Sinking Organic Matter Research Articles

Main drivers of transparent exopolymer particle distribution across the surface Atlantic Ocean

Abstract. Transparent exopolymer particles (TEPs) are a class of gel particles, produced mainly by microorganisms, which play important roles in biogeochemical processes such as carbon cycling and export. TEPs (a) are colonized by carbon-consuming microbes; (b) mediate aggregation and sinking of organic matter and organisms, thereby contributing to the biological carbon pump; and (c) accumulate in the surface microlayer (SML) and affect air–sea gas exchange. The first step to evaluate the global influence of TEPs in these processes is the prediction of TEP occurrence in the ocean. Yet, little is known about the physical and biological variables that drive their abundance, particularly in the open ocean. Here we describe the horizontal TEP distribution, along with physical and biological variables, in surface waters along a north–south transect in the Atlantic Ocean during October–November 2014. Two main regions were separated due to remarkable differences: the open Atlantic Ocean (OAO, n=30), and the Southwestern Atlantic Shelf (SWAS, n=10). TEP concentration in the entire transect ranged 18.3–446.8 µg XG eq L−1 and averaged 117.1±119.8 µg XG eq L−1, with the maximum concentrations in the SWAS and in a station located at the edge of the Canary Coastal Upwelling (CU), and the highest TEP to chlorophyll a (TEP:Chl a) ratios in the OAO (183±56) and CU (1760). TEPs were significantly and positively related to Chl a and phytoplankton biomass, expressed in terms of C, along the entire transect. In the OAO, TEPs were positively related to some phytoplankton groups, mainly Synechococcus. They were negatively related to the previous 24 h averaged solar irradiance, suggesting that sunlight, particularly UV radiation, is more a sink than a source for TEP. Multiple regression analyses showed the combined positive effect of phytoplankton and heterotrophic prokaryotes (HPs) on TEP distribution in the OAO. In the SWAS, TEPs were positively related to high nucleic acid-containing prokaryotic cells and total phytoplankton biomass, but not to any particular phytoplankton group. Estimated TEP–carbon constituted an important portion of the particulate organic carbon pool in the entire transect (28 %–110 %), generally higher than the phytoplankton and HP carbon shares, which highlights the importance of TEPs in the cycling of organic matter in the ocean.

Nitrate isotopic gradients in the North Atlantic Ocean and the nitrogen isotopic composition of sinking organic matter

Biogeochemical processes within the Atlantic basin alter the nitrogen (N) and oxygen (O) isotopic compositions (δ15N and δ18O) of nitrate before this nutrient is carried from the upper water column into the deep ocean by the formation of North Atlantic Deep Water (NADW). Here, nitrate δ15N and δ18O measurements along the 2013 CLIVAR/GO-SHIP Atlantic section A16N from 6°S to 61°N are used to calculate the δ15N and δ18O of regenerated nitrate added to the interior at different latitudes and depths along the section. The δ15N of nitrate being regenerated is similar among depths and covaries with δ15N of the nitrate supply to overlying surface waters. These observations are consistent with regenerated nitrate deriving dominantly from sinking N, rather than from dissolved organic or suspended particulate organic N that is circulated through the ocean interior. The δ15N of regenerated nitrate never declines below the δ15N of the nitrate supply, consistent with the dominance of subsurface nitrate over N2 fixation and atmospheric N deposition in fueling net community production. In the low latitudes (<15°N), shallow and mid-depth nitrate δ15N entering from the South in Subantarctic Mode Water (SAMW) is as high as 6.2‰ vs. air due to partial nitrate assimilation in the Southern Ocean surface, while the nitrate δ18O is similar to that measured in deep water. Its low δ18O indicates that nitrate at shallow and mid-depths is mostly regenerated, the low δ18O of which overprints the high δ18O of unused (“preformed”) nitrate from the Southern Ocean. Regenerated nitrate δ18O is calculated to be 1.1–1.2‰ higher than ambient seawater, consistent with previous work. At the southern end of the section, the δ15N of the sinking flux is as high as that of SAMW (~6.2‰) or higher, maintaining the high δ15N of shallow/mid-depth waters and raising the δ15N of nitrate on deeper isopycnals, including those within NADW. SAMW ultimately supplies water for NADW formation, but the high nitrate δ15N of SAMW observed in the southern end of the section does not survive into newly formed NADW, which has a nitrate δ15N of 4.83‰ vs. air. This indicates that the high nitrate δ15N of northward-flowing shallow/mid-depth waters is overprinted by the input of low-δ15N fixed N to the Atlantic, largely by N2 fixation. Our reconstruction of the δ15N of sinking N indicates that most newly fixed N is added south of the North Atlantic subtropical gyre. In the subpolar North Atlantic, the A16N data indicate that the δ15N of the nitrate in NADW does not differ from that of its regional upper ocean sources, for two reasons. First, nitrate is completely consumed in much of the subpolar North Atlantic surface, minimizing the isotopic signal from partial nitrate assimilation. Second, much of the sinking N from the North Atlantic surface is regenerated in newly formed NADW, reuniting the nitrate assimilated in the surface with the preformed nitrate carried down in NADW.

Tracing sinking organic matter sources in the NW Iberian upwelling system (NE Atlantic Ocean): Comparison between elemental, isotopic and molecular indicators

We studied the seasonal fluctuations in the composition of sinking particulate organic matter (POM) in the only upwelling-affected continental margin of Europe, to improve our understanding of the carbon (C) cycle of such systems. The methods employed targeted the elemental, stable isotope (δ13Coc and δ15N) and molecular (Py-GC-MS) composition. The results showed that the sinking POM in this margin is predominantly of marine origin, with δ15N (5.2 ± 0.3‰) and C/N (9.7 ± 1.0) values similar to those of marine plankton, and high abundances of pyrolysis products with isoprenoid structures, N-containing products from proteins (indoles, cyanobenzenes, etc.), and N-acetylated polysaccharides (acetamide, acetamidofurans) from chitin and peptidoglycan. Furthermore, the dataset demonstrates seasonal differences in POM composition under different oceanographic scenarios: i) dominance of most N-compounds, fatty acids and isoprenoid products during highly productive upwelling seasons, and ii) an increase in the relative abundance of linear alkanes/alkenes, phenols, lignin and PAH reflecting terrestrial influence during downwelling periods. In addition, the detection of isoprenoid alkylthiophenes traces (compounds formed under reducing conditions in the sediment) point to resuspended sediment in the trap material due to hydrodynamic effects of currents and waves on the seafloor. The results show that analytical pyrolysis is a useful tool for identifying different types of autochthonous and terrestrial POM, and therefore identification of the sources of sinking POM.

Modelling the biogeochemical effects of heterotrophic and autotrophic N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fixation in the Gulf of Aqaba (Israel), Red Sea

Abstract. Recent studies demonstrate that marine N2 fixation can be carried out without light by heterotrophic N2 fixers (diazotrophs). However, direct measurements of N2 fixation in aphotic environments are relatively scarce. Heterotrophic as well as unicellular and colonial photoautotrophic diazotrophs are present in the oligotrophic Gulf of Aqaba (northern Red Sea). This study evaluates the relative importance of these different diazotrophs by combining biogeochemical models with time series measurements at a 700 m deep monitoring station in the Gulf of Aqaba. At this location, an excess of nitrate, relative to phosphate, is present throughout most of the water column and especially in deep waters during stratified conditions. A relative excess of phosphate occurs only at the water surface during nutrient-starved conditions in summer. We show that a model without N2 fixation can replicate the observed surface chlorophyll but fails to accurately simulate inorganic nutrient concentrations throughout the water column. Models with N2 fixation improve simulated deep nitrate by enriching sinking organic matter in nitrogen, suggesting that N2 fixation is necessary to explain the observations. The observed vertical structure of nutrient ratios and oxygen is reproduced best with a model that includes heterotrophic as well as colonial and unicellular autotrophic diazotrophs. These results suggest that heterotrophic N2 fixation contributes to the observed excess nitrogen in deep water at this location. If heterotrophic diazotrophs are generally present in oligotrophic ocean regions, their consideration would increase current estimates of global N2 fixation and may require explicit representation in large-scale models.

High contribution of Rhizaria (Radiolaria) to vertical export in the California Current Ecosystem revealed by DNA metabarcoding.

Passive sinking of particulate organic matter (POM) is the main mechanism through which the biological pump transports surface primary production to the ocean interior. However, the contribution and variability of different biological sources to vertical export is not fully understood. Here, we use DNA metabarcoding of the 18S rRNA gene and particle interceptor traps (PITs) to characterize the taxonomic composition of particles sinking out of the photic layer in the California Current Ecosystem (CCE), a productive system with high export potential. The PITs included formalin-fixed and 'live' traps to investigate eukaryotic communities involved in the export and remineralization of sinking particles. Sequences affiliated with Radiolaria dominated the eukaryotic assemblage in fixed traps (90%), with Dinophyta and Metazoa making minor contributions. The prominence of Radiolaria decreased drastically in live traps, possibly due to selective consumption by copepods, heterotrophic nanoflagellates, and phaeodarians that were heavily enriched in these traps. These patterns were consistent across the water masses surveyed extending from the coast to offshore, despite major differences in productivity and trophic structure of the epipelagic plankton community. Our findings identify Radiolaria as major actors in export fluxes in the CCE.

The Fate of Carbon and Nutrients Exported Out of the Southern Ocean

Southern Ocean (SO) nutrient export via mode and intermediate waters is known to affect global biological production. The accompanying effects on the CO2 flux outside the SO are less certain. We performed idealized model simulations to separate the transient effects of SO carbon pumps on nutrients, primary production, and CO2 flux outside the SO. The SO biological carbon pump leads to dissolved inorganic carbon and nutrient reduction at the surface and in the exported water masses, and to a dissolved inorganic carbon increase at depth through the sinking of organic matter. When primary production is suppressed in the SO, only 30% of SO export, 43% of SO net primary production, and 50% of biologically driven SO CO2 flux are compensated outside the SO on a 200‐year time scale. In contrast, when the abiotically driven CO2 flux is suppressed, 90% of CO2 outgassing in the SO is compensated by air‐sea CO2 exchange outside the SO. The longer sequestration time scale of the biological carbon pump can be explained by incomplete compensation of primary production. This is a result of almost complete compensation of diatom productivity and no compensation of nanophytoplankton productivity due to feedbacks in phytoplankton community composition. The longer sequestration time scale is further sustained by the sinking and remineralization of particles in the deep ocean that are not in contact with the atmosphere over the 200‐year time scale considered. As SO biologically driven CO2 flux is only partly compensated outside the SO, potential future changes in SO productivity may have an important impact on the global carbon cycle.

Shifting Diatom—Dinoflagellate Dominance During Spring Bloom in the Baltic Sea and its Potential Effects on Biogeochemical Cycling

The Baltic Sea is affected by a range of human induced environmental pressures such as eutrophication. Here we synthesize the ongoing shift from diatom dominance towards more dinoflagellates in parts of the Baltic Sea during the spring bloom and its potential effects on biogeochemical cycling of key elements (e.g. C, N and P). The spring bloom is the period with the highest annual primary production and sinking of organic matter to the sediment. The fate of this organic matter is a key driver for material fluxes, affecting ecosystem functioning and eutrophication feedback loops. The dominant diatoms and dinoflagellates appear to be functionally surrogates as both groups are able to effectively exhaust the wintertime accumulation of inorganic nutrients and produce bloom level biomass that contribute to vertical export of organic matter. However, the groups have very different sedimentation patterns, and the sea floor has variable potential to mineralize the settled biomass in the different sub-basins. While diatoms sink quickly out of the euphotic zone, dinoflagellates sink as inert resting cysts, or lyse in the water column contributing to slowly settling phyto-detritus. The dominance by either phytoplankton group thus directly affects both the summertime nutrient pools of the water column and the input of organic matter to the sediment but to contrasting directions. The proliferation of dinoflagellates with high encystment efficiency could increase sediment retention and burial of organic matter, alleviating the eutrophication problem and improve the environmental status of the Baltic Sea.

Autonomous profiling float observations reveal the dynamics of deep biomass distributions in the denitrifying oxygen minimum zone of the Arabian Sea

Data from 13 autonomous profiling BGC-Argo floats, equipped with biogeochemical and bio-optical sensors deployed between 2011 and 2016, were used to explore the potential of bio-optical methods to map deep biomass distribution in the Arabian Sea oxygen minimum zone (OMZ). Dissolved oxygen sensors revealed concentrations below 5 μmol kg −1 for much of the depth range between 200 and 400 m and below 1 μmol kg −1 in the centre of the OMZ, which is well below climatological values. Optical particle backscatter sensors revealed intensities within the upper OMZ that were nearly as high as within the euphotic zone. The distribution of these particulate scatterers was confined to oxygen concentrations below 1 μmol kg −1 , with vertical distributions that exhibited maxima just below the top of the OMZ and decreased downward with similar attenuation to that expected for sinking organic matter. These distributions were very similar to those of nitrate deficits (ΔNO 3 ) estimated from a float with an ultra-violet nitrate sensor (R 2 = 0.54, p < 0.01), suggesting backscatter may be a useful proxy to investigate the changing dynamics of denitrification. Weak correlations between the upper OMZ particulate backscatter signals and both backscatter and chlorophyll fluorescence in the overlying euphotic zone is consistent with previous suggestions that advection of organic matter from remote sources is likely to be an important control on denitrification in the Arabian Sea. These results offer a useful path for examining denitrification dynamics in OMZs, which can be augmented by additional sensors for pH to quantify carbon respiration and dissolved organic matter to examine its possible importance relative to particulate oxygen demand. They clearly demonstrate the powerful insights that can be gained from arrays of autonomous profiling floats equipped with biogeochemical sensors for understanding globally significant, large-scale biogeochemical patterns and processes. • BGC-Argo data were used to map deep biomass distribution in the Arabian Sea OMZ. • Oxygen concentration in the centre of the OMZ, which is below climatological values • Increased scattering layer was confined to oxygen concentrations below 1 μmol kg −1 . • Optical backscattering may be a useful proxy to investigate denitrification.

First report on vertical distribution of dissolved polyunsaturated aldehydes in marine coastal waters

Abstract Polyunsaturated aldehydes (PUA) are bioactive molecules released by phytoplankton cells which can modify phytoplankton trophic interactions with grazers, bacteria or coexisting phytoplankters. Their ecological significance is still a matter of debate. There have been reports of PUA producers having been investigated at the ocean surface in both coastal and open areas, but there is little information regarding natural ranges and distribution of dissolved PUA ( dPUA) once they are released. In this report, we provide novel information regarding vertical profiles of dPUA for a coastal area under two different hydrodynamical conditions (mixing and stratified waters). Results show significant concentrations of d PUA ranging from 0.061 nM to 1.87 nM during both periods. This data demonstrate a significant consistency of these compounds in coastal areas and may prove an indirect evidence of high turnover rates in seawater . The vertical distribution of d PUA showed high variability in stratification , and for the first time we describe “patches” of relatively high dPUA concentrations in the upper layer. We hypothesize that these patches of dPUA, correlated to the flow of phytoplankton-derived sinking organic matter, could be significant for increasing the turnover rates of nutrients by triggering bacterial metabolism.

Ocean Acidification-Induced Restructuring of the Plankton Food Web Can Influence the Degradation of Sinking Particles

Ocean acidification (OA) is expected to alter plankton community structure in the future ocean. This, in turn, could change the composition of sinking organic matter and the efficiency of the biological carbon pump. So far, most OA experiments involving entire plankton communities have been conducted in meso- to eutrophic environments. However, recent studies suggest that OA effects may be more pronounced during prolonged periods of nutrient limitation. In this study, we investigated how OA-induced changes in low-nutrient adapted plankton communities of the subtropical North Atlantic Ocean may affect particulate organic matter (POM) standing stocks, POM fluxes, and POM stoichiometry. More specifically, we compared the elemental composition of POM suspended in the water column to the corresponding sinking material collected in sediment traps. Three weeks into the experiment, we simulated a natural upwelling event by adding nutrient-rich deep-water to all mesocosms, which induced a diatom-dominated phytoplankton bloom. Our results show that POM was more efficiently retained in the water column in the highest CO2 treatment levels (>800 μatm pCO2) subsequent to this bloom. We further observed significantly lower C:N and C:P ratios in post-bloom sedimented POM in the highest CO2 treatments, suggesting that degradation processes were less pronounced. This trend is most likely explained by differences in micro- and mesozooplankton abundance during the bloom and post-bloom phase. Overall, this study shows that OA can indirectly alter POM fluxes and stoichiometry in subtropical environments through changes in plankton community structure.

Strategies of amino acid supply in mesozooplankton during cyanobacteria blooms: a stable nitrogen isotope approach

AbstractKnowledge of how zooplankton can utilize cyanobacteria to sustain their nitrogen (N) demand for essential compounds like amino acids (AAs) is a key to predicting responses of higher trophic levels in terms of production and food web structure to future enhanced water column stratification. We explored the natural abundances of bulk N and AA‐specific nitrogen isotopes (δ15N) in particulate organic matter and mesozooplankton size‐fraction samples from three vertically separated water bodies in the central Baltic Sea during two summertime cyanobacteria blooms. The combination of plankton community and isotope data together with environmental variables helped to identify a mechanism of diazotrophic AA supply (synthesized during N2 fixation) for mesozooplankton, that largely depended on the sea surface temperature which regulated the access to the diazotrophic N‐based food web in the surface water (SW). We found that in the warmer summer, thermophilic cladocerans (e.g., Bosmina spp.) benefited most from diazotrophic AAs in the SW (19.8°C), while only in the colder summer, temperate copepods (e.g., Temora longicornis) ascended from the subjacent winter water into the SW (16.2°C) and incorporated diazotrophic AAs. Trophic position estimates based on the phenylalanine and glutamic acid δ15N signatures revealed that the diazotrophic AA supply into mesozooplankton was mainly indirect via feeding on mixo‐ and heterotrophic diets. Significantly enriched δ15N signatures in phenylalanine in the deep mesozooplankton (mainly copepods of Pseudo‐ and Paracalanus spp.) from the bottom water (BW) that was a region of the suboxic zone point to a reliance on a local food web. Mesozooplankton in the BW was feeding on diets of heterotrophic origin and probably profited from the heterotrophic re‐synthesis of AAs originating from sinking organic matter, as well as from the indirect incorporation of de novo synthesized AAs that most likely originated from chemoautotrophic bacteria or archaea communities in the suboxic zone. Our findings suggest that indirect feeding on diazotrophs and chemoautotrophs will be principal ways of amino acid supply for zooplankton in future enhanced stratified aquatic systems. Only a relatively small increase in temperature may restrict temperate key species from diazotrophic N‐based food webs in the mixed layer.

Distinct Siderophores Contribute to Iron Cycling in the Mesopelagic at Station ALOHA

The distribution of dissolved iron (Fe), total organic Fe-binding ligands, and siderophores were measured between the surface and 400 m at Station ALOHA, a long term ecological study site in the North Pacific Subtropical Gyre. Dissolved Fe concentrations were low throughout the water column and strong organic Fe-binding ligands exceeded dissolved Fe at all depths; varying from 0.9 nmol L-1 in the surface to 1.6 nmol L-1 below 150 m. Although Fe does not appear to limit microbial production, we nevertheless found siderophores at nearly all depths, indicating some populations of microbes were responding to Fe stress. Ferrioxamine siderophores were most abundant in the upper water column, with concentrations between 0.1-2 pmol L-1, while a suite of amphibactins were found below 200 m with concentrations between 0.8-11 pmol L-1. The distinct vertical distribution of ferrioxamines and amphibactins may indicate disparate strategies for acquiring Fe from dust in the upper water column and recycled organic matter in the lower water column. Amphibactins were found to have conditional stability constants (log ) ranging from 12.0-12.5, while ferrioxamines had much stronger conditional stability constants ranging from 14.0-14.4, within the range of observed L1 ligands by voltammetry. We used our data to calculate equilibrium Fe speciation at Station ALOHA to compare the relative concentration of inorganic and siderophore complexed Fe. The results indicate that the concentration of Fe bound to siderophores was up to two orders of magnitude higher than inorganic Fe, suggesting that even if less bioavailable, siderophores were nevertheless a viable pathway for Fe acquisition by microbes at our study site. Finally, we observed rapid production of ferrioxamine E by particle-associated bacteria during incubation of freshly collected sinking organic matter. Fe-limitation may therefore be a factor in regulating carbon metabolism and nutrient regeneration in the mesopelagic.

Organic matter processing by microbial communities throughout the Atlantic water column as revealed by metaproteomics

The phylogenetic composition of the heterotrophic microbial community is depth stratified in the oceanic water column down to abyssopelagic layers. In the layers below the euphotic zone, it has been suggested that heterotrophic microbes rely largely on solubilized particulate organic matter as a carbon and energy source rather than on dissolved organic matter. To decipher whether changes in the phylogenetic composition with depth are reflected in changes in the bacterial and archaeal transporter proteins, we generated an extensive metaproteomic and metagenomic dataset of microbial communities collected from 100- to 5,000-m depth in the Atlantic Ocean. By identifying which compounds of the organic matter pool are absorbed, transported, and incorporated into microbial cells, intriguing insights into organic matter transformation in the deep ocean emerged. On average, solute transporters accounted for 23% of identified protein sequences in the lower euphotic and ∼39% in the bathypelagic layer, indicating the central role of heterotrophy in the dark ocean. In the bathypelagic layer, substrate affinities of expressed transporters suggest that, in addition to amino acids, peptides and carbohydrates, carboxylic acids and compatible solutes may be essential substrates for the microbial community. Key players with highest expression of solute transporters were Alphaproteobacteria, Gammaproteobacteria, and Deltaproteobacteria, accounting for 40%, 11%, and 10%, respectively, of relative protein abundances. The in situ expression of solute transporters indicates that the heterotrophic prokaryotic community is geared toward the utilization of similar organic compounds throughout the water column, with yet higher abundances of transporters targeting aromatic compounds in the bathypelagic realm.

Occurrence and seasonal dynamics of metalimnetic Deep Chlorophyll Maximum (DCM) in a stratified meromictic tropical lake and its implications for zooplankton community distribution

Deep Chlorophyll Maxima (DCM) is an important feature of stratified lakes and oceans but very little is known about the phenomenon in tropical systems. Proximate factors accounting for DCM presence include light, thermal stratification and nutrients but biotic interactions such as zooplankton grazing can actively support DCM formation, structure and maintenance. We examined DCM characteristics in tropical Lake Bosumtwi at biweekly intervals between April and July of 2005 and 2006 and also assessed zooplankton herbivore interactions with the DCM. The onset and development of the seasonal DCM peak was based on stable water column, steep thermocline, availability of a nutricline and nutrient limitation in the epilimnion. Euphotic depth doubled during DCM formation and increasing transparency tripled DCM Chl a in the metalimnion just below the oxycline. DCM, however, disaggregated with seasonally induced deeper mixing. The range of variability in surface chlorophyll did not differ significantly between seasonal and aseasonal DCM period. There was about one‐fourth difference between mean surface and metalimnetic chlorophyll concentrations but this shift in resource location did not trigger a descent to deeper waters by herbivores. Herbivore grazing could not be linked to surface variations in water clarity and deep‐water irradiance needed for DCM formation and maintenance. DCM presence therefore was not enhanced by surface water grazing activities of zooplankton. This suppressed biotic influence indicated that DCM dynamics is driven by lake clarity, thermal and nutrient gradients which constitute key abiotic drivers. Direct consumption of the DCM by herbivorous zooplankton was not evident and the energy therein is possibly lost to the anoxic deep waters of the lake through sedimentation. The sinking organic matter could be returned to the pelagic food web via the microbial loop represented by bacteria, ciliates and heterotrophic/mixotrophic flagellates inhabiting the lake's deep layers.

Calibrating amino acid δ13C and δ15N offsets between polyp and protein skeleton to develop proteinaceous deep-sea corals as paleoceanographic archives

Compound-specific stable isotopes of amino acids (CSI-AA) from proteinaceous deep-sea coral skeletons have the potential to improve paleoreconstructions of plankton community composition, and our understanding of the trophic dynamics and biogeochemical cycling of sinking organic matter in the Ocean. However, the assumption that the molecular isotopic values preserved in protein skeletal material reflect those of the living coral polyps has never been directly investigated in proteinaceous deep-sea corals. We examined CSI-AA from three genera of proteinaceous deep-sea corals from three oceanographically distinct regions of the North Pacific: Primnoa from the Gulf of Alaska, Isidella from the Central California Margin, and Kulamanamana from the North Pacific Subtropical Gyre. We found minimal offsets in the δ13C values of both essential and non-essential AAs, and in the δ15N values of source AAs, between paired samples of polyp tissue and protein skeleton. Using an essential AA δ13C fingerprinting approach, we show that estimates of the relative contribution of eukaryotic microalgae and prokaryotic cyanobacteria to the sinking organic matter supporting deep-sea corals are the same when calculated from polyp tissue or recently deposited skeletal tissue. The δ15N values of trophic AAs in skeletal tissue, on the other hand, were consistently 3–4‰ lower than polyp tissue for all three genera. We hypothesize that this offset reflects a partitioning of nitrogen flux through isotopic branch points in the synthesis of polyp (fast turnover tissue) and skeleton (slow, unidirectional incorporation). This offset indicates an underestimation, albeit correctable, of approximately half a trophic position from gorgonin protein-based deep-sea coral skeleton. Together, our observations open the door for applying many of the rapidly evolving CSI-AA based tools developed for metabolically active tissues in modern systems to archival coral tissues in a paleoceanographic context.

Buffering of Ocean Export Production by Flexible Elemental Stoichiometry of Particulate Organic Matter

AbstractOne of the most important factors that determine the ocean‐atmosphere carbon partitioning is the sinking of particulate organic matter (POM) from the surface ocean to the deep ocean. The amount of carbon (C) removed from the surface ocean by this POM export production depends critically on the elemental ratio in POM of C to nitrogen (N) and phosphorus (P), two essential elements that limit productivity. Recent observations indicate that P:N:C in marine POM varies both spatially and temporally due to chemical, physical, and ecological dynamics. In a new approach to predicting a flexible P:C ratio, we developed a power law model with a stoichiometry sensitivity factor, which is able to relate P:C of POM to ambient phosphate concentration. The new factor is robust, measurable, and biogeochemically meaningful. Using the new stoichiometry sensitivity factor, we present a first‐order estimate that P:C plasticity could buffer against a generally expected future reduction in global carbon export production by up to 5% under a future warming scenario compared to a fixed, Redfield P:C. Further, we demonstrate that our new stoichiometry model can be implemented successfully and easily in a global model to reproduce the large‐scale P:N:C variability in the ocean.

Preservation and Significance of Extracellular DNA in Ferruginous Sediments from Lake Towuti, Indonesia.

Extracellular DNA is ubiquitous in soil and sediment and constitutes a dominant fraction of environmental DNA in aquatic systems. In theory, extracellular DNA is composed of genomic elements persisting at different degrees of preservation produced by processes occurring on land, in the water column and sediment. Extracellular DNA can be taken up as a nutrient source, excreted or degraded by microorganisms, or adsorbed onto mineral matrices, thus potentially preserving information from past environments. To test whether extracellular DNA records lacustrine conditions, we sequentially extracted extracellular and intracellular DNA from anoxic sediments of ferruginous Lake Towuti, Indonesia. We applied 16S rRNA gene Illumina sequencing on both fractions to discriminate exogenous from endogenous sources of extracellular DNA in the sediment. Environmental sequences exclusively found as extracellular DNA in the sediment originated from multiple sources. For instance, Actinobacteria, Verrucomicrobia, and Acidobacteria derived from soils in the catchment. Limited primary productivity in the water column resulted in few sequences of Cyanobacteria in the oxic photic zone, whereas stratification of the water body mainly led to secondary production by aerobic and anaerobic heterotrophs. Chloroflexi and Planctomycetes, the main degraders of sinking organic matter and planktonic sequences at the water-sediment interface, were preferentially preserved during the initial phase of burial. To trace endogenous sources of extracellular DNA, we used relative abundances of taxa in the intracellular DNA to define which microbial populations grow, decline or persist at low density with sediment depth. Cell lysis became an important additional source of extracellular DNA, gradually covering previous genetic assemblages as other microbial genera became more abundant with depth. The use of extracellular DNA as nutrient by active microorganisms led to selective removal of sequences with lowest GC contents. We conclude that extracellular DNA preserved in shallow lacustrine sediments reflects the initial environmental context, but is gradually modified and thereby shifts from its stratigraphic context. Discrimination of exogenous and endogenous sources of extracellular DNA allows simultaneously addressing in-lake and post-depositional processes. In deeper sediments, the accumulation of resting stages and sequences from cell lysis would require stringent extraction and specific primers if ancient DNA is targeted.

Temperature and oxygen dependence of the remineralization of organic matter

AbstractAccurate representation of the remineralization of sinking organic matter is crucial for reliable projections of the marine carbon cycle. Both water temperature and oxygen concentration are thought to influence remineralization rates, but limited data constraints have caused disagreement concerning the degree of these influences. We analyze a compilation of particulate organic carbon (POC) flux measurements from 19 globally distributed sites. Our results indicate that the attenuation of the flux of particulate organic matter depends on temperature with a Q10 between 1.5 and 2.01, and on oxygen described by a half‐saturation constant between 4 and 12 μmol/L. We assess the impact of the temperature and oxygen dependence in the biogeochemistry model Carbon, Ocean Biogeochemistry, and Lower Trophics, coupled to Geophysical Fluid Dynamics Laboratory's Earth System Model ESM2M. The new remineralization parameterization results in shallower remineralization in the low latitudes but deeper remineralization in the high latitudes, redistributing POC flux toward the poles. It also decreases the volume of the oxygen minimum zones, partly addressing a long‐standing bias in global climate models. Extrapolating temperature‐dependent remineralization rates to the surface (i.e., beyond the depth range of POC flux data) resulted in rapid recycling and excessive surface nutrients. Surface nutrients could be ameliorated by reducing near‐surface rates in a manner consistent with bacterial colonization, suggesting the need for improved remineralization constraints within the euphotic zone. The temperature and oxygen dependence cause an additional 10% decrease in global POC flux at 500 m depth, but no significant change in global POC flux at 2000 m under the RCP8.5 future projection.

Indirect influence of eutrophication on air – water exchange fluxes, sinking fluxes, and occurrence of polycyclic aromatic hydrocarbons

How eutrophication affects biogeochemical processes of hydrophobic organic contaminants (HOCs) in aquatic environments is a pending challenge. Although the direct influence of eutrophication on biogeochemical processes of HOCs in waters has been well addressed, the indirect influence of eutrophication on biogeochemical processes of HOCs remains largely unknown. Here we take the large shallow eutrophic Lake Taihu in China and polycyclic aromatic hydrocarbons (PAHs) as examples to provide novel knowledge on the indirect influence of eutrophication on air – water exchange fluxes, sinking fluxes, and occurrence of HOCs. The air – water exchange fluxes of individual PAHs varied dramatically at different sites in all studied seasons. The sinking flux of ΣPAH16 was 14 855.3 ± 1579.9, 3548.9 ± 650.6, and 5588.4 ± 530.7 ng m−2 d−1 in spring, summer, and winter. The corresponding concentration of ΣPAH16 in surface sediments was 713.1 ± 78.6, 339.7 ± 36.6, and 293.0 ± 35.2 ng g−1 d.w. Our study for the first time suggested that recruitment of cyanobacteria from surface sediments to water column in spring reduced the concentrations of PAHs in surface sediments, but enhanced their concentrations in the bulk water column, and overwintering of cyanobacteria in winter enhanced the concentrations of PAHs in surface sediments. High pH induced indirectly by eutrophication decreased seasonal air – water exchange fluxes (enhanced net volatilization) of PAHs, reduced the aromaticity of surface sediments and the hydrophobicity of phytoplankton cell surface, and reduced the accumulation of PAHs in surface sediments and phytoplankton consequently. Sinking fluxes and daily loss of PAHs from the water column decreased with phytoplankton biomass because the fraction of organic matter sinking from the water column decreased with phytoplankton biomass. Our study provides novel complementary knowledge for the biological pump for HOCs, and has important implications for understanding the coupling between eutrophication and biogeochemical processes of HOCs in subtropical shallow eutrophic waters.

The impact of primary and export production on the formation of the secondary nitrite maximum: A model study

The so-called secondary nitrite maximum (SNM) is a pronounced subsurface feature in many oxygen deficient zones of the ocean. A nitrite layer of up to several hundred meters thickness indicates high microbial activity and nitrogen loss from the system. To study the effects of primary and export production on the SNM, we have developed a one-dimensional ecosystem model for oxygen deficient zones. Our model couples the marine nitrogen and oxygen cycles with physical water column processes, includes euphotic, aphotic, aerobic and anaerobic processes and thereby dynamically describes source and sink processes for nitrite. The model is able to reproduce a number of typical SNM features found in observations from several oxygen deficient zones. Sensitivity experiments indicate that the SNM is particularly sensitive to the amount of labile dead organic matter sinking out of the euphotic zone but rather insensitive to the strength of regenerated primary production. The absence or presence of SNMs in oxygen deficient zones may be explained by the (non-)availability of labile detritus. Our model is applicable to different ocean regions and can be used to quantify sinks of bioavailable nitrogen due to potential changes in oxygen levels and/or detritus composition.