Over the last decades, it has become clear that plastic pollution presents a global societal and environmental challenge given its increasing presence in the oceans. A growing literature has focused ...on the microbial life growing on the surfaces of these pollutants called the "plastisphere," but the general concepts of microbial ecotoxicology have only rarely been integrated. Microbial ecotoxicology deals with (i) the impact of pollutants on microbial communities and inversely (ii) how much microbes can influence their biodegradation. The goal of this review is to enlighten the growing literature of the last 15 years on microbial ecotoxicology related to plastic pollution in the oceans. First, we focus on the impact of plastic on marine microbial life and on the various functions it ensures in the ecosystems. In this part, we also discuss the driving factors influencing biofilm development on plastic surfaces and the potential role of plastic debris as vector for dispersal of harmful pathogen species. Second, we give a critical view of the extent to which marine microorganisms can participate in the decomposition of plastic in the oceans and of the relevance of current standard tests for plastic biodegradability at sea. We highlight some examples of metabolic pathways of polymer biodegradation. We conclude with several questions regarding gaps in current knowledge of plastic biodegradation by marine microorganisms and the identification of possible directions for future research.
Diatoms are one of the major primary producers in the ocean, responsible annually for ~20% of photosynthetically fixed CO
on Earth. In oceanic models, they are typically represented as large (>20 µm) ...microphytoplankton. However, many diatoms belong to the nanophytoplankton (2-20 µm) and a few species even overlap with the picoplanktonic size-class (<2 µm). Due to their minute size and difficulty of detection they are poorly characterized. Here we describe a massive spring bloom of the smallest known diatom (Minidiscus) in the northwestern Mediterranean Sea. Analysis of Tara Oceans data, together with literature review, reveal a general oversight of the significance of these small diatoms at the global scale. We further evidence that they can reach the seafloor at high sinking rates, implying the need to revise our classical binary vision of pico- and nanoplanktonic cells fueling the microbial loop, while only microphytoplankton sustain secondary trophic levels and carbon export.
The thin film of life that inhabits all plastics in the oceans, so-called "plastisphere," has multiple effects on the fate and impacts of plastic in the marine environment. Here, we aimed to evaluate ...the relative influence of the plastic size, shape, chemical composition, and environmental changes such as a phytoplankton bloom in shaping the plastisphere abundance, diversity and activity. Polyethylene (PE) and polylactide acid (PLA) together with glass controls in the forms of meso-debris (18 mm diameter) and large-microplastics (LMP; 3 mm diameter), as well as small-microplastics (SMP) of 100 μm diameter with spherical or irregular shapes were immerged in seawater during 2 months. Results of bacterial abundance (confocal microscopy) and diversity (16S rRNA Illumina sequencing) indicated that the three classical biofilm colonization phases (primo-colonization after 3 days; growing phase after 10 days; maturation phase after 30 days) were not influenced by the size and the shape of the materials, even when a diatom bloom (
sp.) occurred after the first month of incubation. However, plastic size and shape had an effect on bacterial activity (
H leucine incorporation). Bacterial communities associated with the material of 100 μm size fraction showed the highest activity compared to all other material sizes. A mature biofilm developed within 30 days on all material types, with higher bacterial abundance on the plastics compared to glass, and distinct bacterial assemblages were detected on each material type. The diatom bloom event had a great impact on the plastisphere of all materials, resulting in a drastic change in diversity and activity. Our results showed that the plastic size and shape had relatively low influence on the plastisphere abundance, diversity, and activity, as compared to the plastic composition or the presence of a phytoplankton bloom.
The European Parliament recently approved a new law banning single-use plastic items for 2021 such as plastic plates, cutlery, straws, cotton swabs, and balloon sticks. Transition to a bioeconomy ...involves the substitution of these banned products with biodegradable materials. Several materials such as polylactic acid (PLA), polybutylene adipate terephthalate (PBAT), poly(butylene succinate) (PBS), polyhydroxybutyrate-valerate (PHBV), Bioplast, and Mater-Bi could be good candidates to substitute cotton swabs, but their biodegradability needs to be tested under marine conditions. In this study, we described the microbial life growing on these materials, and we evaluated their biodegradability in seawater, compared with controls made of non-biodegradable polypropylene (PP) or biodegradable cellulose. During the first 40 days in seawater, we detected clear changes in bacterial diversity (Illumina sequencing of 16S rRNA gene) and heterotrophic activity (incorporation of
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H-leucine) that coincided with the classic succession of initial colonization, growth, and maturation phases of a biofilm. Biodegradability of the cotton swab sticks was then tested during another 94 days under strict diet conditions with the different plastics as sole carbon source. The drastic decrease of the bacterial activity on PP, PLA, and PBS suggested no bacterial attack of these materials, whereas the bacterial activity in PBAT, Bioplast, Mater-Bi, and PHBV presented similar responses to the cellulose positive control. Interestingly, the different bacterial diversity trends observed for biodegradable vs. non-biodegradable plastics allowed to describe potential new candidates involved in the degradation of these materials under marine conditions. This better understanding of the bacterial diversity and activity dynamics during the colonization and biodegradation processes contributes to an expanding baseline to understand plastic biodegradation in marine conditions and provide a foundation for further decisions on the replacement of the banned single-used plastics.
The circulation of seawater through permeable coastal sediments is increasingly recognized as an important source of nutrients, including dissolved silica (DSi), to the coastal ocean. Here, we ...utilized a Ra isotope (223Ra, 224Raex, 228Ra) mass balance to quantify DSi fluxes driven by water circulation to a small shallow coastal lagoon (La Palme; French Mediterranean) during June 2016, as compared to karstic groundwater spring inputs. The DSi flux driven by lagoon water circulation (derived from 224Raex) was approximately one order of magnitude greater (1900 ± 1700 mol d−1) than the DSi load of the karstic groundwater spring (250 ± 50 mol d−1) and greater than molecular diffusion (970 ± 750 mol d−1). Lagoon water circulation was a negligible source of 228Ra, indicating that circulation-driven DSi inputs occur over a time-scale of days. Offshore transects were studied to quantify fluxes of marine-derived submarine groundwater discharge (SGD) from the permeable sandy coastline adjacent to the lagoon, into the Mediterranean Sea. Surface water transects revealed near-shore enrichments of Ra and DSi, attributed to wave-setup and water exchange through the permeable beach between the lagoon and the sea. Upscaling over the 9.5 km stretch of sandy beaches results in a marine SGD-driven DSi flux of 2.3 ± 1.3 × 104 mol d−1, similar in magnitude to the Têt river during November 2016 (3.3 ± 2.4 × 104 mol d−1), the largest river in the region. A positive relationship between DSi and 224Raex in lagoon water and seawater, but not 228Ra, suggests that 224Raex and DSi enrichments are derived from a similar source, the sediment (i.e. lithogenic particle dissolution), operating on short time-scales. A marine SGD-driven DSi flux to the Gulf of Lions (3.8 ± 2.2 × 105 mol d−1) is likely continuous over time. The relatively constant DSi inputs from water circulation for the shallow lagoons and beaches along the French Mediterranean Sea may sustain primary production in the coastal zone. In comparison, terrestrial groundwater and rivers supply temporally variable nutrient (N, P, Si) inputs via changes in regional precipitation, runoff and aquifer storage.
The North Western Mediterranean Sea exhibits recurrent and significant autumnal and spring phytoplankton blooms. The existence of these two blooms coincides with typical temperate dynamics. To ...determine the potential control of physical and biogeochemical factors on these phytoplankton blooms, data from a multiplatform approach (combining ships, Argo and BGC‐Argo floats, and bio‐optical gliders) were analyzed in association with satellite observations in 2012–2013. The satellite framework allowed a simultaneous analysis over the whole annual cycle of in situ observations of mixed layer depth, photosynthetical available radiation, particle backscattering, nutrients (nitrate and silicate), and chlorophyll‐a concentrations. During the year 2012–2013, satellite ocean color observations, confirmed by in situ data, have revealed the existence of two areas (or bioregions) with comparable autumnal blooms but contrasting spring blooms. In both bioregions, the ratio of the euphotic zone (defined as the isolume 0.415 mol photons m−2 d−1, Z0.415) and the MLD identified the initiation of the autumnal bloom, as well as the maximal annual increase in Chl‐a in spring. In fact, the autumnal phytoplankton bloom might be initiated by mixing of the summer shallowing deep chlorophyll maximum, while the spring restratification (when Z0.415/MLD ratio became >1) might induce surface phytoplankton production that largely overcomes the losses. Finally, winter deep convection events that took place in one of the bioregions induced higher net accumulation rate of phytoplankton in spring associated with a diatom‐dominated phytoplankton community principally. We suggest that very deep winter MLD lead to an increase in surface silicates availability, which favored the development of diatoms.
Key Points
The initiation of the autumnal bloom is linked to the mixing of the shallowing summer DCM
Heat fluxes and restratification of the mixed layer control the spring bloom dynamics
Deep winter convection events enhance the spring bloom's magnitude by favoring diatoms development
The HYdrological cycle in the Mediterranean Experiment (HyMeX) Special Observing Period 2 (SOP2, January 27–March 15, 2013) was dedicated to the study of dense water formation in the Gulf of Lion in ...the northwestern Mediterranean. This paper outlines the deep convection of winter 2012–2013 and the meteorological conditions that produced it. Alternating phases of mixing and restratification are related to periods of high and low heat losses, respectively. High-resolution, realistic, three-dimensional models are essential for assessing the intricacy of buoyancy fluxes, horizontal advection, and convective processes. At the submesoscale, vertical velocities resulting from symmetric instabilities of the density front bounding the convection zone are crucial for ventilating the deep ocean. Finally, concomitant atmospheric and oceanic data extracted from the comprehensive SOP2 data set highlight the rapid, coupled evolution of oceanic and atmospheric boundary layer characteristics during a strong wind event.
To understand and predict the physical, chemical, and biological processes at play in coastal and nearshore marine areas requires an integrated, interdisciplinary approach. This case study of the ...French structuration of coastal ocean and nearshore observing systems provides an original overview on a federative research infrastructure named ILICO. It is a notable example of national structuration and pan-institution efforts to investigate the forefront of knowledge on the processes at work within the critical coastal zone. ILICO comprises, in a pluridisciplinary approach, eight distributed network-systems of observation and data analysis that are accredited and financially supported by French research institutions and the French Ministry for Higher Education, Research, and Innovation. ILICO observation points are implemented along metropolitan and overseas French coasts, where coastline dynamics, sea level evolution, physical and biogeochemical water properties, coastal water dynamics, phytoplankton composition, and health of coral reefs are monitored in order to address a wide range of scientific questions. To give an overview of the diversity and potential of the observations carried out, this paper offers a detailed presentation of three constituting networks: SOMLIT, with homogeneous sampling strategies, DYNALIT, with heterogeneous sampling strategies adapted to different environments, and MOOSE, an integrated, pluri-disciplinary coastal/offshore regional observatory in the north-western Mediterranean Sea. ILICO was conceived using a European framework. It addresses the great challenges of the next decade in terms of sustainability, cost-efficiency, interoperability, and innovation. This paper emphasizes the added-value of federating these systems, and highlights some recommendations for the future.
Aeolian inputs of organic and inorganic nutrients to the ocean are important as they can enhance biological production in surface waters, especially in oligotrophic areas like the Mediterranean. The ...Mediterranean littoral is particularly exposed to both anthropogenic and Saharan aerosol depositions on a more or less regular basis. During the last few decades experimental studies have been devoted to examining the effect of inorganic nutrient inputs from dust on microbial activity. In this study, we performed experiments at two different locations of the NW Mediterranean, where we evaluated the changes in the quality and quantity of dissolved organic matter due to atmospheric inputs of different origin (Saharan and anthropogenic) and its subsequent transformations mediated by microbial activities. In both experiments the humic-like and protein-like substances, and the fluorescence quantum yield increased after addition. In general, these changes in the quality of dissolved organic matter did not significantly affect the prokaryotes. The recalcitrant character of the fluorescent dissolved organic matter (FDOM) associated with aerosols was confirmed, as we found negligible utilization of chromophoric compounds over the experimental period. We framed these experiments within a two-year time series data set of atmospheric deposition and coastal surface water analyses. These observations showed that both Saharan and anthropogenic inputs induced changes in the quality of organic matter, increasing the proportion of FDOM substances. This increase was larger during Saharan dust events than in the absence of Saharan influence.
Determination of submarine groundwater discharge (SGD) from karstic coastal aquifers is important to constrain hydrological and biogeochemical cycles. However, SGD quantification using commonly ...employed geochemical methods can be difficult to constrain under the presence of large riverine inputs, and is further complicated by the determination of the karstic groundwater endmember. Here, we investigated a coastal region where groundwater discharges from a karstic aquifer system using airborne thermal infrared mapping and geochemical sampling conducted along offshore transects. We report radium data (223Ra, 224Ra, 228Ra) that we used to derive fluxes (water, nutrients) associated with terrestrial groundwater discharge and/or seawater circulation through the nearshore permeable sediments and coastal aquifer. Field work was conducted at different periods of the year to study the temporal variability of the chemical fluxes. Offshore transects of 223Ra and 224Ra were used to derive horizontal eddy diffusivity coefficients that were subsequently combined with surface water nutrient gradients (NO2- + NO3-, DSi) to determine the net nutrient flux from SGD. The estimated DSi and (NO2- + NO3-) fluxes are 6.2 ± 5.0 *103 and 4.0 ± 2.0 *103 mol d-1 per kilometer of coastline, respectively. We attempted to further constrain these SGD fluxes by combining horizontal eddy diffusivity and 228Ra gradients. SGD endmember selection in this area (terrestrial groundwater discharge versus porewater) adds further uncertainty to the flux calculation and thus prevented us from calculating a reliable flux. Additionally, the relatively long half-life of 228Ra (5.75 y) makes it sensitive to specific circulation patterns in this coastal region, including sporadic intrusions of Rhône river waters that impact both the 228Ra and nutrient surface water distributions. We show that SGD nutrient fluxes locally reach up to 20 times the nutrient fluxes from a small river (Huveaune River). On a regional scale, DSi fluxes driven by SGD vary between 0.1 and 1.4 % of the DSi inputs of the Rhône River, while the (NO2- + NO3-) fluxes driven by SGD on this 22 km long coastline are between 0.1 – 0.3 % of the Rhône River inputs, the largest river that discharges into the Mediterranean Sea.
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