Bacterial Growth and Mortality after Deposition of Saharan Dust and Mixed Aerosols in the Eastern Mediterranean Sea: A Mesocosm Experiment

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Microplastics increase the marine production of particulate forms of organic matter

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Depth-driven patterns in lytic viral diversity, auxiliary metabolic gene content, and productivity in offshore oligotrophic waters

IntroductionMarine viruses regulate microbial population dynamics and biogeochemical cycling in the oceans. The ability of viruses to manipulate hosts’ metabolism through the expression of viral auxiliary metabolic genes (AMGs) was recently highlighted, having important implications in energy production and flow in various aquatic environments. Up to now, the presence and diversity of viral AMGs is studied using -omics data, and rarely using quantitative measures of viral activity alongside.MethodsIn the present study, four depth layers (5, 50, 75, and 1,000 m) with discrete hydrographic features were sampled in the Eastern Mediterranean Sea; we studied lytic viral community composition and AMG content through metagenomics, and lytic production rates through the viral reduction approach in the ultra-oligotrophic Levantine basin where knowledge regarding viral actions is rather limited.Results and DiscussionOur results demonstrate depth-dependent patterns in viral diversity and AMG content, related to differences in temperature, nutrients availability, and host bacterial productivity and abundance. Although lytic viral production rates were similar along the water column, the virus-to-bacteria ratio was higher and the particular set of AMGs was more diverse in the bathypelagic (1,000 m) than the shallow epipelagic (5, 50, and 75 m) layers, revealing that the quantitative effect of viruses on their hosts may be the same along the water column through the intervention of different AMGs. In the resource- and energy-limited bathypelagic waters of the Eastern Mediterranean, the detected AMGs could divert hosts’ metabolism toward energy production, through a boost in gluconeogenesis, fatty-acid and glycan biosynthesis and metabolism, and sulfur relay. Near the deep-chlorophyll maximum depth, an exceptionally high percentage of AMGs related to photosynthesis was noticed. Taken together our findings suggest that the roles of viruses in the deep sea might be even more important than previously thought as they seem to orchestrate energy acquisition and microbial community dynamics, and thus, biogeochemical turnover in the oceans

Dynamics and enzymatic degradation of exopolymer particles under increasing concentrations of silver ions and nanoparticles during a marine mesocosm experiment

Pollution of the marine environment is an emerging threat. Nowadays, engineered nanoparticles (<100 nm) such as zinc, copper and silver are widely used as antimicrobial agents, therefore often present in daily-life products. Consequently, the demand and production of nanoparticles are expected to increase. Here, we specifically focus on silver nanoparticles (AgNP). Once released into the environment, AgNPs pose an obvious ecotoxicological risk, potentially affecting ecosystem structure and functioning. For instance, phytoplankton-derived exudates, rich in acidic polysaccharides and amino acids, can abiotically aggregate into microgels such as transparent exopolymer particles (TEP) and Coomassie stainable particles (CSP). Hence, microgels can bridge dissolved and particulate size fractions and facilitate aggregate formation with organic and mineral particles. Both physical and chemical properties make TEP and CSP attractive nutrient hotspots for heterotrophic bacterioplankton. Bacteria, in turn, utilize extracellular enzymes to access these carbon and nitrogen pools. However, knowledge about the mechanisms by which AgNPs might interact with and affect the biogeochemical cycling of TEP and CSP is still insufficient. Therefore, we conducted a mesocosm experiment in the Eastern Mediterranean Sea and investigated the effects of environmentally relevant concentrations of silver ions (Ag+) and AgNP on the properties of TEP and CSP (i.e., area and abundance) along with enzymatic activity measurements. Our results showed that cyanobacteria were likely the primary source of CSP in the ultra-oligotrophic Mediterranean Sea. Also, CSP contributed more to the microgel pool than TEP, as indicated by a strong relationship between CSP and heterotrophic microbial dynamics. While silver (i.e., Ag+ or AgNP) had overall only marginal effects, both species affected the relationships between cell-specific LAPase activity and CSP and cell-specific APase activity and phosphate levels. Thus, Ag+ and AgNP have the potential to regulate microgel dynamics. However, future studies are needed to derive a robust understanding of the effects of silver pollution on the coupling of microgel formation and degradation and the follow-on effect on biogeochemical cycles

The potential impact of Saharan dust and polluted aerosols on microbial populations in the East Mediterranean Sea, an overview of a mesocosm experimental approach.

Recent estimates of nutrient budgets for the Eastern Mediterranean Sea (EMS) indicate that atmospheric aerosols play a significant role as suppliers of macro- and micro- nutrients to its Low Nutrient Low Chlorophyll water. Here we present the first mesocosm experimental study that examines the overall response of the oligotrophic EMS surface mixed layer (Cretan Sea, May 2012) to two different types of natural aerosol additions, “pure” Saharan dust (SD, 1.6 mg l-1) and mixed aerosols (A - polluted and desert origin, 1 mg l-1). We describe the rationale, the experimental set-up, the chemical characteristics of the ambient water and aerosols and the relative maximal biological impacts that resulted from the added aerosols. The two treatments, run in triplicates (3 m3 each), were compared to control-unamended runs. Leaching of approximately 2.1-2.8 and 2.2-3.7 nmol PO4 and 20-26 and 53-55 nmol NOx was measured per each milligram of SD and A, respectively, representing an addition of approximately 30% of the ambient phosphate concentrations. The nitrate/phosphate ratios added in the A treatment were twice than those added in the SD treatment. Both types of dry aerosols triggered a positive change (25-600% normalized per 1 mg l-1 addition) in most of the rate and state variables that were measured: bacterial abundance (BA), bacterial production (BP), Synechococcus (Syn) abundance, chlorophyll-a (chl-a), primary production (PP) and dinitrogen fixation (N2-fix), with relative changes among them following the sequence BP>PP≈N2-fix>chl-a≈BA≈Syn. Our results show that the ‘polluted’ aerosols triggered a relatively larger biological change compared to the SD amendments (per a similar amount of mass addition), especially regarding BP and PP. We speculate that despite the co-limitation of P and N in the EMS, the additional N released by the A treatment may have triggered the relatively larger response in most of the rate and state variables as compared to SD. An implication of our study is that a warmer atmosphere in the future may increase dust emissions and influence the intensity and length of the already well stratified water column in the EMS and hence the impact of the aerosols as a significant external source of new nutrients

Impacts of anthropogenic environmental pressures on viral and bacterial communities of coastal ecosystems

In this PhD dissertation, the relationship between lysogeny and four different conditions of anthropogenic environmental pressure was examined. The stucture of viral and bacterial communities was also described in relation to these pressures. The four conditions reflect anthropogenic environmental pressures that are either currently taking place in a large, medium or small scale, or predicted for the future.In particular, the first pressure concerns the input of engineered silver nanoparticles into the marine ecosystem, which is projected to increase in the near future with significant and unexplored impacts on plankton. The second pressure concerns the carbon dioxide input and the induced reduction in seawater pH, which is a chemical change that has already been measured in the oceans and is projected to continue. The third pressure is also a recorded chemical change in the marine ecosystem caused by the influx of contaminated aerosols. In the Mediterranean Sea, these aerosols are mainly of European origin and their entry causes changes in the balance of essential nutrients. Finally, the fourth pressure includes various anthropogenic activities that take place in the coastal waters along the Greek coastline that cause continuous chemical alterations. For example, the release of either heavy metals from industries or nutrients from aquaculture are two representative examples observed in these coastal systems on a continuous basis. Therefore, for the fourth study, natural communities were analyzed by field sampling to map the bacterial and viral community at a given time. In contrast to the fourth study, the experimental tool of the mesocosms, which is extensively applied to study specific pressures in the pelagic ecosystem, was used in the first three chapters, allowing experimentation on a large scale, with good repeatability and with all the trophic levels present. Knowing already that these pressures affect the bacterial communities, the aim of the dissertation was to check whether the influence is also observed at the level of the viral genome and/or on the viral life cycles. A basic hypothesis of the experiments was that if the pressures had a negative effect on planktonic bacteria, the effect would be primarily visible to viruses too, as viruses are a biological entity that controls and often stimulates cellular responses to higher trophic levels. In order to test the hypothesis, quantitative methods (production of lytic and lysogenic cells) and qualitative methods (genomic analysis) were used.Στην παρούσα διδακτορική διατριβή εξετάστηκε η σχέση μεταξύ της λυσιγονίας και τεσσάρων διαφορετικών συνθηκών ανθρωπογενούς περιβαλλοντικής πίεσης. Επίσης, περιγράφηκε η σύνθεση των κοινοτήτων ιών και βακτηρίων σε σχέση με αυτές τις πιέσεις. Οι τέσσερις συνθήκες απεικονίζουν ανθρωπογενείς περιβαλλοντικές πιέσεις που είτε λαμβάνουν χώρα στις μέρες μας σε μεγάλο, μέτριο ή μικρό βαθμό είτε προβλέπονται για το μέλλον.Πιο συγκεκριμένα, η πρώτη πίεση είναι η εισροή τεχνητών νανοσωματιδίων αργύρου στο θαλάσσιο οικοσύστημα που προβλέπεται να αυξηθεί στο κοντινό μέλλον με σημαντικές και ανεξερεύνητες συνέπειες στο πλαγκτόν. Η δεύτερη πίεση αφορά την εισροή διοξειδίου του άνθρακα (CO2) και την επαγόμενη μείωση του pH της θάλασσας που είναι μια χημική αλλαγή που έχει ήδη μετρηθεί στους ωκεανούς και προβλέπεται να συνεχιστεί. Η τρίτη πίεση είναι επίσης μια καταγεγραμμένη χημική αλλαγή στο θαλάσσιο οικοσύστημα που προκαλείται από την εισροή ρυπασμένων αερολυμάτων. Στη Μεσόγειο Θάλασσα, αυτά τα αερολύματα έχουν κυρίως Ευρωπαϊκή προέλευση και η είσοδός τους προκαλεί αλλαγές στο ισοζύγιο βασικών θρεπτικών στοιχείων. Τέλος, η τέταρτη πίεση περιλαμβάνει τις ποικίλες ανθρωπογενείς δραστηριότητες που λαμβάνουν χώρα στα παράκτια νερά της Ελληνικής ακτογραμμής και προκαλούν συνεχείς χημικές αλλοιώσεις. Για παράδειγμα, η απελευθέρωση είτε βαρέων μετάλλων από βιομηχανίες είτε θρεπτικών από υδατοκαλλιέργειες είναι δύο ενδεικτικά παραδείγματα που παρατηρούνται σε αυτά τα παράκτια συστήματα σε συνεχή βάση. Συνεπώς, για την τέταρτη μελέτη αναλύθηκαν φυσικές κοινότητες με δειγματοληψία πεδίου, ώστε να αποτυπωθεί η βακτηριακή και ιική κοινότητα μια δεδομένη στιγμή. Εν αντιθέσει με την τέταρτη μελέτη, στις τρεις πρώτες χρησιμοποιήθηκε το πειραματικό εργαλείο των μεσοκόσμων, που χρησιμοποιούνται εκτενώς για να μελετηθούν συγκεκριμένες μορφές πίεσης στο πελαγικό οικοσύστημα, καθώς επιτρέπουν τον πειραματισμό σε μεγάλη κλίμακα, με καλή επαναληψιμότητα και με όλο το μικροβιακό πελαγικό τροφικό πλέγμα παρόν. Γνωρίζοντας ήδη πως αυτές οι πιέσεις επηρεάζουν τις βακτηριακές κοινότητες, στόχος της διατριβής ήταν να ελεγχθεί αν η επιρροή παρατηρείται επίσης στο επίπεδο του ιικού γονιδιώματος ή/και του ιικού λυσιγονικού κύκλου ζωής. Μια βασική υπόθεση των πειραμάτων ήταν πως αν οι πιέσεις έχουν αρνητική επίδραση στα πλαγκτονικά βακτήρια, τότε η επίδραση θα είναι ορατή πρωτίστως στους ιούς, καθώς οι ιοί είναι ένας βιολογικός παράγοντας που ελέγχει και συχνά υποκινεί κυτταρικές αποκρίσεις των ανώτερων τροφικών επιπέδων. Για να ελεγχθεί η υπόθεση, χρησιμοποιήθηκαν ποσοτικές μέθοδοι (παραγωγή λυτικών και λυσιγονικών κυττάρων) και ποιοτικές μέθοδοι (γονιδιωματική ανάλυση)

Responses of Free-Living Planktonic Bacterial Communities to Experimental Acidification and Warming

Climate change driven by human activities encompasses the increase in atmospheric CO2 concentration and sea-surface temperature. Little is known regarding the synergistic effects of these phenomena on bacterial communities in oligotrophic marine ecosystems that are expected to be particularly vulnerable. Here, we studied bacterial community composition changes based on 16S rRNA sequencing at two fractions (0.1–0.2 and >0.2 μm) during a 10- day fully factorial mesocosm experiment in the eastern Mediterranean where the pH decreased by ~0.3 units and temperature increased by ~3 °C to project possible future changes in surface waters. The bacterial community experienced significant taxonomic differences driven by the combined effect of time and treatment; a community shift one day after the manipulations was noticed, followed by a similar state between all mesocosms at the third day, and mild shifts later on, which were remarkable mainly under sole acidification. The abundance of Synechococcus increased in response to warming, while the SAR11 clade immediately benefited from the combined acidification and warming. The effect of the acidification itself had a more persistent impact on community composition. This study highlights the importance of studying climate change consequences on ecosystem functioning both separately and simultaneously, considering the ambient environmental parameters

Corrigendum to "Ocean acidification effect on prokaryotic metabolism tested in two diverse trophic regimes in the mediterranean sea" [Estuar. Coast shelf sci. 186 (2017) 125-138]

International audienceThe authors regret for the wrong order of the name/surname of the third author. The correct list of authors is therefore as follows: Mauro Celussia,∗, Francesca Malfattia, Annalisa Franzoa, Frédéric Gazeaub,c, Antonia Giannakouroud, Paraskevi Pittae, Anastasia Tsiolae, Paola Del Negroa

Ocean acidification effect on prokaryotic metabolism tested in two diverse trophic regimes in the Mediterranean Sea

International audienc