Phytoplankton guilds are commonly characterised by dominance effects, while the main contribution to biological diversity is given by rare species. Here, we analysed the influence of rare species on ...taxonomic and functional diversity, which is described by taxa richness and composition, cell size, and size–abundance relationships in phytoplankton guilds. We explore these relationships at global and regional scales by analysing phytoplankton guilds from five biogeographical regions: the Northern Atlantic Ocean (Scotland), the South-Western Atlantic Ocean (Brazil), the South-Western Pacific Ocean (Australia), the Indo-Pacific Ocean (Maldives), and the Mediterranean Sea (Greece and Turkey). We have comparatively analysed the phytoplankton taxonomic diversity of the whole dataset and with the datasets obtained by progressively subtracting taxa occurring in the last 1%, 5%, 10%, and 25% of both numerical abundance and overall biomass. Globally, 306 taxa were identified across five ecoregions with only 27 taxa accounting for 75% of overall numerical abundance and biomass; almost 50% of taxa were lost on every step. The removal of 1% of most rare taxa significantly affected the phytoplankton size–abundance relationships and body-size structure, strongly impacting on small taxa. The progressive removal of additional rare taxa did not further affect phytoplankton size–abundance relationships and size structure.
•First record of Azadinium poporum ribotype C from the Mediterranean Sea.•Azadinium poporum ribotype A from Greece produce low level or no AZA.•Azadinium poporum ribotype C from Greece produce ...predominantly AZA-40.
The marine dinoflagellate Azadinium poporum produce azaspiracids (AZA) and has been recorded widely in the world. However, information on its biogeography is still limited, especially in view of the fact that A. poporum comprises several genetically differentiated groups. A total of 18 strains of A. poporum were obtained from the Eastern Mediterranean area by incubating surface sediment collected from Ionian Sea of Greece. The morphology of these strains was examined with light microscopy and scanning electron microscopy. Small subunit ribosomal DNA (SSU rDNA), large subunit ribosomal DNA (LSU rDNA) and internal transcribed spacer (ITS) sequences were obtained from all cultured strains. Molecular phylogeny based on concatenated SSU, LSU and ITS sequences confirmed three ribotypes within A. poporum and revealed two subclades within ribotypes A and C. Greek strains of A. poporum ribotype A were nested within ribotype A2 together with strains from Western Mediterranean Sea and French Atlantic, and Greek strains of A. poporum ribotype C were nested within ribotype C2 together with a strain from the Gulf of Mexico. Growth experiments on four selected strains revealed that ribotypes A and C from Greece differed in their growth at higher temperatures, indicating that they are physiologically differentiated. Azaspiracid profiles were analyzed for 15 cultured A. poporum strains using LCMS/MS and demonstrate that the A. poporum ribotype A from Greece produce low level or no AZA and A. poporum ribotype C from Greece produces predominantly AZA-40 (9.6–30.2 fg cell−1) followed by AZA-2 (2.1–2.6 fg cell−1). The first record of AZA-40 producing A. poporum from the Mediterranean suggests that this species is a potential source for azaspiracid contaminations in shellfish from the Eastern Mediterranean Sea.
Introduction
Marine 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.
Methods
In 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 Discussion
Our 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.
The Mediterranean Sea has been sampled irregularly by research vessels in the past, mostly by national expeditions in regional waters. To monitor the hydrographic, biogeochemical and circulation ...changes in the Mediterranean Sea, a systematic repeat oceanographic survey programme called Med-SHIP was recommended by the Mediterranean Science Commission (CIESM) in 2011, as part of the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP). Med-SHIP consists of zonal and meridional surveys with different frequencies, where comprehensive physical and biogeochemical properties are measured with the highest international standards. The first zonal survey was done in 2011 and repeated in 2018. In addition, a network of meridional (and other key) hydrographic sections were designed: the first cycle of these sections was completed in 2016, with three cruises funded by the EU project EUROFLEETS2. This paper presents the physical and chemical data of the meridional and key transects in the Western and Eastern Mediterranean Sea collected during those cruises.
Barely any data exist on metal speciation in the marine surface microlayer (SML), a rather complex environment, the study of which contributes to enhancement of knowledge on metal speciation and its ...effect to the ocean. Metal speciation is significant since life requires a wide variety of trace metals that are essential for the growth of the organisms. Ligand concentrations (
L
) of copper (Cu), zinc (Zn), and cadmium (Cd) were studied by anodic stripping voltammetry (ASV) in the SML of coastal areas within the Aegean Sea in the Eastern Mediterranean. Complexing capacities in the SML in decreasing order ranged for Cu from 29 to 201 nM (median 101 nM), for Zn 24–149 nM (45 nM), and for Cd 1.0–1.5 nM (1.4 nM). Average enrichment factors (EFs) of SML samples compared to subsurface ones (SSW) were calculated equal to 0.9 ± 0.8, 1.2 ± 0.5, and 1.7 ± 1.6 for
L
Zn
,
L
Cu
, and
L
Cd
, respectively. In five out of the six total paired samples of SML and SSW, lower concentrations of
L
Zn
were measured in the SML, which is not the case for
L
Cu
and
L
Cd
. Due to elevated dissolved Zn concentrations in the SML, its complexation is incomplete, contrary to those of Cu and Cd, which are fully complexed. These trace metals are essential nutrients for biological functions, hence any differences on their concentration and chemical speciation may directly influence the distribution of phytoplankton species in the upper water column and neuston. EFs of SML relatively to subsurface water ranged in average between 1.2 and 2.4 for total organic carbon (TOC), chlorophyll
a
(Chl
a
), and plankton, being generally >2 for most of the amino acids detected, demonstrating a relative enrichment of the SML in organic matter. A significant correlation was found between Cu ligands and dinoflagellates <20 μm, confirming older findings supporting that marine dinoflagellates of
Gymnodinium
genera produce Cu ligands. New insights are provided in the study and the importance of investigating bio-essential metal ions (Cu, Zn, Cd) and their organic complexes in the SML is pointed. Data on Zn and Cd complexing capacities in the SML are the first published so far.
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.
The effect of episodicity of Saharan dust deposition on the pelagic microbial food web was studied in the oligotrophic Eastern Mediterranean by means of a mesocosm experiment in May 2014. Two ...different treatments in triplicates (addition of natural Saharan dust in a single-strong pulse or in three smaller consecutive doses of the same total quantity), and three unamended controls were employed; chemical and biological parameters were measured during a 10-day experiment. Temporal changes in primary (PP) and bacterial (BP) production, chlorophyll a (Chla) concentration and heterotrophic bacteria, Synechococcus and mesozooplankton abundance were studied. The results suggested that the auto- and hetero-trophic components of the food web (at least the prokaryotes) were enhanced by the dust addition (and by the nitrogen and phosphorus added through dust). Furthermore, a 1-day delay was observed for PP, BP and Chla increases when dust was added in three daily doses; however, the maximal values attained were similar in the two treatments. Although the effect was evident in the first osmotrophic level (phytoplankton and bacteria), it was lost further up the food web, masked under the impact of grazing exerted by predators such as heterotrophic flagellates, ciliates and dinoflagellates. This was partly proved by two dilution experiments. This study demonstrates the important role of atmospheric deposition and protist grazing when evaluating the effect on oligotrophic systems characterised by increased numbers of trophic levels.
The atmospheric deposition of gases and particulates from the Sahara Desert and European landmass is an important source of nutrients for the Mediterranean Sea. In this study, we investigated how ...such atmospheric input might affect bacterial metabolic activities and community dynamics in the ultra-oligotrophic Eastern Mediterranean Sea. Thus a mesocosm simulation experiment was conducted using “pure” Saharan dust (SD) and mixed aerosols (A, polluted and desert origin). The cell specific bacterial production (BP) was stimulated soon after the addition of SD and A, with a higher degree of stimulation being observed in the activity of Alphaproteobacteria than in Gammaproteobacteria, and this lead to significant increases in community BP. Subsequently, a shift between these two dominating classes was observed (such that the proportion of Gammaproteobacteria increased while that of Alphaproteobacteria decreased), along with significant increases in bacterial abundance and chlorophyll a concentration. After a few days, although the abundance of bacteria was still significantly higher in the SD- or A-treated groups, differences in the active community composition between the treatment and control groups were reduced. The altered activity of the two dominating Proteobacteria classes observed, might reflect their different strategies in responding to external nutrient input: with Alphaproteobacteria being more responsive to the direct dust input, whereas Gammaproteobacteria seemed to benefit more from the increase in phytoplankton biomass. In addition, the input of A had a stronger immediate effect and longer lasting influence on changing the active bacterial community composition than did that of SD. Our findings show that episodic atmospheric deposition events might affect the microbial community with regards to their abundance, activity and composition over a short period of time, and thus regulate the function of the microbial community and carbon cycling in oligotrophic waters.
Large amounts of dust and atmospheric aerosols, originating from surrounding desert areas (e.g., Sahara and Middle East) are deposited annually on the surface of the Eastern Mediterranean Sea. These ...depositions can provide high amounts of micro (such as Fe, Zn, Co) and macro nutrients (such as P and N) to supplement nutrient-poor surface waters- that typically limit primary productivity and also dinitrogen (N2) fixation in many marine environments. Here, we studied the impact of the atmospheric deposition of dust and aerosols on N2 fixation in the Cretan Sea (Eastern Mediterranean Sea). Mixed polluted aerosols (hereafter A) and Saharan dust (hereafter SD) were added to nine mesocosms (3-m3 each) containing surface mixed layer seawater (~10 m), and N2 fixation was evaluated for 6 days during May 2012 (springtime). The addition of SD triggered a rapid (30 h) and robust (2-4 fold) increase in N2 fixation rates that remained high for 6 days and contributed 3-8% of the primary productivity. The A addition also resulted in higher N2 fixation rates compared to the unamended control mesocosms, although the responses were less profound (1.5-2 fold) and accounted for only 2-4% of the primary productivity. The microbial community responded differently to the two additions. Heterotrophic bacterial N2 fixers dominated the diazotroph community in A and the control mesocosms, while the non-filamentous cyanobacterial group Trichodesmium prevailed in the SD treatment (68% of all the operational taxonomic units, verified by qPCR analyses). Our results indicate that the aerosol source, its route prior to deposition, and its specific chemical composition, can alter the diazotrophic diversity and activity in the Eastern Mediterranean Sea and may thus impact both the N and C dynamics in this impoverished environment.
The effects of atmospheric deposition on plankton community structure were examined during a mesocosm experiment using water from the Cretan Sea (Eastern Mediterranean), an area with a high frequency ...of atmospheric aerosol deposition events. The experiment was carried out under spring-summer conditions (May 2012). The main objective was to study the changes induced from a single deposition event, on the autotrophic and heterotrophic surface microbial populations, from viruses to zooplankton. To this end, the effects of Saharan dust addition were compared to the effects of mixed aerosol deposition on the plankton community over 9 days. The effects of the dust addition seemed to propagate throughout the food-web, with changes observed in nearly all of the measured parameters up to copepods. The dust input stimulated increased productivity, both bacterial and primary. Picoplankton, both autotrophic and heterotrophic capitalized on the changes in nutrient availability and microzooplankton abundance also increased due to increased availability of prey. Five days after the simulated deposition, copepods also responded, with an increase in egg production. The results suggest that nutrients were transported up the food web through autotrophs that were favored by the Nitrogen supplied through both treatments. Although the effects of individual events are generally short lived, increased deposition frequency and magnitude of events is expected in the area, due to predicted reduction in rainfall and increase in temperature, which can lead to more persistent changes in plankton community structure. Here we demonstrate how a single dust deposition event leads to enhancement of phytoplankton and microzooplankton and can eventually, through copepods, transport more nutrients up the food web in the Eastern Mediterranean Sea.
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