Surface Microlayer Research Articles

Higher Apparent Gas Transfer Velocities for CO2 Compared to CH4 in Small Lakes.

Large greenhouse gas emissions occur via the release of carbon dioxide (CO2) and methane (CH4) from the surface layer of lakes. Such emissions are modeled from the air-water gas concentration gradient and the gas transfer velocity (k). The links between k and the physical properties of the gas and water have led to the development of methods to convert k between gases through Schmidt number normalization. However, recent observations have found that such normalization of apparent k estimates from field measurements can yield different results for CH4 and CO2. We estimated k for CO2 and CH4 from measurements of concentration gradients and fluxes in four contrasting lakes and found consistently higher (on an average 1.7 times) normalized apparent k values for CO2 than CH4. From these results, we infer that several gas-specific factors, including chemical and biological processes within the water surface microlayer, can influence apparent k estimates. We highlight the importance of accurately measuring relevant air-water gas concentration gradients and considering gas-specific processes when estimating k.

Sea spray as a secondary source of chlorinated persistent organic pollutants? - Conclusions from a comparison of seven fresh snowfall events in 2019 and 2021

Secondary sources of persistent organic pollutants (POPs) gain in importance worldwide as primary sources decline. In this work, we aim to determine whether sea spray may be a secondary source of chlorinated POPs to the terrestrial Arctic, since a similar mechanism was proposed there only for the more water-soluble POPs. To this end, we have determined polychlorinated biphenyls and organochlorine pesticides concentrations in fresh snow and seawater collected in the vicinity of the Polish Polar Station in Hornsund in two sampling periods covering spring 2019 and 2021. To support our interpretations, we include also metal and metalloid, and stable hydrogen and oxygen isotopes analysis in those samples. A significant correlation was found between the concentrations of POPs and the distance from the sea at the sampling point, yet the confirmation of sea spray impact lies more in capturing an event with negligible long-range transport influence where the detected chlorinated POPs (Cl-POPs) matched in composition the compounds enriched in the sea surface microlayer, which is both a source of sea spray and a seawater microenvironment rich in hydrophobic substances.

Resupply, diffusion, and bioavailability of Hg in paddy soil-water environment with flood-drain-reflood and straw amendment

Mercury (Hg) poses a significant risk in paddy fields, particularly when it is converted to methylmercury (MeHg) and accumulates in rice. However, the bioavailability and resupply kinetics of Hg in the paddy soil-water environment are not well understood. In this study, the diffusive gradients in thin films (DGT) and the ‘DGT-induced fluxes in sediments’ model (DIFS) were first adopted to investigate the Hg resupply kinetics, diffusion fluxes and bioavailability in a paddy environment subjected to flood-drain-reflood treatment and straw amendment. Our results show that although the straw amendment limited the bioavailability of Hg (38.2%–47.9% lower than control) in porewater by decreasing its resupply capacity, especially with smaller straw particles, the net production of MeHg in paddy fields was significantly increased after straw amendment (73.5%–77.9% higher than control). The results of microbial sequencing indicate that enhanced methylators (e.g., family Geobacter) and non-Hg methylators (e.g., Methanosarcinaceae) played a crucial role in MeHg production following straw amendment. Moreover, Hg-containing paddy soils generally tend to release Hg into the overlying water, while drain-reflood treatment changes the direction of Hg diffusion fluxes in the paddy soil-water interface. The drainage-reflooded treatment decreases the Hg reactive and resupply capacity of the paddy soil, thereby hindering the release of Hg from soil into overlying water during the early stages of reflooding. Overall, this study provides novel insights into the behavior of Hg in paddy soil-water surface microlayers.

Hydrocarbons and suspended matter in the atmosphere-water boundary layer in the Barents and Kara Seas

The concentrations and composition of hydrocarbons - HCs (aliphatic - AHCs and polycyclic aromatic hydrocarbons - PAHs) in the Barents and Kara Seas (cruises 80, 83 and 89 of the R/V «Akademik Mstislav Keldysh», August 2020, June 2021, September 2022 respectively) are analyzed in the paper. The concentrations of the HC dissolved and suspended forms were measured in the surface microlayer (SML, 100–1000 μm thick), melting ice and surface waters. Higher concentration of HCs in SML was found in suspended particulate matter (SPM). AHC concentration in SPM in the Barents Sea were lower (31–96, on average 68 μg/L) than in the Kara Sea in June 2021 (187–1051, 573 μg/L on average), where the studies were carried out in the early summer season. The concentrations of AHCs in the SPM in the SML of the Kara Sea were 3.6 times higher than in the dissolved form (127–217, on average 187 μg/L), and they were almost 15 times higher than those in the surface waters. The accumulation of organic compounds also occurs in ice, but to a lesser extent than in the SML. The composition of alkanes in SML and melting ice mainly indicates the slight influence of autochthonous processes shaping HCs. The average content of PAHs in SPM was also 4.8 times higher than that in the dissolved form. According to the markers, the influence of combustion products of marine fuel was traced in the composition of PAHs. In September 2022, AHC and PAH concentrations in the Kara Sea were lower than in June 2021: on average, 159 μg/L and 492 ng/L accordingly, that is due to a decrease in the content of SPM from an average of 1.89 to 1.21 mg/L.

Phylotype resolved spatial variation and association patterns of planktonic Thaumarchaeota in eastern Chinese marginal seas

The majority of marine ammonia oxidizers belong to Thaumarchaeota, a phylum of Archaea, which is distributed throughout the water column. Marine surface waters contain distinct thaumarchaeotal phylotypes compared to the deeper ocean, but spatial dynamics of the surface-associated lineages are largely unsolved. This study of 120 seawater samples from the eastern Chinese marginal seas identified contrasting distribution and association patterns among thaumarchaeotal phylotypes across different dimensions. Horizontally, Nitrosopumilus-like and Nitrosopelagicus-like phylotypes dominated the surface water (3 m) of the Yellow Sea (YS) and East China Sea (ECS), respectively, along with increased abundance of total free-living Thaumarchaeota in ECS. Similar compositional changes were observed in the surface microlayer. The spatial heterogeneity of particle-attached Thaumarchaeota was less clear in surface microlayers than in surface waters. Vertically, the Nitrosopelagicus-like phylotype increased in abundance from surface to 90 m in ECS, which led to an increase in the proportion of Thaumarchaeota relative to total prokaryotes. This occurred mainly in the free-living fraction. These results indicate a clear size-fractionated niche partitioning, which is more pronounced at lower depths than in the surface water/surface microlayer. In addition, associations of Thaumarchaeota with other microbial taxa varied between phylotypes and size fractions. Our results show that a phylotype-resolved and size-fractionated spatial heterogeneity of the thaumarchaeotal community is present in surface oceanic waters and a vertical variation of the Nitrosopelagicus-like phylotype is present in shallow shelf waters.

New Insights into Cation- and Temperature-Driven Protein Adsorption to the Air–Water Interface through Infrared Reflection Studies of Bovine Serum Albumin

The chemistry and structure of the air-ocean interface modulate biogeochemical processes between the ocean and atmosphere and therefore impact sea spray aerosol properties, cloud and ice nucleation, and climate. Protein macromolecules are enriched in the sea surface microlayer and have complex adsorption properties due to the unique molecular balance of hydrophobicity and hydrophilicity. Additionally, interfacial adsorption properties of proteins are of interest as important inputs for ocean climate modeling. Bovine serum albumin is used here as a model protein to investigate the dynamic surface behavior of proteins under several variable conditions including solution ionic strength, temperature, and the presence of a stearic acid (C17COOH) monolayer at the air-water interface. Key vibrational modes of bovine serum albumin are examined via infrared reflectance-absorbance spectroscopy, a specular reflection method that ratios out the solution phase and highlights the aqueous surface to determine, at a molecular level, the surface structural changes and factors affecting adsorption to the solution surface. Amide band reflection absorption intensities reveal the extent of protein adsorption under each set of conditions. Studies reveal the nuanced behavior of protein adsorption impacted by ocean-relevant sodium concentrations. Moreover, protein adsorption is most strongly affected by the synergistic effects of divalent cations and increased temperature.

Microbial communities in the water surface microlayer and associations with microbes in aerosols, beach sand, and bulk water.

The water surface microlayer (SML) serves as a boundary through which microbes can be exchanged. To evaluate exchanges of microbes, this study compared microbial communities within different reservoirs, with an emphasis on the water SML and aerosols. Additionally, the microbial communities during a sewage spill and perigean tides were evaluated and the results were compared to times without these events. Results show that during perigean tides and during the sewage spill, levels of culturable bacteria were highest and showed an increase via sequencing in potential pathogenic bacteria (Corynebacterium and Vibrio, which increased from 3.5%-1800% depending on sample type). In the aerosol samples, Corynebacterium (average of 2.0%), Vibrio (1.6%), and Staphylococcus (10%), were the most abundant genera. Aerosolization factors, which were used to examine the transfer of the microbes, were high for these three genera. Measurements of general marine bacteria (GMB) by culture showed a weak but significant correlation between culturable GMB in aerosol samples versus in water and in the SML. More research is needed to evaluate the exchange of pathogens between the SML and air, given the increase in potentially pathogenic microbes within the SML during rare events and evidence that suggests that microbes maintain viability during transfers across reservoirs.

Plankton Genes and Extracellular Organic Substances in the Ocean

Dissolved organic matter (DOM) in the ocean represents about 662 billion tons of C, 200 times more than the living biomass. It is produced mainly by microbial primary production. The largest fraction of this DOM is old (>weeks to months) and both chemically and biologically recalcitrant. The remainder is young (seconds to weeks), more labile and surface active. Part of the latter fraction changes the rheological properties in the bulk phase of the water and at interfaces including the sea surface microlayer (SML). In order of abundance, this DOM consists of sugars, amino acids, fatty acids and nucleic acids, often incorporated into complex polymers. The DOM molecules are produced by microbial genes, and are further modified by enzymes themselves produced by genes. The properties of ocean water and its interfaces as well as biogeochemical fluxes may thus be modified by ocean plankton genes. These fluxes influence ocean and atmospheric climate, which in return acts on the biota. Viral infection may furthermore modify prokaryotic and eukaryotic genes and their expression. Therefore, the ocean plankton genomes and the fluxes and climates they influence may be subject to Darwinian-type selection. Research programs need to integrate ocean ecology, rheology, biogeochemistry and genomics, to find the associations among them.

Fe(III) Biomineralization in the Surface Microlayer of Acid Mine Waters Catalyzed by Neustonic Fe(II)-Oxidizing Microorganisms

The formation of thin mineral films or encrustations floating on the water surface of low-flow or stagnant zones of acid mine drainage (AMD)-affected streams is probably among the most exotic features that can be found in mining areas. However, most fundamental questions about their origin (biotic vs. abiotic), structure, mineralogy, physical stability and metal-retention capacity remain unanswered. This study aims to reveal the factors promoting their formation and to clarify their composition in detail. With this purpose, the major mineral phases were studied with XRD in surface film samples found in different mine sites of the Iberian Pyrite Belt mining district (SW Spain), and the major oxide and trace metal concentrations were measured with XRF and/or ICP-MS. Fe(III) minerals dominated these formations, with mineralogy controlled by the pH (jarosite at pH~2.0, schwertmannite at pH 2.5–3.5, ferrihydrite at pH > 6.0). Other minerals have also been identified in minor proportions, such as brushite or khademite. These mineral formations show an astounding capacity to concentrate, by orders of magnitude (×102 to ×105), many different trace metals present in the underlying aqueous solutions, either as anionic complexes (e.g., U, Th, As, Cr, V, Sb, P) or as divalent metal cations (e.g., Cu, Zn, Cd, Pb). These floating mineral films are usually formed in Fe(II)-rich acidic waters, so their formation necessarily implies the oxidation of Fe(II) to Fe(III) phases. The potential involvement of Fe(II)-oxidizing microorganisms was investigated through 16S rRNA gene amplicon sequencing of water underneath the Fe(III)-rich floating mineral films. The sequenced reads were dominated by Ferrovum (51.7 ± 0.3%), Acidithiobacillus (18.5 ± 0.9%) and Leptospirillum (3.3 ± 0.1%), three well-known Fe(II)-oxidizing genera. These microorganisms are major contributors to the formation of the ferric mineral films, although other genera most likely also play a role in aspects such as Fe(III) sequestration, nucleation or mineral growth. The floating mineral films found in stagnant acidic mine waters represent hotspots of biosphere/hydrosphere/atmosphere interactions of great value for the study of iron biogeochemistry in redox boundaries.

Highly active bacterial DMSP metabolism in the surface microlayer of the eastern China marginal seas.

The microbial cycling of dimethylsulfoniopropionate (DMSP) and the resulting gaseous catabolites dimethylsulfide (DMS) or methylmercaptan (MeSH) play key roles in the global sulfur cycle and potentially climate regulation. As the ocean-atmosphere boundary, the sea surface microlayer (SML) is important for the generation and emission of DMS and MeSH. However, understanding of the microbial DMSP metabolism remains limited in the SML. Here, we studied the spatiotemporal differences for DMS/DMSP, bacterial community structure and the key bacterial DMSP metabolic genes between SML and subsurface seawater (SSW) samples in the eastern China marginal seas (the East China Sea and Yellow Sea). In general, DMSPd and DMSPt concentrations, and the abundance of total, free-living and particle-associated bacteria were higher in SML than that in SSW. DMSP synthesis (~7.81-fold for dsyB, ~2.93-fold for mmtN) and degradation genes (~5.38-fold for dmdA, ~6.27-fold for dddP) detected in SML were more abundant compared with SSW samples. Free-living bacteria were the main DMSP producers and consumers in eastern Chinese marginal sea. Regionally, the bacterial community structure was distinct between the East China Sea and the Yellow Sea. The abundance of DMSP metabolic genes (dsyB, dmdA, and dddP) and genera in the East China Sea were higher than those of the Yellow Sea. Seasonally, DMSP/DMS level and DMSP metabolic genes and bacteria were more abundant in SML of the East China Sea in summer than in spring. Different from those in spring, Ruegeria was the dominant DMSP metabolic bacteria. In conclusion, the DMSP synthesis and degradation showed significant spatiotemporal differences in the SML of the eastern China marginal seas, and were consistently more active in the SML than in the SSW.

Per- and Polyfluoroalkyl Substances Enrichment in the Surface Microlayer of a Freshwater System Impacted by Aqueous Film-Forming Foams

Accumulation of per- and polyfluoroalkyl substances (PFAS) in the surface microlayer (SML) of open water bodies has received little attention. Various SML sampling methods exist, yet none have been systematically evaluated for PFAS. Three glass-based techniques were deployed for SML sampling at 10 sites on a freshwater system impacted by aqueous film-forming foam (AFFF). Samples of SML and underlying bulk water (3 and 30 cm) were analyzed for 50 target PFAS by liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QToF). Single dipped microscope slides yielded field measurements of SML thicknesses and enrichment factors for eight PFAS and used to compute field-measured air–water adsorption coefficients. Large glass plates collected a greater volume of SML, gave lower variability, and lower limits of detection and quantification. Up to 19 PFAS (concentrations up to 13,000 ng/L) were associated with the SML, while only 15 PFAS were in underlying bulk water. Enrichment factors (EFs) ranged from 1 to 20 and increased with chromatographic retention time, a proxy for relative hydrophobicity. Field-measured air–water adsorption coefficients were consistent with monolayer adsorption as described by the Gibbs adsorption equation, which indicates that only adsorption to the air–water interface is needed to explain the observed PFAS enrichment in the SML.

Roles of phytoplankton, microzooplankton, and bacteria in DMSP and DMS transformation processes in the East China Continental Sea

The trace gas dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) are the most abundant oceanic biogenic organosulfur molecules. Phytoplankton, zooplankton, and bacteria are the most vital biotic factors controlling DMSP and DMS, but their relative effects on the transformation processes still remain uncertain due to their complex biogeochemistry. Here, we provided the relative transformation rates of DMSP and DMS, and their relationships with phytoplankton, microzooplankton, and bacteria in the East China Continental Sea in the summer of 2018. The phytoplankton was the major DMSP producer, contributing 89.8% of DMSP stock with the particulate DMSP production rate at 42.0 nmol L−1 d−1. The DMSP lyase activity was 140.5 nmol DMS L−1h−1, resulting in a short DMSP fate (0.8 d). The microzooplankton grazing resulted in DMSP loss and DMS production at the rates of −49.3 and 8.2 nmol L−1 d−1, equaling 60.0% of total DMSP loss and 71.6% of DMS microbial production. The dissolved DMSP degradation and DMS microbial production rates were at −23.4 and 11.5 nmol L−1 d−1, largely controlled by bacterial abundance. Microbial consumption, photolysis, and ventilation accounted for 41%, 43%, and 16% of surface DMS removal, indicating that most of the DMS was consumed within the surface. Finally, DMS was enriched in the sediment but not significantly enriched in the surface microlayer, with enrichment factors at 3.7 and 1.0, respectively. The budget model in this study could help better estimate the roles of phytoplankton, microzooplankton, and bacteria in DMSP and DMS transformation processes in the continental shelf seas.

Enrichment of saccharides at the air–water interface: a quantitative comparison of sea surface microlayer and foam

Environmental context Saccharides contribute substantially to dissolved organic carbon in the ocean and are enriched at the ocean surface. In this study, we demonstrate that saccharides are more enriched in persistent whitecap foam compared to the sea surface. The maturation of bubbles at the air–water interface is thus expected to enhance the enrichment of organic matter at the ocean surface and ultimately in the sea spray aerosol that forms when bubbles burst at the ocean surface. Rationale Organic matter accumulates at the ocean surface. Herein, we provide the first quantitative assessment of the enrichment of dissolved saccharides in persistent whitecap foam and compare this enrichment to the sea surface microlayer (SSML) during a 9 day mesocosm experiment involving a phytoplankton bloom generated in a Marine Aerosol Reference Tank (MART). Methodology Free monosaccharides were quantified directly, total saccharides were determined following mild acid hydrolysis and the oligo/polysaccharide component was determined as the difference between total and free monosaccharides. Results Total saccharides contributed a significant fraction of dissolved organic carbon (DOC), accounting for 13% of DOC in seawater, 27% in SSML and 31% in foam. Median enrichment factors (EFs), calculated as the ratio of the concentrations of saccharides relative to sodium in SSML or foam to that of seawater, ranged from 1.7 to 6.4 in SSML and 2.1–12.1 in foam. Based on median EFs, xylitol, mannitol, glucose, galactose, mannose, xylose, fucose, rhamnose and ribose were more enriched in foam than SSML. Discussion The greatest EFs for saccharides coincided with high chlorophyll levels, indicating increasing ocean surface enrichment of saccharides during phytoplankton blooms. Higher enrichments of organic matter in sea foam over the SSML indicate that surface active organic compounds become increasingly enriched on persistent bubble film surfaces. These findings help to explain how marine organic matter becomes highly enriched in sea spray aerosol that is generated by bursting bubbles at the ocean surface.

Review on the physical chemistry of iodine transformations in the oceans

The transformation between iodate (IO3−), the thermodynamically stable form of iodine, and iodide (I-), the kinetically stable form of iodine, has received much attention because these species are often dependent on the oxygen concentration, which ranges from saturation to non-detectable in the ocean. As suboxic conditions in the ocean’s major oxygen minimum zones indicate that IO3− is minimal or non-detectable, the incorporation of IO3− into carbonate minerals has been used as a redox proxy to determine the O2 state of the ocean. Here, I look at the one and two electron transfers between iodine species with a variety of oxidants and reductants to show thermodynamics of these transformations. The IO3− to IO2− conversion is shown to be the controlling step in the reduction reaction sequence due to thermodynamic considerations. As IO3− reduction to IO2− is more favorable than NO3− reduction to NO2− at oceanic pH values, there is no need for nitrate reductase for IO3− reduction as other reductants (e.g. Fe2+, Mn2+) and dissimilatory IO3− reduction by microbes during organic matter decomposition can affect the transformation. Unfortunately, there is a dearth of information on the kinetics of reductants with IO3−; thus, the thermodynamic calculations suggest avenues for research. Conversely, there is significant information on the kinetics of I- oxidation with various oxygen species. In the environment, I- oxidation is the controlling step for oxidation. The oxidants that can lead to IO3− are reactive oxygen species with O3 and •OH being the most potent as well as sedimentary oxidized Mn, which occurs at lower pH than ocean waters. Recent work has shown that iodide oxidizing bacteria can also form IO3−. I- oxidation is more facile at the sea surface microlayer and in the atmosphere due to O3.

Monitoring the Sea Surface Microlayer (SML) on Sentinel images

Slicks on the sea surface are usually related to oil spills, algal blooms or organic runoff around coastlines. An extensive network of slicks extending across the English Channel is seen on Sentinel 1 and Sentinel 2 images and are identified as comprising a film of natural surfactant material within the sea surface microlayer (SML). As the SML represents the interface between ocean and atmosphere, controlling the vital exchange of gases and aerosols, identification of the slicks on images can add a new dimension to climate modelling. Current models use primary productivity often combined with wind speed, but quantifying the global extent of surface films spatially and temporally is difficult due to their patchy nature. The slicks are shown to be visible on Sentinel 2 optical images affected by sun glint, due to the wave dampening effect of the surfactants. On a Sentinel 1 SAR image of the same day, they can be identified using the VV polarised band. The paper investigates the nature and spectral properties of the slicks in relation to sun glint, and evaluates the performance of chlorophyll-a, floating algae and floating debris indices on the slick-affected areas. No index was able to distinguish slicks from non-slick areas as successfully as the original sun glint image. This image was used to devise a tentative Surfactant Index (SI) which indicates over 40 % of the study area covered by slicks. As ocean sensors have lower spatial resolution and are generally designed to avoid sun glint, Sentinel 1 SAR may offer a useful alternative for monitoring the global spatial extent of surface films, until dedicated sensors and algorithms can be developed.

Finishing the surface micro-layer of additively manufactured TiAl alloy using electro-thermal discharge assisted post-processing

Selective laser melting (SLM) of titanium–aluminium (TiAl) alloy components has gained significant attention in the modern industrial world. The flexibility of the SLM process in producing complex shapes with minimum utilization of material and energy makes it dominant over other manufacturing techniques. As aerospace and biomedical industries demand complex-shaped TiAl alloy components, part fabrication using SLM becomes the ultimate solution. However, the unacceptable level of surface integrity and anisotropic behavior of SLM components demand post processing operations such as laser polishing, chemical polishing, and conventional polishing methods. In this study, a recently developed polishing method called wire electrical discharge polishing (WEDP) is performed on TiAl alloys for obtaining a smooth and defect-free surface. This study aims to investigate the micro-layer modification occurring to the WEDP-processed surface in detail. The experimental results establish the effectiveness of WEDP method in terms of improved surface integrity. The surface finish (Sa) got enhanced by ~88% after WEDP processing. In addition, the thickness of recast layer formed by WEDP was found to be minimum. Moreover, post-processing of TiAl alloy resulted in better surface morphology specifically at lower settings of peak current. It is noteworthy that the migration of wire material was minimum with zinc-coated brass electrode compared to the normal brass electrode. Hence, coated wire electrodes are recommended for WEDP process. In short, an excellent surface integrity can be achieved using WEDP process through favorable surface modification aided by lower peak current and coated wire electrodes. Furthermore, less electrode wear observed in WEDP process enables the deployment of lower feed rates leading to minimal electrode consumption.

Usage of antibiotics in aquaculture and the impact on coastal waters

For decades, coastal marine ecosystems have been threatened by a wide range of anthropogenic pollutants. Recently, there has been increasing concern about the accumulation and impacts of antibiotic compounds on marine ecosystems. However, information regarding the accumulation of antibiotics and the impacts they may have on microbial communities in coastal water bodies and on human health is sparse in literature. Antibiotics from aquacultures are constantly discharged into marine environments via rivers. Large rivers transport tons of antibiotics every year into coastal waters, e.g., 12 tons of sulfonamide by the river Mekong. Here, we discuss a potential influence of such imported antibiotics on bacterial communities in coastal waters. Potential accumulation of antibiotics in the uppermost surface layer of aquatic ecosystems, the so-called sea surface microlayer (SML), is of interest. Because of the ability of the SML to accumulate anthropogenic pollutants, it may serve as a pool for antibiotics and correspondingly also for resistant organisms. Also, due to its biofilm-like structure, the SML could serve as a hotspot for horizontal gene transfer, speeding up the spread of antibiotic resistant strains to encompassing marine environments. The emergence of antibiotic resistant bacteria is a global threat and scientists projected that it could pave the way for the next pandemic that could ravage the world in the next decades. For this reason, it is time to focus research on understanding and minimizing the impact of antibiotics on the sustainability of coastal waters and on the health of humans who depend on coastal resources for food and recreational purposes. Also, knowledge about antibiotics in the SML is necessary to understand the effects they are likely to have on bacterial abundance, diversity, and metabolic activities in coastal water bodies.

Dimethyl sulfide cycling in the sea surface microlayer in the southwestern Pacific – Part 1: Enrichment potential determined using a novel sampler

Abstract. Elevated dimethyl sulfide (DMS) concentrations in the sea surface microlayer (SML) have been previously related to DMS air–sea flux anomalies in the southwestern Pacific. To further address this, DMS, its precursor dimethylsulfoniopropionate (DMSP), and ancillary variables were sampled in the SML and also subsurface water at 0.5 m depth (SSW) in different water masses east of New Zealand. Despite high phytoplankton biomass at some stations, the SML chlorophyll a enrichment factor (EF) was low (< 1.06), and DMSP was enriched at one station with DMSP EF ranging from 0.81 to 1.25. DMS in the SML was determined using a novel gas-permeable tube technique which measured consistently higher concentrations than with the traditional glass plate technique; however, significant DMS enrichment was present at only one station, with the EF ranging from 0.40 to 1.22. SML DMSP and DMS were influenced by phytoplankton community composition, with correlations with dinoflagellate and Gymnodinium biomass, respectively. DMSP and DMS concentrations were also correlated between the SML and SSW, with the difference in ratio attributable to greater DMS loss to the atmosphere from the SML. In the absence of significant enrichment, DMS in the SML did not influence DMS emissions, with the calculated air–sea DMS flux of 2.28 to 11.0 µmol m−2 d−1 consistent with climatological estimates for the region. These results confirm previous regional observations that DMS is associated with dinoflagellate abundance but indicate that additional factors are required to support significant enrichment in the SML.

HYDROCARBONS IN THE SURFACE MICROLAYER AND ICE OF THE KARA SEA

Concentrations and composition of hydrocarbons – HCs (aliphatic – AHCs and polycyclic aromatic hydrocarbons – PAHs) in the Kara Sea (cruise 83 of the R/V Academic Mstislav Keldysh, June 2021) in the sea surface microlayer – SML (about 300 µm thick), ice and surface waters weree defined. The concentration of AHCs in SML occurs in suspended particulate matter (SPM), where their content varied in the range of 197–1051 µg/l, on average 621 and was almost 4 times higher than in the dissolved form: 89–270, on average 158 µg/l, and according to compared with the suspended form of surface waters – almost 22 times higher (on average 33 µg/l). The accumulation of organic compounds also occurs in ice, but to a lesser extent than in the SML. According to the composition of alkanes, the influence of autochthonous processes on the formation of AHCs in surface microlayer and melting ice is mostly insignificant, and this distinguishes their behavior from the ice that forms. The content of PAHs in surface microlayer in SPM was also 4.8 times higher on average than in the dissolved form. Despite the different sources of these hydrocarbon classes, for all samples taken, a dependence was observed in the distribution of AHCs and PAHs in SPM (r = 0.79, n = 52, at p 0.01). In the composition of PAHs, according to the markers, the influence of combustion products of marine fuel is traced, since, in addition to phenanthrene, fluoranthene and pyrene dominated in all samples.

Iodine emission from the reactive uptake of ozone to simulated seawater.

The heterogeneous reaction of ozone and iodide is both an important source of atmospheric iodine and dry deposition pathway of ozone in marine environments. While the iodine generated from this reaction is primarily in the form of HOI and I2, there is also evidence of volatile organoiodide compound emissions in the presence of organics without biological activity occuring [M. Martino, G. P. Mills, J. Woeltjen and P. S. Liss, A new source of volatile organoiodine compounds in surface seawater, Geophys. Res. Lett., 2009, 36, L01609, L. Tinel, T. J. Adams, L. D. J. Hollis, A. J. M. Bridger, R. J. Chance, M. W. Ward, S. M. Ball and L. J. Carpenter, Influence of the Sea Surface Microlayer on Oceanic Iodine Emissions, Environ. Sci. Technol., 2020, 54, 13228-13237]. In this study, we evaluate our fundamental understanding of the ozonolysis of iodide which leads to gas-phase iodine emissions. To do this, we compare experimental measurements of ozone-driven gas-phase I2 formation in a flow tube to predictions made with the kinetic multilayer model for surface and bulk chemistry (KM-SUB). The KM-SUB model uses literature rate coefficients used in current atmospheric chemistry models to predict I2(g) formation in pH-buffered solutions of marine composition containing chloride, bromide, and iodide compared to solutions containing only iodide. Experimentally, I2(g) formation was found to be suppressed in solutions containing seawater levels of chloride compared to solutions containing only iodide, but the model does not predict this effect using literature rate constants. However, the model is able to predict this trend upon adjustment of two specific reaction rate constants. To more closely represent true oceanic conditions, we add an organic component to the proxy seawater solutions using material generated from Thalassiosira pseudonana phytoplankton cultures. Whereas the rate of ozone deposition is unaffected, the formation rate of I2(g) is strongly suppressed in the presence of biological organic material, indicative of a sink or reduction of reactive iodine formed during the oxidation process.