominant. The consistency in the stable carbon isotope signature between POC and macrophytes indicates that stable carbon is an ideal bio-indicator for tracking macrophyte detritus destination and transformation after it has been laterally exported. Based on robust scientific methods, these findings provide valuable insights into the lateral export of macrophyte detritus to the deep sea in POC, influenced by habitat species, and shaped by distinct oceanographic physics around NSCS.

Asunto(s)

RESUMEN

Carbon export efficiency is a key indicator of the capacity of biological pump, but the controlling mechanism of the efficiency remains unclear. Our findings revealed that interannual variations in seasonal carbon export efficiency are determined by direct factors including riverine nutrient fluxes, stratification, residence time. These direct factors are finally attributed to two indirect factors (human activities and climate change). We quantified the absolute contributions of direct and indirect factors to carbon export efficiency. The results showed that the carbon export efficiency in the northern Gulf of Mexico in spring (summer; autumn; winter) was driven by human activities, which accounted for an absolute contribution of 16.02% (7.20%; 4.00%; 8.49%, respectively) through riverine nutrient fluxes, and by climate change, which accounted for an absolute contribution of 33.51% (21.43%; 25.73%; 15.80%, respectively) through stratification and water residence time. Moreover, carbon export efficiency could be predicted by MEI of 8 months earlier.

Asunto(s)

RESUMEN

Subtropical oceans contribute significantly to global primary production, but the fate of the picophytoplankton that dominate in these low-nutrient regions is poorly understood. Working in the subtropical Mediterranean, we demonstrate that subduction of water at ocean fronts generates 3D intrusions with uncharacteristically high carbon, chlorophyll, and oxygen that extend below the sunlit photic zone into the dark ocean. These contain fresh picophytoplankton assemblages that resemble the photic-zone regions where the water originated. Intrusions propagate depth-dependent seasonal variations in microbial assemblages into the ocean interior. Strikingly, the intrusions included dominant biomass contributions from nonphotosynthetic bacteria and enrichment of enigmatic heterotrophic bacterial lineages. Thus, the intrusions not only deliver material that differs in composition and nutritional character from sinking detrital particles, but also drive shifts in bacterial community composition, organic matter processing, and interactions between surface and deep communities. Modeling efforts paired with global observations demonstrate that subduction can flux similar magnitudes of particulate organic carbon as sinking export, but is not accounted for in current export estimates and carbon cycle models. Intrusions formed by subduction are a particularly important mechanism for enhancing connectivity between surface and upper mesopelagic ecosystems in stratified subtropical ocean environments that are expanding due to the warming climate.

Asunto(s)

RESUMEN

Microbial community dynamics on sinking particles control the amount of carbon that reaches the deep ocean and the length of time that carbon is stored, with potentially profound impacts on Earth's climate. A mechanistic understanding of the controls on sinking particle distributions has been hindered by limited depth- and time-resolved sampling and methods that cannot distinguish individual particles. Here, we analyze microbial communities on nearly 400 individual sinking particles in conjunction with more conventional composite particle samples to determine how particle colonization and community assembly might control carbon sequestration in the deep ocean. We observed community succession with corresponding changes in microbial metabolic potential on the larger sinking particles transporting a significant fraction of carbon to the deep sea. Microbial community richness decreased as particles aged and sank; however, richness increased with particle size and the attenuation of carbon export. This suggests that the theory of island biogeography applies to sinking marine particles. Changes in POC flux attenuation with time and microbial community composition with depth were reproduced in a mechanistic ecosystem model that reflected a range of POC labilities and microbial growth rates. Our results highlight microbial community dynamics and processes on individual sinking particles, the isolation of which is necessary to improve mechanistic models of ocean carbon uptake.

Asunto(s)

RESUMEN

The genus Phaeocystis is globally distributed, with blooms commonly occurring on continental shelves. This unusual phytoplankter has two major morphologies: solitary cells and cells embedded in a gelatinous matrix. Only colonies form blooms. Their large size (commonly 2 mm but up to 3 cm) and mucilaginous envelope allow the colonies to escape predation, but data are inconsistent as to whether colonies are grazed. Cultured Phaeocystis can also inhibit the growth of co-occurring phytoplankton or the feeding of potential grazers. Colonies and solitary cells use nitrate as a nitrogen source, although solitary cells can also grow on ammonium. Phaeocystis colonies might be a major contributor to carbon flux to depth, but in most cases, colonies are rapidly remineralized in the upper 300 m. The occurrence of large Phaeocystis blooms is often associated with environments with low and highly variable light and high nitrate levels, with Phaeocystis antarctica blooms being linked additionally to high iron availability. Emerging results indicate that different clones of Phaeocystis have substantial genetic plasticity, which may explain its appearance in a variety of environments. Given the evidence of Phaeocystis appearing in new systems, this trend will likely continue in the near future.

Asunto(s)

RESUMEN

Radionuclides can provide key information on the temporal dimension of environmental processes, given their well-known rates of radioactive decay and production. Naturally occurring radionuclides, such as 234Th and 210Po, have been used as powerful particle tracers in the marine environment to study particle cycling and vertical export. Since their application to quantify the magnitude of particulate organic carbon (POC) export in the 1990s, 234Th and, to a lesser extent, 210Po have been widely used to characterize the magnitude of the biological carbon pump (BCP). Combining both radionuclides, with their different half-lives, biogeochemical behaviors, and input sources to the ocean, can help to better constrain POC export and capture BCP dynamics that would be missed by a single tracer. Here, we review the studies that have simultaneously used 234Th and 210Po as tracers of POC export, emphasizing what can be learned from their joint application, and provide recommendations and future directions.

Asunto(s)

RESUMEN

Mesozooplankton is a very diverse group of small animals ranging in size from 0.2 to 20 mm not able to swim against ocean currents. It is a key component of pelagic ecosystems through its roles in the trophic networks and the biological carbon pump. Traditionally studied through microscopes, recent methods have been however developed to rapidly acquire large amounts of data (morphological, molecular) at the individual scale, making it possible to study mesozooplankton using a trait-based approach. Here, combining quantitative imaging with metabarcoding time-series data obtained in the Sargasso Sea at the Bermuda Atlantic Time-series Study (BATS) site, we showed that organisms' transparency might be an important trait to also consider regarding mesozooplankton impact on carbon export, contrary to the common assumption that just size is the master trait directing most mesozooplankton-linked processes. Three distinct communities were defined based on taxonomic composition, and succeeded one another throughout the study period, with changing levels of transparency among the community. A co-occurrences' network was built from metabarcoding data revealing six groups of taxa. These were related to changes in the functioning of the ecosystem and/or in the community's morphology. The importance of Diel Vertical Migration at BATS was confirmed by the existence of a group made of taxa known to be strong migrators. Finally, we assessed if metabarcoding can provide a quantitative approach to biomass and/or abundance of certain taxa. Knowing more about mesozooplankton diversity and its impact on ecosystem functioning would allow to better represent them in biogeochemical models.

Asunto(s)

RESUMEN

The 13C isotope composition (δ13C) of leaf dry matter is a useful tool for physiological and ecological studies. However, how post-photosynthetic fractionation associated with respiration and carbon export influences δ13C remains uncertain. We investigated the effects of post-photosynthetic fractionation on δ13C of mature leaves of Cleistogenes squarrosa, a perennial C4 grass, in controlled experiments with different levels of vapour pressure deficit and nitrogen supply. With increasing leaf age class, the 12C/13C fractionation of leaf organic matter relative to the δ13C of atmosphere CO2 (ΔDM) increased while that of cellulose (Δcel) was almost constant. The divergence between ΔDM and Δcel increased with leaf age class, with a maximum value of 1.6‰, indicating the accumulation of post-photosynthetic fractionation. Applying a new mass balance model that accounts for respiration and export of photosynthates, we found an apparent 12C/13C fractionation associated with carbon export of -0.5‰ to -1.0‰. Different ΔDM among leaves, pseudostems, daughter tillers, and roots indicate that post-photosynthetic fractionation happens at the whole-plant level. Compared with ΔDM of old leaves, ΔDM of young leaves and Δcel are more reliable proxies for predicting physiological parameters due to the lower sensitivity to post-photosynthetic fractionation and the similar sensitivity in responses to environmental changes.

Asunto(s)

RESUMEN

The oceans play a fundamental role in the global carbon cycle, providing a sink for atmospheric carbon. Key to this role is the vertical transport of organic carbon from the surface to the deep ocean. This transport is a product of a diverse range of physical and biogeochemical processes that determine the formation and fate of this material, and in particular how much carbon is sequestered in the deep ocean. Models can be used to both diagnose biogeochemical processes and predict how the various processes will change in the future. Global biogeochemical models use simplified representations of food webs and processes but are converging on values for the export of organic carbon from the surface ocean. Other models concentrate on understanding specific processes and can be used to develop parameterizations for global models. Model development is continuing by adding representations and parameterizations of higher trophic levels and mesopelagic processes, and these are expected to improve model performance.

Asunto(s)

RESUMEN

A vast literature has examined the empirical link between gross exports and total carbon emissions for different country groups. However, countries' increasing participation in global value chains (GVCs) challenges this traditional approach since the gross measures neglect trade-embodied carbon emissions and intermediates-driven value-added trade. Therefore, this study scrutinizes how backward participation (foreign contents in domestic exports) and forward participation (domestic contents in foreign exports) in GVCs affect per capita net exports of trade-embodied carbon dioxide emissions. The study adopts input-output accounting and value-added decomposition framework for Group of Seven (G7) and Emerging Seven (E7) countries over the 1995-2018 period. (i) Pre-estimation analyses reveal that the net carbon importer G7 group had a comparative advantage in high-tech exports and a lower export product concentration level, while the net carbon exporter E7 group had a comparative advantage in resource-intensive exports and a higher export product concentration level, albeit significant within-group heterogeneities. (ii) The augmented mean group estimates reveal that increasing backward participation raises net carbon exports for both G7 and E7. The forward participation-net carbon exports nexus is negative for G7 but positive for E7. (iii) While economic growth reduces net carbon exports in both groups, the effects of comparative advantages in resource-intensive and high-tech exports differ. Practitioners should be aware of the GVCs-driven carbon circle when assessing decarbonization performances and obligations of countries.

Asunto(s)

RESUMEN

Discarded surgical masks have become a new source of plastic waste in seawater capable of releasing numerous micro and nano plastic fragments. However, little information is available about how this waste impacts the ecological state of marine phytoplankton. Here, we exposed two model marine diatoms (Phaeodactylum tricornutum and Thalassiosira weissflogii) to mask-released debris (MD) that is characterized by various differently-charged functional groups. Although MD could only bind loosely to diatoms, it still inhibited their growth and significantly altered cell surface physicochemical properties. At the nanoscale, MD-exposed cell walls showed enhanced roughness and modulus, besides declined electrical potential, adhesion, and proportion of oxygen-containing compounds. As a result, diatom ingestion by copepods was reduced, and the sinking rate of the carbon pool consisting of MD plus diatoms decreased as well. Our study indicated that MD effects on diatoms have the potential to slow down carbon export from surface seawater to the deep sea. Since oxidation and generation of functional groups are common during the aging process of microplastics (MPs) in nature, the interactions between the diatom cell surface and MD have important environmental significance.

Asunto(s)

RESUMEN

The biological carbon pump (BCP) in the Southern Ocean is driven by phytoplankton productivity and is a significant organic matter sink. However, the role of particle-attached (PA) and free-living (FL) prokaryotes (bacteria and archaea) and their diversity in influencing the efficiency of the BCP is still unclear. To investigate this, we analyzed the metagenomes linked to suspended and sinking marine particles from the Sub-Antarctic Southern Ocean Time Series (SOTS) by deploying a Marine Snow Catcher (MSC), obtaining suspended and sinking particulate material, determining organic carbon and nitrogen flux, and constructing metagenome-assembled genomes (MAGs). The suspended and sinking particle-pools were dominated by bacteria with the potential to degrade organic carbon. Bacterial communities associated with the sinking fraction had more genes related to the degradation of complex organic carbon than those in the suspended fraction. Archaea had the potential to drive nitrogen metabolism via nitrite and ammonia oxidation, altering organic nitrogen concentration. The data revealed several pathways for chemoautotrophy and the secretion of recalcitrant dissolved organic carbon (RDOC) from CO2, with bacteria and archaea potentially sequestering particulate organic matter (POM) via the production of RDOC. These findings provide insights into the diversity and function of prokaryotes in suspended and sinking particles and their role in organic carbon/nitrogen export in the Southern Ocean. IMPORTANCE The biological carbon pump is crucial for the export of particulate organic matter in the ocean. Recent studies on marine microbes have shown the profound influence of bacteria and archaea as regulators of particulate organic matter export. Yet, despite the importance of the Southern Ocean as a carbon sink, we lack comparable insights regarding microbial contributions. This study provides the first insights regarding prokaryotic contributions to particulate organic matter export in the Southern Ocean. We reveal evidence that prokaryotic communities in suspended and sinking particle fractions harbor widespread genomic potential for mediating particulate organic matter export. The results substantially enhance our understanding of the role played by microorganisms in regulating particulate organic matter export in suspended and sinking marine fractions in the Southern Ocean.

Asunto(s)

RESUMEN

Through biological activity, marine dissolved inorganic carbon (DIC) is transformed into different types of biogenic carbon available for export to the ocean interior, including particulate organic carbon (POC), dissolved organic carbon (DOC), and particulate inorganic carbon (PIC). Each biogenic carbon pool has a different export efficiency that impacts the vertical ocean carbon gradient and drives natural air-sea carbon dioxide gas (CO2) exchange. In the Southern Ocean (SO), which presently accounts for ~40% of the anthropogenic ocean carbon sink, it is unclear how the production of each biogenic carbon pool contributes to the contemporary air-sea CO2 exchange. Based on 107 independent observations of the seasonal cycle from 63 biogeochemical profiling floats, we provide the basin-scale estimate of distinct biogenic carbon pool production. We find significant meridional variability with enhanced POC production in the subantarctic and polar Antarctic sectors and enhanced DOC production in the subtropical and sea-ice-dominated sectors. PIC production peaks between 47°S and 57°S near the "great calcite belt." Relative to an abiotic SO, organic carbon production enhances CO2 uptake by 2.80 ± 0.28 Pg C y-1, while PIC production diminishes CO2 uptake by 0.27 ± 0.21 Pg C y-1. Without organic carbon production, the SO would be a CO2 source to the atmosphere. Our findings emphasize the importance of DOC and PIC production, in addition to the well-recognized role of POC production, in shaping the influence of carbon export on air-sea CO2 exchange.

RESUMEN

Warming in mountains is known to intensify aridity and threaten water availability globally. Its impacts on water quality, however, have remained poorly understood. Here we collate long-term (multi-year to decadal mean), baseline stream concentrations and fluxes of dissolved organic and inorganic carbon, two essential indicators of water quality and soil carbon response to warming, across more than 100 streams in the United States Rocky Mountains. Results show a universal pattern of higher mean concentrations in more arid mountain streams with lower mean discharge, a long-term climate measure. A watershed reactor model revealed less lateral export of dissolved carbon (via less water flow) out of the watersheds in more arid sites, leading to more accumulation and higher concentrations. Lower concentrations typically occur in cold, steep, and compact mountains with higher snow fraction and lower vegetation cover, which generally have higher discharge and carbon fluxes. Inferring from a space-for-time perspective, the results indicate that as warming intensifies, lateral fluxes of dissolved carbon will decrease but concentrations will increase in these mountain streams. This indicates deteriorating water quality and potentially elevated CO2 emission directly from the land (instead of streams) in the Rockies and other mountain areas in the future climate.

Asunto(s)

RESUMEN

Boreal rivers transport and process large amounts of organic and inorganic materials derived from their catchments, yet quantitative estimates and patterns of carbon (C) transport and emissions in these large rivers are scarce relative to those of high-latitude lakes and headwater streams. Here, we present the results of a large-scale survey of 23 major rivers in northern Québec sampled during the summer period of 2010, which aimed to determine the magnitude and spatial variability of different C species (carbon dioxide - CO2, methane - CH4, total carbon - TC, dissolved organic carbon - DOC and inorganic carbon - DIC), as well as to identify their main drivers. In addition, we constructed a first order mass balance of total riverine C emissions to the atmosphere (outgassing from the main river channel) and export to the ocean over summer. All rivers were supersaturated in pCO2 and pCH4 (partial pressure of CO2 and CH4), and the resulting fluxes varied widely among rivers, especially the CH4. There was a positive relationship between DOC and gas concentrations, suggesting a common watershed source of these C species. DOC concentrations declined as a function of % land surface covered by water (lentic + lotic systems) in the watershed, suggesting that lentic systems may act as a net sink of organic matter in the landscape. The C balance suggests that the export component is higher than atmospheric C emissions in the river channel. However, for heavily dammed rivers, C emissions to the atmosphere approaches the C export component. Such studies are highly important for the overall efforts to effectively quantify and incorporate major boreal rivers into whole-landscape C budgets, to determine the net role of these ecosystems as C sinks or sources, and to predict how these might shift under anthropogenic pressures and dynamic climate conditions.

RESUMEN

Despite growing attention on the contribution of macroalgae to carbon cycling and sequestration (blue carbon), more observational data is needed to constrain current estimates. In this study, we estimate the floating macroalgal carbon flux within and beyond a large sub-Arctic fjord system, Nuup Kangerlua, Greenland, which could potentially reach carbon sinks. Our study estimates 1) the fjord-scale area with macroalgal coverage and barrens caused by sea urchin grazing, 2) the floating macroalgal biomass in the fjord, and 3) the annual export flux of floating macroalgae out of the fjord system. ROV surveys documented that macroalgal habitats cover 32 % of the seafloor within the photic zone (0-30 m) with an average coverage of 39.6, 22, and 7.2 % in the depth intervals 0-10, 10-20, and 20-30 m, respectively. 15 % of the area suitable for macroalgae was denuded by sea urchin grazing. Floating macroalgae were common with an average biomass of 55 kg wet weight km-2. Densities and species composition varied seasonally with the highest levels after storms. The floating biomass was composed of intertidal macroalgal species (58 %) (Fucus vesiculosus, Fucus distichus, and Ascophyllum nodosum) and kelps (42 %) (Saccharina longicruris, S. latissima, and Alaria esculenta). We deployed surface GPS drifters to simulate floating macroalgal trajectories and velocity. Data indicated that 80 % of the floating biomass is retained in the fjord where its fate in relation to long-term sequestration is unknown. Export beyond the fjord was limited and indicated an annual floating macroalgal export beyond the fjord of only 6.92 t C yr-1, which is equal to ~0.02 % of the annual net primary production. Our findings suggest that floating macroalgae support a limited blue carbon potential beyond this fjord and that future research should focus on the fate of retained floating macroalgae and subsurface export to resolve the connectivity between macroalgal habitats and long-term carbon sinks.

Asunto(s)

RESUMEN

The biological pump transports organic matter, created by phytoplankton productivity in the well-lit surface ocean, to the ocean's dark interior, where it is consumed by animals and heterotrophic microbes and remineralized back to inorganic forms. This downward transport of organic matter sequesters carbon dioxide from exchange with the atmosphere on timescales of months to millennia, depending on where in the water column the respiration occurs. There are three primary export pathways that link the upper ocean to the interior: the gravitational, migrant, and mixing pumps. These pathways are regulated by vastly different mechanisms, making it challenging to quantify the impacts of the biological pump on the global carbon cycle. In this review, we assess progress toward creating a global accounting of carbon export and sequestration via the biological pump and suggest a path toward achieving this goal.

Asunto(s)

RESUMEN

Many coastal areas are hotspots of aquaculture expansion, where the overuse of artificial feeds results in the accumulation of organic carbon in nearshore aquaculture ponds. In rural areas, wastewater from the aquaculture ponds is discharged to the nearshore waters through artificial ditches causing lateral carbon export from the land to the ocean. Such flux may be meaningful in coastal carbon budgets since aquaculture is the hotspot of carbon sequestration and storage. To quantify the magnitude and temporal dynamics of lateral carbon export from aquaculture ponds, we used high-frequency in-situ monitoring of turbidity, fluorescent dissolved organic matter, etc. across different temporal scales. We measured water levels and velocity profiles in a ditch cross-section to obtain year-round water exchange. Carbon export was integrated from water fluxes and organic carbon concentrations. Our results suggested that aquaculture ponds were a source of particular organic carbon (POC) and dissolved organic carbon (DOC). The net lateral flux of POC and DOC was 148 ± 38 kg yr-1 and 296 ± 18 kg yr-1. Temporally, the export of POC and DOC is influenced by both tides and wastewater discharge. Under the disturbance with aquaculture wastewater discharge, the mean DOC export in the ditch increased by 497 kg, which was 1.5 times that of the undisturbed; the mean POC export increased by 190 kg, which was 1.8 times that of the undisturbed. Thus, aquaculture activities can considerably disturb the coastal carbon balance by facilitating carbon-rich fluid exchange from onshore farms to nearshore estuaries. As aquaculture expands across Asia and the globe, this study provides important insights into the impacts of aquaculture on coastal carbon budgets.

Asunto(s)

RESUMEN

Picocyanobacteria are the most abundant primary producers in the ocean and play a fundamental role in marine carbon cycling. Quantification of picocyanobacteria on sinking particles and in sediments is essential to understanding their contribution to the biological carbon pump. We designed a primer set targeting the 16S-23S rRNA internal transcribed spacer (ITS) sequence of cyanobacteria and established a quantitative PCR (qPCR) method for quantifying the ITS sequence abundance. High-throughput sequencing confirmed that this primer set can cover broad diversities of marine picocyanobacteria and avoid amplification of other marine cyanobacteria such as Trichodesmium and Crocosphaera. Amplification efficiencies were slightly different when seven marine Synechococcus and Prochlorococcus strains were assayed. The qPCR results were comparable with flow cytometry for water samples. Using this method, we found that, in the dark ocean, picocyanobacterial ITS sequence abundances were 10 to 100 copies/mL in the size fraction of 0.2 to 3 µm, which were 1 to 3 orders of magnitude more abundant than on the >3-µm particles. We also found that picocyanobacterial ITS abundance in sediment ranged from 105 to 107 copies/g along two nearshore-to-offshore transects in the northern South China Sea. These results further explain the important role of picocyanobacteria in carbon export. Collectively, we provide a qPCR method quantifying the total abundance of marine picocyanobacteria on water column particles and in sediments. Moreover, this newly designed primer set can be also applied to investigate the community of picocyanobacteria via high-throughput sequencing. IMPORTANCE Picocyanobacteria are the most abundant primary producers in the ocean. However, quantification of picocyanobacteria on the sinking particles and in sediments remains challenging using flow cytometry or epifluorescence microscopy. Here, we developed a real-time PCR method to quantify picocyanobacteria using a newly designed primer set specifically targeting the 16S-23S rRNA ITS sequence of cyanobacteria. We showed that in the dark ocean, picocyanobacteria are 1 to 3 orders of magnitude more abundant in small particles (0.2 to 3 µm) than in larger particles (>3 µm). This result supports the important role of direct sinking free-living picocyanobacteria cells in the carbon export to deep ocean. We also found that the picocyanobacterial ITS sequence abundance were 105 to 107 copies per gram in sediments, suggesting significant accumulation of sinking picocyanobacteria in the benthic ecosystem. This qPCR method can be used to quantify the contribution of picocyanobacteria to the biological carbon pump.

Asunto(s)