Oligotrophic Eastern Mediterranean Sea Research Articles

Relationships of pelagic ciliates with the microbial food web components at a coastal station in the oligotrophic Eastern Mediterranean Sea: temporal and vertical variability.

The annual/temporal and vertical dynamics of the microbial food web (MFW) was studied in a coastal station of the oligotrophic Eastern Mediterranean Sea. The present study analyzed the changes of all components of the MFW with a specific focus on the relationships between different size classes of heterotrophic and mixotrophic ciliates with their potential prey. The MFW was dominated by heterotrophic picoplankton in all months and depths analyzed, whereas autotrophic nanoplankton took advantage in cold months with higher nutrient availability. On the other hand, mixotrophic microplankton biomass was higher in summer when nutrients and chlorophyll-a were scarce.As part of the mixotrophic biomass, mixotrophic ciliates were correlated with their “potential” prey at the surface and deep chlorophyll maximum. Large mixotrophic ciliates (L. strobila) were more selective in terms of potential prey, showing a correlation with Synechococcus. On the other hand, mixotrophic nanociliates (Strombidium dalum) were correlated differently with different potential prey according to depth, supporting the idea that nanociliates could be more generalists in terms of prey selection. Because the relationships between mixotrophic ciliates and their potential prey are still poorly studied, this work represents the start for further investigation.

Deep Water Nutrient Supply for an Offshore Ulva sp. Cultivation Project in the Eastern Mediterranean Sea: Experimental Simulation and Modeling

Offshore cultivation of marine macroalgae is a potential sustainable resource for fuel, food, and chemicals. Offshore, the high productivity of macroalgae cultivation depends on external nitrogen supply. The current work examines the idea of supplying nitrogen for Ulva sp. cultivation in the oligotrophic Eastern Mediterranean Sea (EMS) by artificial upwelling of nutrient-rich deep seawater (DSW). Growth rates, protein, and starch contents of Ulva sp. were measured for time varying fertilizations with nitrate concentrations corresponding to nutrient concentrations of DSW at increasing depths of the EMS. A maximal relative growth rate of 7.4% was measured for fertilizing ten times per week with 5.8 μM, which corresponds to the artificial upwelling from the depth of 700 m at EMS. Protein and starch contents ranged between 1–6 and 8–15% of dry weight. Finally, yields and energetic costs of DSW pumping were modeled for an example case of 10-ha offshore farm. The model predicts a high productivity but low energetic efficiency, which can be improved by coupling the biomass production with offshore power sources such as ocean thermal energy conversion.

Trophic positioning of prominent copepods in the epi- and mesopelagic zone of the ultra-oligotrophic eastern Mediterranean Sea

Combined analyses of Fatty Acid Trophic Markers (FATM) and Stable Isotopes (SI) were used to characterize food preferences among copepod species/taxa and to trace their food sources in the ultra-oligotrophic Cretan Passage of the Eastern Mediterranean Sea (EMS). FATMs are based on the conservation and transfer of specific source Fatty Acids (FAs) through the food web, providing a qualitative indicator of assimilated food types. SI provide information about trophic levels and food sources of specific organisms. Most species/taxa exhibited low δ15N ratios caused by low δ15N-PON base values at the study site. A cluster analysis based on the mean Fatty Acid (FA) compositions of nine copepods from all stations identified three distinct groups (group 1: Clausocalanus lividus and Euchaeta spp.; group 2: Pleuromamma spp., Haloptilus longicornis, Lucicutia spp., Clausocalanus spp. and Corycaeus spp.; group 3: Pareucalanus attenuatus and Oncaea spp.), coinciding with similarities in feeding behavior. H. longicornis, however, revealed similar trophic positions but different food resources by FATM and SIA analyses. Overall, we conclude that omnivory was the prevailing feeding mode, demonstrating a high degree of opportunistic feeding of copepods in these ultra-oligotrophic waters. This study shed first light on the zooplankton taxa/species life strategies with regard to feeding preferences and lipid storage mechanisms in the oligotrophic EMS, where lack of relevant information exists. Although further investigation is required, the good agreement between FA and SI in some species of copepods emphasize the applicability of lipid trophic markers. Results benefit from the coupling of these indices even in oligotrophic regions of the world ocean.

The deep water of Eastern Mediterranean Sea is a hotspot for bacterial activity

We investigated the abundance and production of mesopelagic (150–650 m) and bathypelagic (>650 m) bacteria from the poorly-studied Levantine basin (Eastern Mediterranean Sea) during the summers of 2014–2017. We show that the mesopelagic and bathypelagic microbial populations have relatively high cell specific activity, amongst the highest worldwide, likely due to the relatively warm temperatures characterizing these depths in comparison to other oceanic regions. Furthermore, in order to investigate if mesopelagic and bathypelagic microbial populations are limited by the scarce organic carbon substrates in the area, we performed microcosm addition assays of pure glucose (Glu) and/or a mixture of dissolved free amino acids (DFAA). Both the mesopelagic and bathypelagic bacterial populations responded positively to the additions, with bacterial production rapidly increasing within a few hours following the Glu or DFAA amendments. Our results demonstrate that deep-water microbes in the Levantine Basin may be important nutrient recyclers, especially when labile organic carbon is introduced. Our observations may have implications on carbon cycling and sequestration in the oligotrophic Eastern Mediterranean Sea.

Composition and sources of sedimentary organic matter in the deep eastern Mediterranean Sea

Abstract. Surface sediments collected from deep slopes and basins (1018–4087 m depth) of the oligotrophic eastern Mediterranean Sea have been analysed for bulk elemental and isotopic composition of organic carbon, total nitrogen and selected lipid biomarkers, jointly with grain size distribution and other geochemical proxies. The distribution and sources of sedimentary organic matter (OM) have been subsequently assessed and general environmental variables, such as water column depth and physical circulation patterns, have been examined as causative factors of deep-sea sediment characteristics. Lithogenic and biogenic carbonates are the dominant sedimentary fractions, accounting for up to 85.4 and 66.5 % of the total weight respectively. The low OC and TN contents in the surface sediments of the study area, which ranged from 0.15 to 1.15 % and 0.06 to 0.11 % respectively, reflect the oligotrophic character of the eastern Mediterranean Sea. Both bulk and molecular organic tracers reflect a mixed contribution from autochthonous and allochthonous sources for the sedimentary OM, as indicated by relatively degraded marine OM, terrestrial plant waxes and anthropogenic OM (e.g. degraded petroleum by-products) respectively. Wide regional variations have been observed amongst the studied proxies, which reflect the multiple factors controlling sedimentation in the deep eastern Mediterranean Sea. Our findings highlight the role of deep eastern Mediterranean basins as depocentres of organic-rich fine-grained sediments (mean 5.4 ± 2.4 μm), with OM accumulation and burial being attributed to aggregation mechanisms and hydrodynamic sorting. A multi-proxy approach is applied aiming to investigate the biogeochemical composition of sediment samples, which sheds new light on the sources and transport mechanisms along with the impact of preservation vs. diagenetic processes on the composition of sedimentary OM in the deep basins of the oligotrophic eastern Mediterranean Sea.

Nutrient Limitation in Surface Waters of the Oligotrophic Eastern Mediterranean Sea: an Enrichment Microcosm Experiment.

The growth rates of planktonic microbes in the pelagic zone of the Eastern Mediterranean Sea are nutrient limited, but the type of limitation is still uncertain. During this study, we investigated the occurrence of N and P limitation among different groups of the prokaryotic and eukaryotic (pico-, nano-, and micro-) plankton using a microcosm experiment during stratified water column conditions in the Cretan Sea (Eastern Mediterranean). Microcosms were enriched with N and P (either solely or simultaneously), and the PO4 turnover time, prokaryotic heterotrophic activity, primary production, and the abundance of the different microbial components were measured. Flow cytometric and molecular fingerprint analyses showed that different heterotrophic prokaryotic groups were limited by different nutrients; total heterotrophic prokaryotic growth was limited by P, but only when both N and P were added, changes in community structure and cell size were detected. Phytoplankton were N and P co-limited, with autotrophic pico-eukaryotes being the exception as they increased even when only P was added after a 2-day time lag. The populations of Synechococcus and Prochlorococcus were highly competitive with each other; Prochlorococcus abundance increased during the first 2 days of P addition but kept increasing only when both N and P were added, whereas Synechococcus exhibited higher pigment content and increased in abundance 3 days after simultaneous N and P additions. Dinoflagellates also showed opportunistic behavior at simultaneous N and P additions, in contrast to diatoms and coccolithophores, which diminished in all incubations. High DNA content viruses, selective grazing, and the exhaustion of N sources probably controlled the populations of diatoms and coccolithophores.

Uncoupling between dinitrogen fixation and primary productivity in the eastern Mediterranean Sea

In the nitrogen (N)‐impoverished photic zones of many oceanic regions, prokaryotic organisms fixing atmospheric dinitrogen (N2; diazotrophs) supply an essential source of new nitrogen and fuel primary production. We measured dinitrogen fixation and primary productivity (PP) during the thermally stratified summer period in different water regimes of the oligotrophic eastern Mediterranean Sea, including the Cyprus Eddy and the Rhodes Gyre. Low N2 fixation rates were measured (0.8–3.2 µmol N m−2 d−1) excluding 10‐fold higher rates in the Rhodes Gyre and Cyprus Eddy (~20 µmol N m−2 d−1). The corresponding PP increased from east to west (200–2500 µmol C m−2 d−1), with relatively higher productivity recorded in the Rhodes Gyre and Cyprus Eddy (2150 and 2300 µmol C m−2 d−1, respectively). These measurements demonstrate that N2 fixation in the photic zone of the eastern Mediterranean Sea contributes only negligibly by direct inputs to PP (i.e., cyanobacterial diazotrophs) and is in fact uncoupled from PP. By contrast, N2 fixation is significantly coupled to bacterial productivity and to net heterotrophic areas, suggesting that heterotrophic N2 fixation may in fact be significant in this ultraoligotrophic system. This is further substantiated by the high N2 fixation rates we measured from aphotic depths and by the results of phylogenetic analysis in other studies showing an abundance of heterotrophic diazotrophs.

Thalassospira sp. Isolated from the Oligotrophic Eastern Mediterranean Sea Exhibits Chemotaxis toward Inorganic Phosphate during Starvation

The eastern Mediterranean Sea represents an ultraoligotrophic environment where soluble phosphate limits the growth of bacterioplankton. Correspondingly, genes coding for high-affinity phosphate uptake systems and for organophosphonate utilization are highly prevalent in the plankton metagenome. Chemotaxis toward inorganic phosphate constitutes an alternative strategy to cope with phosphate limitation, but so far has only been demonstrated for two bacterial pathogens and an archaeon, and not in any free-living planktonic bacterium. In the present study, bacteria affiliated with the genus Thalassospira were found to constitute a regular, low-abundance member of the bacterioplankton that can be detected throughout the water column of the eastern Mediterranean Sea. A representative (strain EM) was isolated in pure culture and exhibited a strong positive chemotaxis toward inorganic phosphate that was induced exclusively in phosphate-starved cultures. Phosphate-depleted cells were 2-fold larger than in exponentially growing cultures, and 43% of the cells retained their motility even during prolonged starvation over 10 days. In addition, Thalassospira sp. strain EM was chemotactically attracted by complex substrates (yeast extract and peptone), amino acids, and 2-aminoethylphosphonate but not by sugar monomers. Similarly to the isolate from the eastern Mediterranean, chemotaxis toward phosphate was observed in starved cultures of the other two available isolates of the genus, T. lucentensis DSM 14000T and T. profundimaris WP0211T. Although Thalassospira sp. represents only up to 1.2% of the total bacterioplankton community in the water column of the eastern Mediterranean Sea, its chemotactic behavior potentially leads to an acceleration of nutrient cycling and may also explain the persistence of marine copiotrophs in this extremely nutrient-limited environment.

Sediment trap and in-situ pump size-fractionated POC/ 234Th ratios in the Mediterranean Sea and Northwest Atlantic: Implications for POC export

Measurements of particle size-fractionated POC/ 234Th ratios and 234Th and POC fluxes were conducted using surface-tethered, free-floating, sediment traps and large-volume in-situ pumps during four cruises in 2004 and 2005 to the oligotrophic eastern Mediterranean Sea and the seasonally productive western Mediterranean and northwest Atlantic. Analysis of POC/ 234Th ratios in sediment trap material and 10, 20, 53, 70, and 100 μm size-fractionated particles indicate, for most stations, decreasing ratios with depth, a weak dependence on particle size, and ratios that converge to ∼1–5 μmol dpm −1 below the euphotic zone (∼100–150 m) throughout the contrasting biogeochemical regimes. In the oligotrophic waters of the Aegean Sea, 234Th and POC fluxes estimated using sediment traps were consistently higher than respective fluxes estimated from water-column 234Th– 238U disequilibrium, observations that are attributed to terrigenous particle scavenging of 234Th. In the more productive western Mediterranean and northwest Atlantic, 234Th and POC fluxes measured by sediment trap and 234Th– 238U disequilibrium agreed within a factor of 2–4 throughout the water column. An implication of these results is that estimates of POC export by sediment traps and 234Th– 238U disequilibrium can be biased differently because of differential settling speeds of POC and 234Th-carrying particles.

Depth-specific distribution of Bacteroidetes in the oligotrophic Eastern Mediterranean Sea

Representatives of the phylum Bacteroidetes are repeatedly reported to be abundant members of oceanic bacterioplankton, probably because of their ability to degrade complex organic matter. 16S rDNA analysis was performed in order to address the importance of this phylum in a highly oligotrophic region such as the Eastern Mediterranean Sea and to investigate its distribution patterns and community composition. A new antisense primer was designed for PCR and used in combination with the general eubacterial sense primer 27F to specifically target Bacteroidetes representatives. Data were correlated with water depth and water mass properties. Denaturant gradient gel electrophoresis (DGGE) analysis and sequencing of environmental clone libraries revealed specific distribution patterns. A major fraction of the sequences was assigned to the AGG58 cluster, a branch of yet uncultured members of the Bacteroidetes lineage. Our results demonstrate a considerable diversity within the Bacteroidetes lineage in the oligotrophic Eastern Mediterranean Sea. Differing water mass properties are considered to be of major relevance for the spatial distribution with depth; several environmental clone sequence clusters could be specifically assigned to a defined water mass or to the deep waters of investigated locations. Depth-specific distribution of Bacteroidetes is demonstrated for the first time by the results of this study.

Low bacterial growth efficiency in the oligotrophic eastern Mediterranean Sea: a modelling analysis

Bacterial growth efficiency (BGE) is generally related to the trophic status of marine systems, with values as low as 0.15 or less associated with the most nutrient-depleted areas. A simple steady-state model is used to examine whether the observed ratio of bacterial to primary production (BP:PP) of 0.22 in the eastern Mediterranean Sea, an ultra-aligotrophic system, can be sustained given a BGE of ≤0.15. There is no a priori reason to believe that BGE should be higher in this area relative to other open ocean environments, although accurate measurements are required to investigate this possibility. The model includes primary production [both particulate and exudation of dissolved material expressed as a percentage of extracellular release (PER)] and terriginous dissolved organic carbon (DOC) as sources, cycling via zooplankton, viruses and bacteria, and bacterial and zooplankton respiration as sinks for carbon. Results indicate that a BP:PP of 0.22 cannot be maintained if bacterial carbon demand is met solely with DOC of autochthonous origin, given a BGE of ≤0.15. Sufficient carbon is, however, supplied to maintain this ratio if a high phytoplankton exudation of DOC (PER = 40%) is permitted, in conjunction with a BGE of 0.16. A BGE of 0.28 is required if PER takes on a more typical value of 10%. The possibility of BP being significantly enhanced by DOC of terrestrial origin is discounted, there being no major riverine sources near the Cretan Sea. Our analysis emphasizes the need for accurate estimates of BGE, and an improved understanding of sources and sinks of organic carbon, in marine systems.

Intraspecific Variations in Size- and Age-at-maturity for Red Bandfish, Cepola macrophthalma

Intraspecific variations in size- and age-at-maturity were studied in red bandfish, Cepola macrophthalma, in two adjacent gulfs of the western Aegean Sea, in the southern of which the population of red bandfish is stunted. Samples were collected with a commercial trawler over a grid of 34 stations at depths ranging from 22 to 222 m. The hypothesis tested was that length and age at 50% maturity, Lm50 and tm50 respectively, for males and females do not differ in the two regions. The results showed that the Lm50 of both males and females in the northern area was by 3.5 cm larger than that in the southern area and the 95% confidence intervals of Lm50 in the two areas did not overlap. Although the tm50 of males was larger in the northern area, the 95% confidence intervals of tm50 overlapped in the two areas whereas for females, the tm50 was larger by 0.4 years in the northern area and the 95% confidence intervals of tm50 in the two areas did not overlap. Stunting of the red bandfish growth in the southern area is the result of the combination of an extremely low food availability with higher temperatures prevailing in that area. Implications of these fine spatial scale intraspecific differences for the fisheries management of the highly oligotrophic eastern Mediterranean Sea are also discussed.