Do bromine and surface-active substances influence the coastal atmospheric particle growth?

New particle formation (NPF) is considered a major source of aerosol particles and cloud condensation nuclei (CCN); however, our understanding of NPF and the subsequent particle growth mechanisms in coastal areas remains limited. This study provides evidence of frequent NPF events followed by particle growth in the middle Adriatic Sea during the summer months at the coastal station of Rogoznica in Croatia. To our knowledge, this is the first study to report such events in this region. Our research aims to improve the understanding of NPF by investigating particle growth through detailed physicochemical characterization and event classification. We used a combination of online measurements and offline particle collection, followed by a thorough chemical analysis. Our results suggest the role of bromine in the particle growth process and provide evidence for its involvement in combination with organic compounds. In addition, we demonstrated the significant influence of surface-active substances (SAS) on particle growth. NPF and particle growth events have been observed in air masses originating from the Adriatic Sea, which can serve as an important source of volatile organic compounds (VOC). Our study shows an intricate interplay between bromine, organic carbon (OC), and SAS in atmospheric particle growth, contributing to a better understanding of coastal NPF processes. In this context, we also introduced a new approach using the semi-empirical 1st derivative method to determine the growth rate for each time point that is not sensitive to the nonlinear behavior of the particle growth over time. We observed that during NPF and particle growth event days, the OC concentration measured in the ultrafine mode particle fraction was higher compared to non-event days. Moreover, in contrast to non-event days, bromine compounds were detected in the ultrafine mode atmospheric particle fraction on nearly all NPF and particle growth event days. Regarding sulfuric acid, the measured sulfate concentration in the ultrafine mode atmospheric particle fraction on both NPF event and non-event days showed no significant differences. This suggests that sulfuric acid may not be the primary factor influencing the appearance of NPF and the particle growth process in the coastal region of Rogoznica.

A new tool for the determination of humic substances in natural waters: Pulsed voltammetry approach.

Humic substances (HS) in natural waters can be determined with a new, simple and sensitive method based on their influence on the background current in a differential pulse - adsorptive cathodic stripping voltammetry. The proposed method, termed PB-HS (pulsed background - humic substances) is discussed in detail, including its application in natural samples from the Krka River estuary. The method was additionally compared with absorbance measurements as well as with the typical electrochemical HS quantification in natural waters based on HS complexation with molybdenum (Mo). A good correlation between methods was observed, with PB-HS showing slightly better sensitivity to humic compounds than classical spectrophotometry. Higher HS concentrations measured with the Mo-method may be due to the enhanced hydrophobicity reached at pH 2 that is required by the method. Advantages of the proposed PB-HS method, compared to existing voltammetric methods for HS quantification, are that it does not require any reagent addition (except buffer) and that it can be used at the natural pH of water as well as in a wide salinity range, which is crucial for its application in estuarine waters.

Phytoplankton pigment dynamics in marine lake fluctuating between stratified and holomictic euxinic conditions.

Biomass dynamics in the marine lake are strongly dependent on seasonal variability in vertical stratification, indicating rapid adaptation of phytoplankton to short-term changes in the water column. A small marine lake (Rogoznica Lake, Croatia), which fluctuates between stably stratified and holomictic euxinic conditions, was used as a model to study the phytoplankton responses to environmental perturbations, in particular the anoxic stress, caused by periodic holomixia. The epilimnion showed significant temporal and vertical variability with a chlorophyll a subsurface maximum with the highest biomass near the chemocline. Fucoxanthin-containing biomass (diatoms) dominated in the epilimnion in colder seasons and was first to recover after holomictic euxinic events. The shift towards the smaller groups prevailed during highly stratified water column conditions in warmer seasons. Results for the hypolimnion were more enigmatic, with high concentrations of alloxanthin, zeaxanthin, and violaxanthin indicating the presence of a viable small-size mixotrophic community under extreme conditions.

Dynamics of organic matter in the changing environment of a stratified marine lake over two decades.

The marine lake (Rogoznica Lake), which fluctuates between stratified and holomictic conditions, is a unique environment on the eastern Adriatic coast affected by environmental changes. These changes are reflected in the warming of the water column, the apparent deoxygenation of the epilimnion, and the accumulation of organic matter (OM), toxic sulfide, and ammonium in the anoxic hypolimnion. Since the early 1990s, the volume of anoxic water has increased as the chemocline has moved to the surface water layer. A trend toward enrichment of refractory dissolved organic carbon (DOC) was observed in the anoxic hypolimnion, while a decreasing trend was observed in the oxic epilimnion in the spring DOC. At the same time, the most reactive surface-active fraction of DOC showed the opposite trend. In addition, there is evidence of accumulation of particulate organic carbon (POC) in the water column, followed by an increase in the fraction of POC in total organic carbon (TOC). On a multi-year scale (1996-2020), this work presents a unique time series of the dynamics of OM in the stratified marine system, showing a significant change in its quantity and quality due to climate and environmental variability. DOC-normalized surfactant activity is shown to be a good indicator of environmental change.

Bacterioneuston and Bacterioplankton Structure and Abundance in Two Trophically Distinct Marine Environments - a Marine Lake and the Adjacent Coastal Site on the Adriatic Sea.

Marine surface microlayer (SML) is a large and extreme marine environment with an important role in biogeochemical cycling and climate regulation. We explored the seasonal structure and abundance of bacterial assemblages in SML (bacterioneuston) and underlying water layer (ULW) (bacterioplankton) in eutrophic marine Rogoznica Lake and more oligotrophic coastal area of the adjacent Adriatic Sea. SML and ULW in each site were similar in pH, salinity, dissolved oxygen, oxygen saturation, and temperature. Rogoznica Lake was colder in winter and warmer in summer compared to the Adriatic Sea. Regarding nutrients, SML and ULW were notably different environments. SML was consistently enriched in nitrate, nitrite, orthophosphate, and total organic carbon than ULW in both investigated environments. Except in spring in Rogoznica Lake, bacterial abundance in SML was also significantly higher (p < 0.05) than in ULW. Both layers and sites show prominent seasonal variability. High-throughput 16S rRNA gene sequencing of DNA and cDNA revealed a considerable difference in bacterial assemblage structure, although study sites were < 200 m apart. Heterotrophs were predominant in both layers with pronounced spatial and temporal structural differences, except in autumn in Rogoznica Lake when, autotrophs became the dominant fraction under oxygen-deprived conditions. All these variations were driven by in situ conditions, the most important ones being total organic carbon and temperature (and additionally dissolved oxygen in Rogoznica Lake). This is especially important in terms of ongoing eutrophication, warming and deoxygenation, noticed not only in the Adriatic Sea and Rogoznica Lake but globally as well. Therefore, further structural and physiological changes in bacterioneuston and bacterioplankton assemblages can be expected.

The possible role of the surface active substances (SAS) in the airborne transmission of SARS-CoV-2.

Surface active substances (SAS) have the potential to form films at different interfaces, consequently influencing the interfacial properties of atmospheric particulate matter (PM). They can be derived from both human activities and natural processes and can be found in an indoor and outdoor environment. This paper's fundamental question is the possible role of the SAS in stabilizing respiratory aerosols in the closed space. In that context, we discuss results of preliminary measurements of the SAS and dissolved organic carbon (DOC) concentrations in the water-soluble fractions of PM2.5 and PM10 that were sampled simultaneously in primary school inside and outside of the building. The concentrations of SAS were determined using highly sensitive electrochemical measurements. It was observed that SAS and DOC concentrations have been enhanced indoor in both PM fractions. Consistent with these results, a discussion arises on the possibility that SAS could play a crucial role in respiratory droplet dispersion as stabilizers, especially in a closed space. At the same time, we assume that they could prolong the lifetime of respiratory aerosols and as well viability of some (possible SARS-CoV-2) virus inside of the droplets.

Dissolved organic carbon and surface active substances in the northern Adriatic Sea: Long-term trends, variability and drivers.

The paper presents a unique time series of organic matter content (dissolved organic carbon, DOC, and its surface active substances fraction, SAS) collected in the northern Adriatic along the Po-Rovinj transect between 1998 and 2017. The data were collected on a monthly or bimonthly basis. Seasonal variance of organic matter content does not exceed 30% of its total variance, while the DOC and the SAS trends are significantly negative and positive, respectively, over the whole transect. The organic matter content, however, exhibits pronounced interannual and decadal changes, with periods of high and low carbon content and evident changes in composition of the SAS content. The changes indicate altering episodes between eutrophication and oligotrophication, embedded in the overall oligotrophication trend in the considered period. Both series were correlated with the potential local and regional yearly-averaged drivers in both atmosphere and sea. DOC is most strongly (significant at 99%) correlated with the Po River discharges, at the phase lag of -1 to -2 years. For the SAS, the largest correlations (significant at 99%) are obtained with the Adriatic-Ionian Bimodal Oscillating System index (BiOS index), at the phase lag of -3 to -4 years. Correlations between the organic matter content and the hemispheric or the regional climate indices (North Atlantic Oscillation, East Atlantic/West Russia, East Atlantic, Scandinavian, and Mediterranean Oscillation) are much lower and only sparsely significant at 95% at some phase lags. The same was found for the other local drivers (precipitation and net heat flux). Our analysis highlights the importance of remote processes, like the BiOS, that weren't previously considered to shape the biogeochemical properties of such shallow coastal region impacted by freshwater load. To properly assess such impacts, long-term ecological monitoring and homogenized data series are required.

Chemical characterization of fine aerosols in respect to water-soluble ions at the eastern Middle Adriatic coast.

Fine particulate matter (PM2.5) concentrations at the Middle Adriatic coastal site of Croatia were affected by different air-mass inflows and/or local sources and meteorological conditions, and peaked in summer. More polluted continental air-mass inflows mostly affected the area in the winter period, while southern marine pathways had higher impact in spring and summer. Chemical characterization of the water-soluble inorganic and organic ionic constituents is discussed with respect to seasonal trends, possible sources, and air-mass inputs. The largest contributors to the PM2.5 mass were sea salts modified by the presence of secondary sulfate-rich aerosols indicated also by principal component analysis. SO42- was the prevailing anion, while the anthropogenic SO42- (anth-nssSO42-) dominantly constituted the major non-sea-salt SO42- (nssSO42-) fraction. Being influenced by the marine origin, its biogenic fraction (bio-nssSO42-) increased particularly in the spring. During the investigated period, aerosols were generally acidic. High Cl- deficit was observed at Middle Adriatic location for which the acid displacement is primarily responsible. With nssSO42- being dominant in Cl- depletion, sulfur-containing species from anthropogenic pollution emissions may have profound impact on atmospheric composition through altering chlorine chemistry in this region. However, when accounting for the neutralization of H2SO4 by NH3, the potential of HNO3 and organic acids to considerably influence Cl- depletion is shown to increase. Intensive open-fire events substantially increased the PM2.5 concentrations and changed the water-soluble ion composition and aerosol acidity in summer of 2015. To our knowledge, this work presents the first time-resolved data evaluating the seasonal composition of water-soluble ions and their possible sources in PM2.5 at the Middle Adriatic area. This study contributes towards a better understanding of atmospheric composition in the coastal Adriatic area and serves as a basis for the comparison with future studies related to the air quality at the coastal Adriatic and/or Mediterranean regions.

Microbial diversity and long-term geochemical trends in the euxinic zone of a marine, meromictic lake.

Hypoxic and anoxic niches of meromictic lakes are important sites for studying the microbial ecology of conditions resembling ancient Earth. The expansion and increasing global distribution of such environments also means that information about them serves to understand future phenomena. In this study, a long-term chemical dataset (1996-2015) was explored together with seasonal (in 2015) information on the diversity and abundance of bacterial and archaeal communities residing in the chemocline, monimolimnion and surface sediment of the marine meromictic Rogoznica Lake. The results of quantitative PCR assays, and high-throughput sequencing, targeting 16S rRNA genes and transcripts, revealed a clear vertical structure of the microbial community with Gammaproteobacteria (Halochromatium) and cyanobacteria (Synechococcus spp.) dominating the chemocline, Deltaproteobacteria and Bacteroidetes dominating the monimolimnion, and significantly more abundant archaeal populations in the surface sediment, most of which affiliated to Nanoarchaeota. Seasonal changes in the community structure and abundance were not pronounced. Diversity in Rogoznica Lake was found to be high, presumably as a consequence of stable environmental conditions accompanied by high dissolved carbon and nutrient concentrations. Long-term data indicated that Rogoznica Lake exhibited climate changes that could alter its physico-chemical features and, consequently, induce structural and physiological changes within its microbial community.

Dissolved organic carbon as potential indicator of global change: A long-term investigation in the northern Adriatic.

Dissolved organic carbon (DOC) is an essential component of the biogeochemical marine system, effecting biological and chemical reactions that take place in the sea. DOC represents a dynamic component of the global carbon cycle. This paper reports 25years of measurements of DOC content and distribution at seven stations along the transect Po River delta - Rovinj in the northern Adriatic (NA). The results show strong temporal and spatial variability: (1) The highest average DOC concentrations were observed in 1998 and 2002 (143µmol/L and 137µmol/L, respectively); (2) The minimum average DOC was recorded in 2006 (88µmol/L) and (3) The short-term DOC accumulation (up to 203, average 102µmol/L) for the years 2009 to 2012, was observed during the summer and autumn months followed by unusually low DOC concentrations during the winter and spring. The DOC results from the more recent monitoring at the same stations indicate primarily oligotrophic characteristics of the NA seawater (88µmol/L). The results of DOC variability and distribution in the NA appears to be strongly influenced by complex circulation patterns. This paper provides a "link" between the Ionian circulation and the NA ecosystem as a part of the recently identified Adriatic-Ionian Bimodal Oscillating System (BiOS). A good agreement between the BiOS oscillation and other variables related to the DOC concentration, like the NA A and B winter types, the Po River discharge, salinity, chlorophyll a, occurrence of hypoxic-anoxic conditions, eutrophication and oligotrophication, suggests that DOC might be a good tool and indicator of global change.

Enhanced viral activity and dark CO2 fixation rates under oxygen depletion: the case study of the marine Lake Rogoznica.

Global change is determining the expansion of marine oxygen-depleted zones, which are hot spots of microbial-driven biogeochemical processes. However, information on the functioning of the microbial assemblages and the role of viruses in such low-oxygen systems remains largely unknown. Here, we used the marine Rogoznica Lake as a natural model to investigate the possible consequences of oxygen depletion on virus-prokaryote interactions and prokaryotic metabolism in pelagic and benthic ecosystems. We found higher bacterial and archaeal abundances in oxygen-depleted than in oxic conditions, associated with higher heterotrophic carbon production, enzymatic activities and dark inorganic carbon fixation (DCF) rates. The oxygen-depleted systems were also characterized by higher viral abundance, production and virus-induced prokaryotic mortality. The highest DCF relative contribution to the whole total C production (> 30%) was found in oxygen-depleted systems, at the highest virus-induced prokaryotic mortality values (> 90%). Our results suggest that the higher rates of viral lysis in oxygen-depleted conditions can significantly enhance DCF by accelerating heterotrophic processes, organic matter cycling, and hence the supply of inorganic reduced compounds fuelling chemosynthesis. These findings suggest that the expansion of low-oxygen zones can trigger higher viral impacts on prokaryotic heterotrophic and chemoautotrophic metabolism, with cascading effects, neglected so far, on biogeochemical processes.

Community shift from phototrophic to chemotrophic sulfide oxidation following anoxic holomixis in a stratified seawater lake.

Most stratified sulfidic holomictic lakes become oxygenated after annual turnover. In contrast, Lake Rogoznica, on the eastern Adriatic coast, has been observed to undergo a period of water column anoxia after water layer mixing and establishment of holomictic conditions. Although Lake Rogoznica's chemistry and hydrography have been studied extensively, it is unclear how the microbial communities typically inhabiting the oxic epilimnion and a sulfidic hypolimnion respond to such a drastic shift in redox conditions. We investigated the impact of anoxic holomixis on microbial diversity and microbially mediated sulfur cycling in Lake Rogoznica with an array of culture-independent microbiological methods. Our data suggest a tight coupling between the lake's chemistry and occurring microorganisms. During stratification, anoxygenic phototrophic sulfur bacteria were dominant at the chemocline and in the hypolimnion. After an anoxic mixing event, the anoxygenic phototrophic sulfur bacteria entirely disappeared, and the homogeneous, anoxic water column was dominated by a bloom of gammaproteobacterial sulfur oxidizers related to the GSO/SUP05 clade. This study is the first report of a community shift from phototrophic to chemotrophic sulfide oxidizers as a response to anoxic holomictic conditions in a seasonally stratified seawater lake.

Electrochemical and colorimetric measurements show the dominant role of FeS in a permanently anoxic lake.

Recent publications have shown that the anodic reaction between FeS and Hg can be used for electrochemical detection of colloidal and particulate FeS in natural waters. Anodic waves that were recorded around -0.45 V (vs Ag/AgCl) in model solutions correspond to the electrochemical transformation of nanoparticulate FeS to HgS. Here, as a further step, the proposed approach is tested on anoxic, sulfidic, and iron-rich samples of a meromictic freshwater lake (Lake Pavin, France). Based on new and more comprehensive work on FeS electrochemistry in model and anoxic Lake Pavin samples, a new interpretation is given for previously recorded voltammetric signals in sulfide and iron rich environment, usually designated FeS(aq), and its role in controlling solubility of different FeS phases. A comparison of the depth profiles of S(-II) measured by voltammetry and the methylene blue method showed that the majority of S(-II) is in the form of FeS. In the monimolimnion layer, thermodynamic calculations based on total Fe(II) and S(-II) concentration, measured by ferrozine and the methylene blue method, predict precipitation of FeS with log K(s) values between -3.6 and -3.8, very close to mackinawite's K(s) value. In the upper part of the same layer, precipitation of greigite is predicted. It is shown that modification of a Hg electrode by surface-formed FeS has a significant influence on voltammetric Fe(II) determination, since reduction of Fe(II) under such conditions occurs both on bare (-1.4 V) and on FeS modified Hg surfaces (-1.1 V); Fe(II) may be underdetermined when only the -1.4 V peak is measured.

Voltammetric characterization of metal sulfide particles and nanoparticles in model solutions and natural waters.

Voltammetric scans in sulfidic natural waters often reveal reduction peaks in the range -0.9 to -1.35 V versus Ag/AgCl. These peaks have been attributed to iron sulfide complexes or clusters. However, sols containing CuS nanoparticles now also are known to produce reduction peaks in this range. Here we investigate the voltammetric behavior of two additional metal sulfides at the Hg electrode in 0.55 M NaCl + 0.03 M NaHCO3 electrolyte, pH=8.5. We show that Pb and Hg sulfides, either as suspended powders or as precipitated nanoparticles, also yield cathodic peaks between -0.9 and -1.35 V, similar to peaks obtained with CuS and FeS. For precipitated nanoparticles, the position and shape of these reduction peaks change with ageing. Freshly formed nanoparticles produce less negative reduction peaks than aged nanoparticles. Peaks from aged nanoparticles often consist of two or more superimposed reduction peaks. When all other experimental parameters are held constant, the amount of nanoparticle analyte accumulated on the electrode increases with the amount of ageing (< or = 1 h). Addition of EDTA or acidification followed by purging can be used to distinguish PbS nanoparticles and Fe sulfide clusters from CuS and HgS nanoparticles or from colloidal S. This test was applied to interpret -0.9 to -1.35 V reduction peaks observed in two meromictic lakes. In conjunction with other evidence, this test suggests that FeS clusters are present in one case whereas colloidal S is present in the other. Interpreting -0.9 to -1.35 V voltammetric peaks observed in sulfidic natural waters requires caution, but these peaks are potentially rich sources of information about trace metal speciation.

Voltammetry of copper sulfide particles and nanoparticles: investigation of the cluster hypothesis.

An association of Cu with sulfide in aerobic natural waters has been attributed to these components' coexistence in clusters of sizes intermediate between mononuclear complexes and colloidal particles. This hypothesis is investigated here. Copper sulfide solid phases display size-related voltammetric behavior at Hg electrodes. Suspensions of copper sulfide powders held at accumulation potentials of 0 to -0.2 V (vs Ag/AgCl) produce voltammetric peaks near -0.15, -0.65, and -0.95 V during subsequent cathodic scans. The first two peaks arise from electrochemically generated Cu-oxyhydroxides and HgS; the -0.95 V peak arises from reduction of sorbed copper sulfide particles. Nanoparticles of radius approximately 10(-8) m produce the third peak even without stirring or accumulation. Still smaller analytes give only the first two peaks. Published evidence alleging production of subnanometer copper sulfide clusters during titrations of Cu2+ and HS- was not reproduced when sulfide oxidation was avoided. Instead, such titrations apparently generate nanoparticles. The titration stoichiometry is 1/1, consistent with previous descriptions of this process: Cu2+ + HS- --> 1/2Cu2S x S0 (brown sol) --> CuS (green sol). Titrating Cu2+ into organic-rich (muscilaginous) Adriatic Sea water, which contains 10(-7) M natural thiols and sulfide, produces solid products. In the future, voltammetry might prove useful for studying semiconductor sulfide nanoparticles in nature.