Marine microalgae obtain essential sulfur through energetically costly, assimilatory reduction of sea-salt sulfate, so the ability for osmotrophic incorporation of reduced sulfur would confer them adaptability in energy-limiting conditions. Dimethylsulfoniopropionate (DMSP) is a common marine osmolyte whose pervasive occurrence in seawater makes it as good a candidate for supplying reduced sulfur to marine eukaryotes as it is to prokaryotes. Despite earlier recognition of DMSP uptake by some phytoplankters, the mechanism and its taxonomic and geographic distributions have remained unknown. Here we show that DMSP uptake occurs in taxonomically diverse eukaryotic phytoplankton harbouring PaDT (for picoalgal DMSP transporter), a gene within the betaine/choline/carnitine transporter (BCCT) family whose transcription was upregulated by DMSP additions. Confirmation that PaDT encodes a functional DMSP transporter was obtained by restoring DMSP uptake in a BCCT-deficient mutant of Escherichia coli complemented with picoalgal PaDT. Increase of picoalgal DMSP uptake and PaDT transcription under decreasing light supports exogenous DMSP incorporation as an energy-saving strategy. The global and taxonomic spread of PaDT expression in the surface ocean indicates that eukaryotic phytoplankton are not only the main DMSP source but also a ubiquitous sink, reinforcing their role in regulating marine sulfur emissions.

Paralytic shellfish toxins (PST) are detected in seafood globally. Resource managers and seafood producers must make decisions in the face of toxic events in order to protect public health and operate businesses. The coast of Maine in the US sees the occurrence of PSTs annually, and fosters a robust shellfish industry composed of both wild harvest and recently increasing aquaculture. Leveraging monitoring data generated from chemical analysis of PST composition in shellfish, we present a method for grouping together consecutive samples to build a machine-learning training set and make short-term, site-specific predictions towards toxicity of shellfish one week in the future. Over five seasons of operational deployment, based on a process co-developed with end-users, the forecast has consistently achieved >90% accuracy at predicting whether or not sites being currently monitored will surpass the regulatory threshold for PST toxicity.

A natural plankton community from oligotrophic subtropical waters of the Atlantic near Gran Canaria, Spain, was subjected to varying degrees of ocean alkalinity enhancement (OAE) to assess the potential physiological effects in the context of the application of ocean carbon dioxide removal (CDR) techniques. We employed nine mesocosms with sediment traps attached to the bottom, each enclosing a volume of 8.3 m3, to create a gradient in total alkalinity (TA). OAE was based on the addition of carbonates (NaHCO3 and Na2CO3). The lowest point on this gradient was 2400 µmol L−1, which corresponded to the natural alkalinity of the environment, and the highest point was 4800 µmol L−1. Over the course of the 33 d experiment, the plankton community exhibited two distinct phases. In phase I (days 5–20), a notable decline in the photosynthetic efficiency (Fv/Fm) was observed. This change was accompanied by substantial reductions in the abundances of picoeukaryotes, small-size nanoeukaryotes (nanoeukaryotes-1), and microplankton. The cell viability of picoeukaryotes, as indicated by fluorescein diacetate hydrolysis by cellular esterases (FDA green fluorescence), slightly increased by the end of phase I, whilst the viability of nanoeukaryotes-1 and Synechococcus spp. did not change. Reactive oxygen species levels (ROS green fluorescence) showed no significant changes for any of the functional groups. In contrast, in phase II (days 21–33), a pronounced community response was observed. Increases in Fv/Fm in the intermediate OAE treatments of Δ900 to Δ1800 µmol L−1 and in chlorophyll a (Chl a), chlorophyll c2 (Chl c2), fucoxanthin, and divinyl Chl a were attributed to the emergence of blooms of large-size nanoeukaryotes (nanoeukaryotes-2) from the genera Chrysochromulina, as well as picoeukaryotes. Synechococcus spp. also flourished towards the end of this phase. In parallel, we observed a significant change of 20 % in the overall metaproteome of the phytoplankton community. This is considered a significant alteration in protein expression, having a substantial impact on cellular functions and the physiology of the organisms. Medium levels of ΔTA showed more upregulated and less-downregulated proteins than higher ΔTA treatments. Under these conditions, cell viability significantly increased in pico- and nanoeukaryotes-1 at intermediate alkalinity levels, while in Synechococcus spp., nanoeukaryotes-2 and microplankton remained stable. ROS levels did not significantly change in any functional group. The pigment ratios DD+DT : FUCO and DD+DT : Chl a increased in medium ΔTA treatments, supporting the idea of nutrient deficiency alleviation and the absence of physiological stress. When all data are taken together, this study shows that OAE did not cause cellular stress in the phytoplankton community studied, and physiological fitness was not impaired. The drawdown in phytoplankton cell numbers that was observed at times seemed to have been most likely caused by nutrient limitation.

Volatile organic compounds (VOCs) have been proposed to indicate coral reef health, but little is known about their cycling processes in coral reefs and the roles of reef components. We studied the distribution and cycling of ocean-leaving VOCs (dimethylsulfide (DMS), carbonyl sulfide (COS), CS2, dimethyl disulfide, isoprene, CH3I, CH2ClI, and bromomethanes CH2Br2 and CHBr3) across a coral reef in Mo’orea (French Polynesia) that has a fast and unidirectional water flow. Repeated sampling of transects between the open ocean and the reef outflow channel, across the shelf, the reef crest, and the back-reef lagoon, showed that reef waters were depleted in dissolved organic carbon, chlorophyll, phytoplankton, and bacteria, and enriched in nutrients. All studied VOCs increased in concentration after oceanic waters crossed the reef crest, with bromomethanes showing the largest increase. Incubation experiments of back-reef waters around midday suggested that: (a) photochemical reactions were a major source for COS and major sink for DMS; (b) microbial plankton were the main daytime source for DMS, isoprene, and CH3I, and an important sink for COS; (c) seaweeds were the main source of CH2ClI, CHBr3, and CH2Br2; and (d) carbonate sediments were a major source for CS2 and CH2ClI, an important source for DMS and isoprene, and the main sink for COS. The dominant coral Pocillopora sp. was a source only for DMS and COS. Decomposing seaweed rafts were an important but unquantified source for all VOCs except CH3I. In April 2018, the reef was a net producer of VOCs compared to the ocean, with the anticipation that production would increase if corals were lost and replaced by seaweeds. Using VOCs and other chemical tracers of reef waters, we estimated that one third of the water entering the reef is recirculated water from the same reef, with implications for ecosystem self-recruitment and genetic maintenance.

Enteric methane emissions from ruminant livestock are a significant source of atmospheric methane. Efforts to address rising atmospheric methane concentrations have led to an expansion of research into mitigating enteric methane production. One of the most effective approaches utilizes bromoform-containing feed supplements, such as the algae Asparagopsis spp., to inhibit methanogenesis in the rumen. Understanding the fate and persistence of bromoform in the rumen is important for developing safe, effective products and feeding strategies. This study conducted a series of in vitro rumen fluid experiments monitoring bromoform, dibromomethane, and bromomethane concentrations, methane production and several biochemical parameters to understand the inhibitory thresholds and degradation processes of these compounds. Analysis of the rumen fluid confirmed bromoform is rapidly dehalogenated. The half-life of bromoform was 26 min, coinciding with the production of dibromomethane accumulating to 22.1% of the initial bromoform amendment, but no bromomethane was detected. Dibromomethane demonstrated a considerably longer half-life of 775 min. In separate dose-response experiments, bromoform, dibromomethane and bromomethane all exhibited anti-methanogenic activity. Bromoform and dibromomethane produced sigmoidal-relationships between concentration and inhibition at approximately 1-2 µM, and yielded similar effective concentration values (EC50s) for antimethanogenic activity. Experiments using Asparagopsis taxiformis algae revealed less accumulation of bromoform and formation of dibromomethane, likely driven by a slower release from the seaweed material. The A. taxiformis dose response was less effective at inhibiting methane per mole of bromoform added compared with direct bromoform additions. These results have significant implications for understanding the dynamics of bromoform-mediated methane inhibition and will aid the development of effective halocarbon additives, feeding strategies, and testing protocols for bromoform and its degradation byproducts.

Photochemical release of iodine from snow has been suggested as a source of reactive iodine to the Arctic atmosphere, however understanding of the underlying mechanism and potential source strength is hindered by a lack of measurements of iodine concentration and speciation in snow. Moreover, the origin of snow iodine is also unknown. Here, we report iodine speciation measurements in Arctic snow on sea ice at a range of snow depths from 177 samples, representing 80 sampling events, from December 2019 to October 2020 collected during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. We demonstrate that while there appears to be a source of iodine, in particular iodate, to the base of the snow over first year ice, this does not influence iodine concentration in the surface snow. There is instead evidence of a top-down source of iodine, potentially from iodine-enriched marine aerosol, as well as some evidence for episodic influx of iodate with dust. The potential for photochemical release of molecular iodine (I2) from iodide in surface snow was investigated, and it was demonstrated that this could provide an iodine emission flux to the Arctic atmosphere comparable to oceanic fluxes. Knowledge of the prevalence and speciation of iodine in Arctic snow will contribute to better understanding of its contribution to observed concentrations of polar iodine oxide (IO), and hence its contribution to the depletion of tropospheric ozone in the Arctic.

This paper presents a cost model for implementing a deployment scale effort for conducting ocean iron fertilization (OIF) for marine-based carbon dioxide removal (CDR). The model incorporates basic oceanographic parameters critical for estimating the effective export of newly fixed CO2 into biomass that is stimulated by Fe addition to an Fe-limited region of the Southern Ocean. Estimated costs can vary by nearly a 100-fold between best-case and worst-case scenarios with best-case values of $7/net ton C captured versus worst-case $1500/net ton C captured, without accounting for verification costs. Primary oceanographic factors that influence cost are the net primary productivity increases achieved via OIF, the amount of carbon exported into the deep ocean, and the amount of CO2 ventilated back to the atmosphere. The model compares ship-based versus aerial delivery of iron to the ocean, and estimates aerial delivery can be 30 – 40% more cost effective; however, the specific requirements for aerial delivery require additional research and development. The model also estimates costs associated with verification and environmental monitoring of OIF. These costs increase $/net ton C captured by 3 – 4-fold. Best, intermediate, and worst cases for aerial delivery and ship delivery are $23, $83, $1,735, and $25, $94, $4481, respectively, inclusive of verification costs. The primary goal of this model is to demonstrate the variability in cost of OIF as a CDR method, to better understand where additional research is needed to determine the major factors that may make OIF a tractable, nature-based CDR method.

IntroductionArtificial upwelling has been discussed as a nature-based solution to fertilize currently unproductive areas of the ocean to enhance food web productivity and atmospheric CO2 sequestration. The efficacy of this approach may be closely tied to the nutrient stoichiometry of the upwelled water, as Si-rich upwelling should benefit the growth of diatoms, who are key players for primary production, carbon export and food web efficiency.MethodsWith a mesocosm experiment in subtropical waters, we assessed the physiological and functional responses of an oligotrophic phytoplankton community to artificial upwelling under varying Si:N ratios (0.07-1.33).ResultsDeep water fertilization led to strongly enhanced primary productivity rates and net autotrophy across Si scenarios. At the community level, Si-rich upwelling50 temporarily increased primary production and consistently enhanced diatom growth, producing up to 10-fold higher abundances compared to Si-deficient upwelling. At the organism level, contrasting effects were observed. On the one hand, silicification and size of diatom cells remained unaffected by Si:N, which is surprising given the direct dependency of these traits on Si. On the other hand, diatom Chlorophyll a density and carbon density were strongly reduced and particulate matter C:N was elevated under Si-rich upwelling.DiscussionThis suggests a reduced nutritional value for higher trophic levels under high Si:N ratios. Despite these strong qualitative changes under high Si, diatom cells appeared healthy and showed high photosynthetic efficiency. Our findings reveal great physiological plasticity and adaptability in phytoplankton under artificial upwelling, with Si-dependent trade-offs between primary producer quantity and quality.

Kuwaiti hypersaline soil samples were contaminated with 5 % (w/w) weathered Kuwaiti light crude oil and bioaugmented with autochthonous halophilic hydrocarbonoclastic archaeal and bacterial strains, two each, individually and as consortia. Residual oil contents were determined, and microbial communities were analyzed by culture-dependent and culture-independent approaches initially and seasonally for one year. After one year of the bioremediation process, the mean oil degradation rate was similar across all treated soils including the controlled unbioaugmented one. Oil hydrocarbons were drastically reduced in all soil samples with values ranging from 82.7 % to 93 %. During the bioremediation process, the number of culturable oil-degrading bacteria increased to a range of 142 to 344 CFUx10(4) g(-1) after 12 months of bioaugmentation. Although culture-independent analysis showed a high proportion of inoculants initially, none could be cultured throughout the bioremediation procedure. Within a year, microbial communities changed continually, and 33 species of halotolerant/halophilic hydrocarbonoclastic bacteria were isolated and identified belonged mainly to the three major bacterial phyla Actinobacteria, Proteobacteria, and Firmicutes. The archaeal phylum Halobacterota represented <1 % of the microbial community's relative abundance, which explains why none of its members were cultured. Improving the biodegradability of an already balanced environment by autochthonous bioaugmentation is more involved than just adding the proper oil degraders. This study emphasizes the possibility of a relatively large resistant population, a greater diversity of oil-degrading microorganisms, and the highly selective impacts of oil contamination on hypersaline soil bacterial communities.

In the central Arctic, warm and moist air intrusions (WAMIs) are increasingly prevalent during winter and spring, significantly impacting the near-surface energy budget. This study investigates WAMI-induced transport and wet deposition of black carbon (BC) and its subsequent influence on snow properties and melting. Using a modeling approach combined with extensive observational data from the 2019–2020 Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, we document several episodes of elevated BC wet deposition during winter and spring (Nov–Apr). These events, driven by WAMI-transported pollution and moisture, result in an average 41% increase in BC wet deposition compared to typical winter/spring conditions. Furthermore, this enhanced BC deposition contributes to increased solar energy absorption (+3.4–4.6 W/m2 on average) and accelerated snowpack melt rate (+24–36%) during the subsequent summer. Despite their episodic occurrence (only 15% of the time), this study underscores the critical role of WAMIs in shaping the central Arctic environment and emphasizes the importance of the BC snow albedo effect. Our findings highlight the need for further investigation into the broader climatic impacts of WAMI-induced pollution transport in the Arctic.

Ocean alkalinity enhancement (OAE) is a negative emissions technology (NET) that shows significant potential for climate change mitigation. By increasing the bicarbonate ion concentration in ocean water, OAE could enhance long-term carbon storage and mitigate ocean acidification. However, the side effects and/or potential co-benefits of OAE on natural planktonic communities remain poorly understood. To address this knowledge gap, a mesocosm experiment was conducted in the oligotrophic waters of Gran Canaria. A CO2-equilibrated total alkalinity (TA) gradient was employed in increments of 300 µmol L−1, ranging from ∼ 2400 to ∼ 4800 µmol L−1. This study represents the first attempt to evaluate the potential impacts of OAE on planktonic communities under natural conditions. The results show that net community production (NCP), gross production (GP), community respiration (CR) rates, and the metabolic balance (GP:CR) did not exhibit a linear response to the whole alkalinity gradient. Instead, significant polynomial and linear regression models were observed for all rates up to ΔTA 1800 µmol L−1, in relation to the dissolved inorganic carbon (DIC) concentrations. Notably, the ΔTA 1500 and 1800 µmol L−1 treatments showed peaks in NCP shifting from a heterotrophic to an autotrophic state, with NCP values of 4 and 8 µmol O2 kg−1 d−1, respectively. These peaks and the optimum curve were also reflected in the nanoplankton abundance, size-fractionated chlorophyll a, and 14C uptake data. Furthermore, abiotic precipitation occurred in the highest treatment after day 21, but no impact on the measured parameters was detected. Overall, a damaging effect of CO2-equilibrated OAE in the range applied here on phytoplankton primary production, community metabolism, and composition could not be inferred. In fact, a potential co-benefit to OAE was observed in the form of the positive curvilinear response to the DIC gradient up to the ΔTA 1800 treatment. Further experimental research at this scale is key to gain a better understanding of the short- and long-term effects of OAE on planktonic communities.

The marine trace gas dimethylsulfide (DMS) supplies sulfur to the atmosphere at a rate of 15–40 Tg S per year, contributing to the production of atmospheric sulfate aerosols that influence cloud radiative properties and thereby climate. The resulting climate cooling effect of DMS is an estimated −1.7 to −2.3 W m − 2 , which is similar in magnitude to the warming effect of anthropogenic CO 2 emissions (1.83 ± 0.2 W m − 2 ). In this Review, we describe the production and cycling of marine DMS and its fate in the atmosphere. Advances in molecular genetics and large-scale biogeochemical measurements have revealed the global prevalence of DMS-related processes, including in previously overlooked environments and organisms, such as sediment-dwelling bacteria. Most marine DMS (>90%) is degraded or consumed in the water column, but the remainder is emitted to the atmosphere, where it contributes to the formation of cloud condensation nuclei. Large uncertainties (up to ±10 W m − 2 ) associated with the global impact of DMS emissions arise from the use of crudely defined biological parameters, such as total chlorophyll, in models. Constraining and modelling the biogeochemical processes that control DMS production are key to better estimating the influence of DMS on climate.

The goal of the Sea2Cloud project is to study the interplay between surface ocean biogeochemical and physical properties, fluxes to the atmosphere, and ultimately their impact on cloud formation under minimal direct anthropogenic influence. Here we present an interdisciplinary approach, combining atmospheric physics and chemistry with marine biogeochemistry, during a voyage between 41 degrees and 47 degrees S in March 2020. In parallel to ambient measurements of atmospheric composition and seawater biogeochemical properties, we describe semicontrolled experiments to characterize nascent sea spray properties and nucleation from gas-phase biogenic emissions. The experimental framework for studying the impact of the predicted evolution of ozone concentration in the Southern Hemisphere is also detailed. After describing the experimental strategy, we present the oceanic and meteorological context including provisional results on atmospheric thermodynamics, composition, and flux measurements. In situ measurements and flux studies were carried out on different biological communities by sampling surface seawater from subantarctic, subtropical, and frontal water masses. Air-Sea-Interface Tanks (ASIT) were used to quantify biogenic emissions of trace gases under realistic environmental conditions, with nucleation observed in association with biogenic seawater emissions. Sea spray continuously generated produced sea spray fluxes of 34% of organic matter by mass, of which 4% particles had fluorescent properties, and which size distribution resembled the one found in clean sectors of the Southern Ocean. The goal of Sea2Cloud is to generate realistic parameterizations of emission flux dependences of trace gases and nucleation precursors, sea spray, cloud condensation nuclei, and ice nuclei using seawater biogeochemistry, for implementation in regional atmospheric models.

The rapid melt of snow and sea ice during the Arctic summer provides a significant source of low-salinity meltwater to the surface ocean on the local scale. The accumulation of this meltwater on, under, and around sea ice floes can result in relatively thin meltwater layers in the upper ocean. Due to the small-scale nature of these upper-ocean features, typically on the order of 1 m thick or less, they are rarely detected by standard methods, but are nevertheless pervasive and critically important in Arctic summer. Observations during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in summer 2020 focused on the evolution of such layers and made significant advancements in understanding their role in the coupled Arctic system. Here we provide a review of thin meltwater layers in the Arctic, with emphasis on the new findings from MOSAiC. Both prior and recent observational datasets indicate an intermittent yet long-lasting (weeks to months) meltwater layer in the upper ocean on the order of 0.1 m to 1.0 m in thickness, with a large spatial range. The presence of meltwater layers impacts the physical system by reducing bottom ice melt and allowing new ice formation via false bottom growth. Collectively, the meltwater layer and false bottoms reduce atmosphere-ocean exchanges of momentum, energy, and material. The impacts on the coupled Arctic system are far-reaching, including acting as a barrier for nutrient and gas exchange and impacting ecosystem diversity and productivity.

Understanding the mercury cycle in the Arctic is important due to the harmful bioaccumulation of its toxic form, methylmercury, in wildlife and ultimately Arctic residents. Gaseous elemental mercury (Hg(0)) is relatively well-mixed across the northern hemisphere atmosphere due to its long atmospheric lifetime. Hg(0) can be oxidized, especially in the Arctic spring during halogen-driven depletion events. The resulting gaseous oxidized mercury (Hg(II)) is relatively quickly deposited onto snow, either directly or via condensing onto particles, forming particulate mercury (PHg). It is generally understood that a large fraction of the deposited Hg(II) and PHg is photoreduced to Hg(0) and re-emitted to the atmosphere. However, mercury remaining in the snowpack till melt can become bioavailable through entering the ocean.There is a severe lack of Hg(II) and PHg observations in the central Arctic, particularly over sea ice, limiting our understanding of the mercury cycle in that region and inhibiting us from quantifying mercury budgets in all environmental compartments and particularly where it unfolds its harmful neurotoxic effects. Moreover, most of the observational efforts aiming at creating process understanding focused on spring during mercury depletion events or the snow melt period, leaving large knowledge gaps for fall and winter.Here, we show atmospheric observations of PHg during MOSAiC, measured with an aerosol mass spectrometer in fall and spring over the central Arctic pack ice. In both seasons, PHg concentrations correlate strongly with wind speed and chloride, suggesting a mechanical (wind-driven) process behind atmospheric PHg related partly to blowing snow. In addition, there are significant differences between fall and spring observations (e.g. no atmospheric mercury depletion events in fall), suggesting that various processes are at play.This wind-driven process has hitherto not been reported and is different from observations at land-based stations as well as previous measurements over sea ice that ascribed the formation of PHg to adsorption of Hg(II) onto pre-existing aerosols or diamond dust rather than aerosolization from the snow pack. We hypothesize, based on snow chemical analyzes and literature, that the elevated halide content in snow on sea ice creates complexes of PHg, which are much harder to photoreduce than Hg(II), leading to a larger PHg content in snow. These processes of forming PHg and wind-driven aerosolization have implications for the mercury content of snow and the distances over which PHg is re-deposited after atmospheric transport given that the lifetime of PHg is about one order of magnitude larger than that of Hg(II) in the atmosphere.

Microplastics are subject to environmental forces that can change polymer organization on a molecular scale. However, it is not clear to what extent these changes occur in the environment and whether microplastics in the atmospheric and water environment differ. Here we identify structural differences between microplastics in the atmosphere and water environment from Japan and New Zealand, representing two archipelagos differing in their proximity to nearby countries and highly populated areas. We first highlight the propensity for smaller microplastics to arrive via air masses from the Asian continent to the Japan Sea coastal area, while New Zealand received larger, locally derived microplastics. Analyses of polyethylene in the Japanese atmosphere indicate that microplastics transported to the Japanese coastal areas were more crystalline than polyethylene particles in the water, suggesting that the plastics arriving by air were relatively more aged and brittle. By contrast, polypropylene particles in New Zealand waters were more degraded than the microplastic particles in the air. Due to the lack of abundance, both polyethylene and polypropylene could not be analyzed for both countries. Nevertheless, these findings show the structural variation in microplastics between environments in markedly different real-world locations, with implications for the toxic potential of these particles.

Concentration data of soluble radionuclides in the southern South Indian Ocean and Southern Ocean transition zone are rare or insufficient for the study of its current system. We examined the lateral surface variations in soluble natural (226Ra and 228Ra) and anthropogenic (134Cs and 137Cs) radionuclide activity concentrations in the surface waters in this area from November 2021 to March 2022. The surface distributions of 226Ra and 137Cs concentrations were classified into Subantarctic Mode Water-, Antarctic Intermediate Water-, and Upper Circumpolar Deep Water (UCDW)-dominated areas along latitudinal band (40°S-65°S, 110°E-120°E). Notably, the highest 226Ra concentrations occurred along the longitudinal band (60°S-65°S, 40°E-120°E). Significantly lower 137Cs concentrations in the Southern Ocean than those in surface waters in other global oceans were observed along with depletion of 228Ra. Additionally, 226Ra and 137Cs concentrations appeared to show small variations between eastern and western areas (2.5-3.0 mBq/L and 0.06-0.03 mBq/L, respectively). Lateral profiles in the Southern Ocean are governed by a large contribution from deep/old waters (e.g., UCDW), with a small effect from southward transport of Subantarctic Mode Water.

Dry deposition to the surface is one of the main removal pathways of tropospheric ozone (O3). We quantified for the first time the impact of O3 deposition to the Arctic sea ice on the planetary boundary layer (PBL) O3 concentration and budget using year-round flux and concentration observations from the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) campaign and simulations with a single-column atmospheric chemistry and meteorological model (SCM). Based on eddy-covariance O3 surface flux observations, we find a median surface resistance on the order of 20,000 s m−1, resulting in a dry deposition velocity of approximately 0.005 cm s−1. This surface resistance is up to an order of magnitude larger than traditionally used values in many atmospheric chemistry and transport models. The SCM is able to accurately represent the yearly cycle, with maxima above 40 ppb in the winter and minima around 15 ppb at the end of summer. However, the observed springtime ozone depletion events are not captured by the SCM. In winter, the modelled PBL O3 budget is governed by dry deposition at the surface mostly compensated by downward turbulent transport of O3 towards the surface. Advection, which is accounted for implicitly by nudging to reanalysis data, poses a substantial, mostly negative, contribution to the simulated PBL O3 budget in summer. During episodes with low wind speed (<5 m s−1) and shallow PBL (<50 m), the 7-day mean dry deposition removal rate can reach up to 1.0 ppb h−1. Our study highlights the importance of an accurate description of dry deposition to Arctic sea ice in models to quantify the current and future O3 sink in the Arctic, impacting the tropospheric O3 budget, which has been modified in the last century largely due to anthropogenic activities.

Host gut microbiomes play an important role in animal health and resilience to conditions, such as malnutrition and starvation. These host-microbiome relationships are poorly understood in the marine mussel Perna canaliculus, which experiences significant variations in food quantity and quality in coastal areas. Prolonged starvation may be a contributory factor towards incidences of mass mortalities in farmed mussel populations, resulting in highly variable production costs and unreliable market supplies. Here, we examine the gut microbiota of P. canaliculus in response to starvation and subsequent re-feeding using high-throughput amplicon sequencing of the 16S rRNA gene. Mussels showed no change in bacterial species richness when subjected to a 14-day starvation, followed by re-feeding/recovery. However, beta bacteria diversity revealed significant shifts (PERMANOVA p-value < 0.001) in community structure in the starvation group and no differences in the subsequent recovery group (compared to the control group) once they were re-fed, highlighting their recovery capability and resilience. Phylum-level community profiles revealed an elevation in dominance of Proteobacteria (ANCOM-BC p-value <0.001) and Bacteroidota (ANCOM-BC p-value = 0.04) and lower relative abundance of Cyanobacteria (ANCOM-BC p-value = 0.01) in the starvation group compared to control and recovery groups. The most abundant genus-level shifts revealed relative increases of the heterotroph Halioglobus (p-value < 0.05) and lowered abundances of the autotroph Synechococcus CC9902 in the starvation group. Furthermore, a SparCC correlation network identified co-occurrence of a cluster of genera with elevated relative abundance in the starved mussels that were positively correlated with Synechococcus CC9902. The findings from this work provide the first insights into the effect of starvation on the resilience capacity of Perna canaliculus gut microbiota, which is of central importance to understanding the effect of food variation and limitation in farmed mussels.