The bactericidal effect of TiO2 photocatalysis involves adsorption onto catalyst and the loss of membrane integrity

The bactericidal effect of photocatalysis with TiO2 is well recognized, although its mode of action is still poorly characterized. It may involve oxidation, as illuminated TiO2 generates reactive oxygen species. Here we analyze the bactericidal effect of illuminated TiO2 in NaCl-KCl or sodium phosphate solutions. We found that adsorption of bacteria on the catalyst occurred immediately in NaCl-KCl solution, whereas it was delayed in the sodium phosphate solution. We also show that the rate of adsorption of cells onto TiO2 is positively correlated with its bactericidal effect. Importantly, adsorption was consistently associated with a reduction or loss of bacterial membrane integrity, as revealed by flow cytometry. Our work suggests that adsorption of cells onto aggregated TiO2, followed by loss of membrane integrity, is key to the bactericidal effect of photocatalysi

A two-component parameterization of marine ice-nucleating particles based on seawater biology and sea spray aerosol measurements in the Mediterranean Sea

Ice-nucleating particles (INPs) have a large impact on the climate-relevant properties of clouds over the oceans. Studies have shown that sea spray aerosols (SSAs), produced upon bursting of bubbles at the ocean surface, can be an important source of marine INPs, particularly during periods of enhanced biological productivity. Recent mesocosm experiments using natural seawater spiked with nutrients have revealed that marine INPs are derived from two separate classes of organic matter in SSAs. Despite this finding, existing parameterizations for marine INP abundance are based solely on single variables such as SSA organic carbon (OC) or SSA surface area, which may mask specific trends in the separate classes of INP. The goal of this paper is to improve the understanding of the connection between ocean biology and marine INP abundance by reporting results from a field study and proposing a new parameterization of marine INPs that accounts for the two associated classes of organic matter. The PEACETIME cruise took place from 10 May to 10 June 2017 in the Mediterranean Sea. Throughout the cruise, INP concentrations in the surface microlayer (INPSML) and in SSAs (INPSSA) produced using a plunging aquarium apparatus were continuously monitored while surface seawater (SSW) and SML biological properties were measured in parallel. The organic content of artificially generated SSAs was also evaluated. INPSML concentrations were found to be lower than those reported in the literature, presumably due to the oligotrophic nature of the Mediterranean Sea. A dust wet deposition event that occurred during the cruise increased the INP concentrations measured in the SML by an order of magnitude, in line with increases in iron in the SML and bacterial abundances. Increases in INPSSA were not observed until after a delay of 3 days compared to increases in the SML and are likely a result of a strong influence of bulk SSW INPs for the temperatures investigated (T=−18 ∘C for SSAs, T=−15 ∘C for SSW). Results confirmed that INPSSA are divided into two classes depending on their associated organic matter. Here we find that warm (T≥−22 ∘C) INPSSA concentrations are correlated with water-soluble organic matter (WSOC) in the SSAs, but also with SSW parameters (particulate organic carbon, POCSSW and INPSSW,−16C) while cold INPSSA (T<−22 ∘C) are correlated with SSA water-insoluble organic carbon (WIOC) and SML dissolved organic carbon (DOC) concentrations. A relationship was also found between cold INPSSA and SSW nano- and microphytoplankton cell abundances, indicating that these species might be a source of water-insoluble organic matter with surfactant properties and specific IN activities. Guided by these results, we formulated and tested multiple parameterizations for the abundance of INPs in marine SSAs, including a single-component model based on POCSSW and a two-component model based on SSA WIOC and OC. We also altered a previous model based on OCSSA content to account for oligotrophy of the Mediterranean Sea. We then compared this formulation with the previous models. This new parameterization should improve attempts to incorporate marine INP emissions into numerical models

Analyse à haute fréquence spatiale et temporelle du phytoplancton à l'aide de la cytométrie en flux automatisée et immergeable

AIX-MARSEILLE2-BU Sci.Luminy (130552106) / SudocSudocFranceF

Temporal and Spatial High-Frequency Monitoring of Phytoplankton by Automated Flow Cytometry and Pulse-Shape Analysis

Phytoplankton was investigated with automated high frequency flow cytometry to address their patchiness and short-term variability. To document the latter, we deployed a submersible flow cytometer (CytoSub, www.cytobuoy.com) in the Bay of Marseille (North Mediterranean), at 1.5 m depth. This instrument involves pulse shape analysis and can analyse cells (1-1000 µm) and even chains at a flow rate of 8 µl.s-1. Phytoplankton was monitored in situ every 30 min during summer 2005. The seven clusters resolved in the size range 1-50 µm, behaved as independent entities, suggesting that they could be considered as functional groups. The spatial heterogeneity of oceanic phytoplankton distribution was addressed by running the CytoSub on board a 33 m schooner (Fetia Ura, www.seanergies.com) between Azores and French Britany in April 2007. The flow cytometric analysis was triggered every 15 min (spatial resolution of 2.8 km). Five clusters were resolved in the pumped surface water and specific relationships were determined between their distributions within the different water masses sampled during the cruise. The evidenced variabilities are critical to explain the impact of intrinsic and extrinsic factors on phytoplankton spatial and temporal distributions. Automated in situ flow cytometry appears as a powerful tool to investigate phytoplankton assemblages at high frequency and at the single cell level. Undergoing technological developments are extending this capacity to the whole microbial ecosystem

Microbial community structure along 18°W (39°N–44.5°N) in the NE Atlantic in late summer 2001 (POMME programme)

International audienc

Sub meso scale phytoplankton distribution in the North East Atlantic surface waters determined with an automated flow cytometer

International audiencePhytoplankton cells in the size range ∼1-50 µm were analysed in surface waters using an automated flow cytometer, the Cytosub (http://www.cytobuoy.com), from the Azores to the French Brittany during spring 2007. The Cytosub records the pulse shape of the optical signals generated by phytoplankton cells when intercepted by the laser beam. A total of 6 distinct optical groups were resolved during the whole transect, and the high frequency sampling (15 min) provided evidence for the cellular cycle (based on cyclic changes in cell size and fluorescence) and distribution changes linked to the different water characteristics crossed in the North East Atlantic provinces. Nutrient concentrations and mixed layer depth varied from west to east, with a decrease in the mixed layer depth and high nutrient concentrations in the middle of the transect as well as near the French coast. Data provided a link between the sub meso scale processes and phytoplankton patchiness, some abundance variations due to the cellular cycle can be pointed out. The high frequency spatial sampling encompasses temporal variations of the phytoplankton abundance, offering a better insight into phytoplankton distribution

Short-term vaiation of phytoplankton assemblages in Mediterranean coastal waters recorded with an automated submerged flow cytometer

International audienc

Ultraphytoplankton basin-scale distribution in the eastern Mediterranean Sea in winter : link to hydrodynamism and nutrients

International audienceThe basin-scale distribution of ultraphytoplankton (<10 µm) was determined in the upper 200 m of the eastern Mediterranean Sea during the winter season. Four clusters were resolved by flow cytometry on the basis of their optical properties and identified as Synechococcus, Prochloro-coccus, pico-(<3 µm) and nanoeukaryotes (3-10 µm). Syne-chococcus was the most abundant population (maximum abundance of about 37 000 cells cm −3) and contributed up to 67.7% to the overall ultraphytoplanktonic carbon biomass, whereas the contribution of Prochlorococcus never exceeded 6.5%. The maximum integrated carbon biomass was 1763, 453, 58 and 571 mg C m −2 for nanoeukaryotes, picoeukary-otes, Prochlorococcus and Synechococcus respectively. Water mass properties were analyzed on the basis of temperature and salinity distributions in order to account for the general circulation and locate the main hydrodynamic structures (fronts, gyres, transition between western and eastern basins). The effect of the main hydrodynamic structures and nutrients on the ultraphytoplankton distribution was investigated. No positive correlation between nutrients and phytoplankton could be established when considering large scales. However, below 50 m depth, nutrient ratios between particular stations were correlated to corresponding density ratios. In contrast, significant relationships were found be-Correspondence to: M. Denis ([email protected]) tween Synechococcus abundance and density, resulting from the impact of a gyre in southern Adriatic basin and a ther-mohaline front in the Ionian basin. A significant relationship was also found between picoeukaryotes and salinity in the comparison of western and eastern Mediterranean Sea