Quantification, extractability and stability of dissolved domoic acid within marine dissolved organic matter

The widespread diatom Pseudo-nitzschia can produce domoic acid (DA). DA is a compound with well described neurotoxic effects on vertebrates including humans known as amnesic shellfish poisoning (ASP) syndrome. It has also been suggested to serve as an organic ligand that binds to iron and copper. By binding these trace elements, DA may increase their solubility and bioavailability. In order to serve this function, DA has to be excreted and reabsorbed by the cells. Only few records of dissolved domoic acid (dDA) concentrations in the ocean exist. To accomplish quantification by ultra performance liquid chromatography (UPLC), samples have to be pre-concentrated and desalted using solid-phase extraction, a procedure commonly applied for dissolved organic matter. Our major goals were to quantify dDA in a basin-wide assessment in the East Atlantic Ocean, to determine extraction efficiencies for complexed and uncomplexed dDA, and to assess whether domoic acid is represented by its molecular formula in direct-infusion high resolution mass spectrometry. Our results showed that dDA was extracted almost quantitatively and occurred ubiquitously in the ocean surface but also in deeper (and older) water, indicating surprisingly high stability in seawater. The maximum concentration measured was 173 pmol L−1 and the average molar dDA carbon yield was 7.7 ppm. Both carbon yield and dDA concentration decreased with increasing water depth. Providing quantification of dDA in the water column, we seek to improve our understanding of toxic bloom dynamics and the mechanistic understanding of DA production

Glacial meltwater effects on the carbon cycle of Scoresby Sund (Greenland), the world's largest fjord system

Climate change induced mass loss of the Greenland Ice Sheet increases the amount of meltwater, which is mainly released into the numerous fjords along the coast of Greenland. Due to its low salinity and high silt load, meltwater can profoundly affect the biogeochemical cycling of carbon. We visited the world’s largest fjord system, Scoresby Sund at the eastern coast of Greenland, and its northernmost branch, Nordvestfjord, in the summer of 2016 for investigating biogeochemical carbon cycling. The data reveal that meltwater limited the productivity by inhibiting the resupply of nutrients to the surface and by shadowing the upper part of the water column by the introduction of silts. These silts, though, increased the export of organic carbon to depth by ballasting the sinking organic particles. While the region close to the fjord entrance was influenced by shelf waters, the water column within Nordvestfjord was significantly modulated by meltwater input from a number of marine- and land-terminating glaciers. Our results show that there was a clear gradient from a productive system with efficient remineralization at the mouth of the fjord to a less productive system with a high carbon export towards the inner fjord parts. These results imply that Scoresby Sund can be seen as a hotspot of carbon burial

Influence of Glacial Meltwater on Summer Biogeochemical Cycles in Scoresby Sund, East Greenland

Greenland fjords receive considerable amounts of glacial meltwater discharge from the Greenland Ice Sheet due to present climate warming. This impacts the hydrography, via freshening of the fjord waters, and biological processes due to altered nutrient input and the addition of silts. We present the first comprehensive analysis of the summer carbon cycle in the world's largest fjord system situated in southeastern Greenland. During a cruise onboard RV Maria S. Merian in summer 2016, we visited Scoresby Sund and its northernmost branch, Nordvestfjord. In addition to direct measurements of hydrography, biogeochemical parameters and sediment trap fluxes, we derived net community production (NCP) and full water column particulate organic carbon (POC) fluxes, and estimated carbon remineralization from vertical flux attenuation. While the narrow Nordvestfjord is influenced by subglacial and surface meltwater discharge, these meltwater effects on the outer fjord part of Scoresby Sund are weakened due to its enormous width. We found that subglacial and surface meltwater discharge to Nordvestfjord significantly limited NCP to 32–36 mmol C m−2 d−1 compared to the outer fjord part of Scoresby Sund (58–82 mmol C m−2 d−1) by inhibiting the resupply of nutrients to the surface and by shadowing of silts contained in the meltwater. The POC flux close to the glacier fronts was elevated due to silt-ballasting of settling particles that increases the sinking velocity and thereby reduces the time for remineralization processes within the water column. By contrast, the outer fjord part of Scoresby Sund showed stronger attenuation of particles due to horizontal advection and, hence, more intense remineralization within the water column. Our results imply that glacially influenced parts of Greenland's fjords can be considered as hotspots of carbon export to depth. In a warming climate, this export is likely to be enhanced during glacial melting. Additionally, entrainment of increasingly warmer Atlantic Water might support a higher productivity in fjord systems. It therefore seems that future ice-free fjord systems with high input of glacial meltwater may become increasingly important for Arctic carbon sequestration

Biogeochemie gelöster Domoinsäure im Ozean: Quellen, Verteilung und Funktion

Domoic acid (DA) is primarily produced by the ubiquitous marine diatom genus Pseudo-nitzschia. DA is primarily known for its neurotoxic effect on higher trophic level organisms and particulate DA (pDA) has thus been well studied. Dissolved DA (dDA) can, however, substantially contribute to the total DA of toxic blooms since it can be released into the water in high amounts. So far, dDA has not been as extensively studied, leaving knowledge gaps about its large-scale distribution, its carbon contribution to marine dissolved organic matter (DOM) and its ecological function. The motivation of this thesis was to fill in these gaps by elucidating the ecological and biogeochemical role of dDA in the Atlantic Ocean with a spatial focus on the Arctic and Antarctic sectors. I aimed at (i) developing a sensitive quantification method for dDA in seawater, (ii) investigating its broad-scale distribution and contribution to DOM, and (iii) improve our understanding about the ecological role of dDA. The highly sensitive quantification method allowed the determination of dDA concentrations in the East Atlantic Ocean, where it was ubiquitous and decreased with water depth. Together with the radiocarbon dates of dissolved organic carbon this points to a fairly high persistence of dDA in the ocean. In a standard DOM solid-phase extraction method DA showed high recovery rates, allowing a chemical identification within DOM. In the Southern Ocean, a high-nutrient, low-chlorophyll (HNLC) area due to low iron bioavailability, dDA also occurs. I thus tested its suggested ligand function for the Antarctic species P. subcurvata. Although dDA availability did not increase iron uptake, intracellular copper levels of iron-depleted treatments increased, potentially due to dDA. I also studied dDA’s occurrence in three iron-replete Arctic fjords, differing in their glaciation state. Pseudo-nitzschia counts and pDA correlated with dDA, which was influenced by macronutrient availability. This thesis contains the first verification of the in situ presence of DA biosynthesis genes, which points to an active DA production. Overall, this thesis provides new insights into the distribution of dDA in the East Atlantic Ocean, its contribution to DOM, its connection to environmental parameters, and its potential ligand function

Quantification and molecular characterisation of dissolved organic sulphur at the benthopelagic interface in the Weddell Sea

The Weddell Sea is an important region for Antarctic bottom water formation and thereby affecting global ocean currents and biogeochemical cycles. Few previous studies revealed that elevated dissolved organic carbon (DOC) concentrations could be found in the bottom water of the Weddell Sea. Such elevated concentrations could either originate from downwelling of water masses or a sedimentary DOC efflux. Furthermore, little is known about the molecular identity of dissolved organic sulphur (DOS) in the ocean. This study aims at a DOC and DOS source identification in the Southern Weddell Sea using bulk and molecular analytical approaches. In addition, the reproducibility of solid phase extraction (SPE) for dissolved organic matter concentration and desalting using different sample volumes were assessed. The water column was sampled at 40 stations in the outflow region of the Filchner Depression in the Weddell Sea. At 14 stations sediment pore water was sampled in addition. SPE was performed using different sample volumes for water column and sediment pore water. Quantification of DOC and SPE-extractable DOC (SPE-DOC) was performed via high-temperature catalytic oxidation. Quantification of SPE-DOS was performed via inductively coupled plasma mass spectrometry, molecular characterisation via Fourier transform ion cyclotron resonance mass spectrometry. Elevated DOC and partly also elevated SPE-DOS concentrations (average water column DOC: 52 ± 11 µmol L-1, SPE-DOS: 0.26 ± 0.22 µmol L-1) were found in surface water (DOC up to 69 ± 13 µmol L-1, SPE-DOS up to 1.5 ± 0.5 µmol L-1) and also the lower pelagic zone within water layers of high potential density (DOC up to 61 ± 13 µmol L-1, SPE-DOS up to 1.2 ± 0.4 µmol L-1). In sediment pore water both DOC and SPE-DOS concentrations were higher (DOC: 205 ± 202 µmol L-1, SPE-DOS: 4.2 ± 6.2 µmol L-1) and showed more molecular differences than water column samples. Pore water and water column samples were clearly distinguishable on a molecular level. The water layer above the sediment showed a high molecular similarity to the pore waters but lower concentrations of DOC and SPE-DOS than the pore water. The increase of the extraction volume led to a decrease in SPE-DOC extraction efficiency and a relative decrease of SPE-DOS. Elevated DOC concentrations in the lower pelagic zone were attributed to downwelling of water masses rather than sedimentary efflux. Still, the sediment most likely contributes DOC and DOS to the lower water column, which could be observed on a molecular level. Differences in the extraction volume of pore water and water column samples led to significant changes in SPE-DOC/SPE-DOS ratios (larger volume: 76 ± 39, smaller volume: 24 ± 12). The extraction volume was furthermore likely to affect the molecular composition of the samples

Dissolved domoic acid in the East Atlantic

Domoic acid, a neurotoxin to vertebrates predominantly produced by the diatom Pseudo-nitzschia has been suggested to serve as an organic ligand. By binding iron and copper, it could increase their solubility and bioavailability. Domoic acid has to be released by the cells to serve this function and thus occur dissolved in sea water. Samples were pre-concentrated and desalted using solid-phase extraction, a procedure commonly applied for dissolved organic matter. Dissolved domoic acid was quantified in the East Atlantic, where it occurred ubiquitously, especially in the ocean surface. The maximum domoic acid concentration measured was 173 pmol L-1 and the average domoic acid carbon yield was 7.7 ppm. Both, carbon yield and dissolved domoic acid concentration, decreased with increasing water depth. Samples were taken during the cruise PS73 (ANT-XXV) on RV Polarstern. The extraction efficiency of domoic acid was 91%. The detection limit for solid-phase extractable domoic acid (DA-SPE) was 10 pmol L-1 and limit of quantification was 26 pmol L-1. Domoic acid concentrations below the limit of detection are marked as <LOD and concentrations below limit of quantification are marked as <LOQ in the data set