Hera Karayanni

Dynamic Characteristics of Prochlorococcus and Synechococcus Consumption by Bacterivorous Nanoflagellates

We compared the characteristics of ingestion of Prochlorococcus and Synechococcus by the marine heterotrophic nanoflagellate Pseudobodo sp. and by a mixed nanoflagellate culture (around 3 microm in size) obtained from an open sea oligotrophic area. Maximum ingestion rate on Synechococcus (2.7 Syn flagellate(-1) h(-1)) was reached at concentrations of 5 x 10(5) Syn mL(-1) and decreased between 6 x 10(5) and 1.5 x 10(6) Syn mL(-1). In order to validate laboratory data, one set of data on Synechococcus grazing was obtained during a field study in the oligotrophic northeastern Mediterranean Sea. Ingestion rates by heterotrophic nanoflagellates were related to Synechococcus abundance in the water, and the feeding rate showed a clear diel rhythm with consumption being highest during the night, declining during the day hours, and being lowest at dusk. Ingestion rates on Prochlorococcus increased linearly for the whole range of prey density used (i.e., from 1 x 10(3) to 3 x 10(6) Proc mL(-1)), with maximum ingestion of 6.7 Proc flagellate(-1) h(-1). However, for prey concentrations in the range of 10(3)-10(5), which are usually encountered in aquatic systems, ingestion rates were significantly less than on Synechococcus. In our experiments, both Prochlorococcus and Synechococcus proved to be poor food items for support of nanoflagellate growth.

Influence of ciliated protozoa and heterotrophic nanoflagellates on the fate of primary production in the northeast Atlantic Ocean

showed seasonal variation, with maximal values in spring (866 mg C m � 2 and 637 mg C m � 2 , respectively). Oligotrichs dominated the ciliate assemblages, except at one site visited during spring, where a tintinnid bloom was observed. Ingestion of photosynthetic cells less than 10 mm in size was positively correlated (r = 0.7, p < 0.05, n = 12) with primary production and accounted for 2–94% of this. Phytoplankton consumption reflected differences in the evolution of the phytoplankton bloom and in the structure of the microbial food web, both associated with the strong mesoscale hydrodynamic variability of the study area. In that context it is worthy to note that when tintinnids reached high abundances locally (1260 cells L � 1 ), their impact as phytoplankton grazers was important and reached 69% of primary production. Generally, heterotrophic nanoflagellates and ciliates were relatively more important in determining the fate of phytogenic carbon during spring. Another interesting feature of primary production consumption was that during the autumn, when Prochlorococcus dominated the phytoplankton community, the protozoan grazing activity was ineffective in regulating the fate of primary producers.

Plankton Microorganisms Coinciding with Two Consecutive Mass Fish Kills in a Newly Reconstructed Lake

Lake Karla, Greece, was dried up in 1962 and its refilling started in 2009. We examined the Cyanobacteria and unicellular eukaryotes found during two fish kill incidents, in March and April 2010, in order to detect possible causative agents. Both microscopic and molecular (16S/18S rRNA gene diversity) identification were applied. Potentially toxic Cyanobacteria included representatives of the Planktothrix and Anabaena groups. Known toxic eukaryotes or parasites related to fish kill events were Prymnesium parvum and Pfiesteria cf. piscicida, the latter being reported in an inland lake for the second time. Other potentially harmful microorganisms, for fish and other aquatic life, included representatives of Fungi, Mesomycetozoa, Alveolata, and Heterokontophyta (stramenopiles). In addition, Euglenophyta, Chlorophyta, and diatoms were represented by species indicative of hypertrophic conditions. The pioneers of L. Karlas plankton during the first months of its water refilling process included species that could cause the two observed fish kill events.

Microbial community production, respiration, and structure of the microbial food web of an ecosystem in the northeastern Atlantic Ocean

increased from winter (101 ± 24 mmol O2 m 2 d 1 ) to spring (153 ± 27 mmol O2 m 2 d 1 ) and then decreased from spring to late summer (44 ± 18 mmol O2 m 2 d 1 ). DCR rates increased from winter (47 ± 18 mmol O2 m 2 d 1 ) to spring (97 ± 7 mmol O2 m 2 d 1 ) and then decreased from spring to late summer (50 ± 7 mmol O2 m 2 d 1 ). The onset of stratification depended on latitude as well as on the presence of mesoscale structures (eddies), and this largely contributed to the variability of GCP. The trophic status of the POMME area was defined as net autotrophic, with a mean annual net community production rate of +38 ± 18 mmol O2 m 2 d 1 , exhibiting a seasonal variation from +2 ± 20 mmol O2 m 2 d 1 to +57 ± 20 mmol O2 m 2 d 1 . This study highlights that small organisms (picoautotrophs, nanoautotrophs, and bacteria) are the main organisms contributing to biological fluxes throughout the year and that episodic blooms of microphytoplankton are related to mesoscale structures.

Anthropogenic effects on bacterial diversity and function along a river-to-estuary gradient in Northwest Greece revealed by metagenomics

Studies assessing the effects of anthropogenic inputs on the taxonomic and functional diversity of bacterioplankton communities in lotic ecosystems are limited. Here, we applied 16S rRNA gene amplicon and whole-genome shotgun sequencing to examine the microbial diversity in samples from the Kalamas River (Northwest Greece), a mid-size river that runs through agricultural and NATURA-protected areas, but also receives urban sewage from a large city through a manmade ditch. Samples from three different locations between the exit of the ditch and the estuary, during three different months showed that temporal differences of taxonomic and functional diversity were more pronounced than spatial ones, and <1% of total taxa were shared among all samples, revealing a highly dynamic ecosystem. Comparisons of gene diversity with other aquatic habitats showed that only the high flow winter samples resembled more to freshwater environments while samples during the decreased water flow months were dominated by sewage inputs and soil-related organisms. Notably, microbial human gut signals were detectable over background freshwater and soil/runoff related signals, even at tens of kilometers downstream the city. These findings revealed the significance of allochthonous inputs on the composition and dynamics of river bacterial communities, and highlighted the potential of metagenomics for source tracking purposes.

Spatially uniform but temporally variable bacterioplankton in a semi-enclosed coastal area.

Studies focusing on the temporal and spatial dynamics of bacterioplankton communities within littoral areas undergoing direct influences from the coast are quite limited. In addition, they are more complicated to resolve compared to communities in the open ocean. In order to elucidate the effects of spatial vs. temporal variability on bacterial communities in a highly land-influenced semi-enclosed gulf, surface bacterioplankton communities from five coastal sites in Igoumenitsa Gulf (Ionian Sea, Greece) were analyzed over a nine-month period using 16S rDNA 454-pyrosequencing. Temporal differences were more pronounced than spatial ones, with lower diversity indices observed during the summer months. During winter and early spring, bacterial communities were dominated by SAR11 representatives, while this pattern changed in May when they were abruptly replaced by members of Flavobacteriales, Pseudomonadales, and Alteromonadales. Additionally, correlation analysis showed high negative correlations between the presence of SAR11 OTUs in relation to temperature and sunlight that might have driven, directly or indirectly, the disappearance of these OTUs in the summer months. The dominance of SAR11 during the winter months further supported the global distribution of the clade, not only in the open-sea, but also in coastal systems. This study revealed that specific bacteria exhibited distinct succession patterns in an anthropogenic-impacted coastal system. The major bacterioplankton component was represented by commonly found marine bacteria exhibiting seasonal dynamics, while freshwater and terrestrial-related phylotypes were absent.

Inter-comparison of the potentially active prokaryotic communities in the halocline sediments of Mediterranean deep-sea hypersaline basins

The sediment microbiota of the Mediterranean deep-sea anoxic hypersaline basins (DHABs) are understudied relative to communities in the brines and halocline waters. In this study, the active fraction of the prokaryotic community in the halocline sediments of L’ Atalante, Urania, and Discovery DHABs was investigated based on extracted total RNA and 454 pyrosequencing of the 16S rRNA gene. Bacterial and archaeal communities were different in the sediments underlying the halocline waters of the three habitats, reflecting the unique chemical settings of each basin. The relative abundance of unique operational taxonomic units (OTUs) was also different between deep-sea control sediments and sediments underlying DHAB haloclines, suggesting adaptation to the steep DHAB chemical gradients. Only a few OTUs were affiliated to known bacterial halophilic and/or anaerobic groups. Many OTUs, including some of the dominant ones, were related to aerobic taxa. Archaea were detected only in few halocline samples, with lower OTU richness relative to Bacteria, and were dominated by taxa associated with methane cycling. This study suggests that, while metabolically active prokaryotic communities appear to be present in sediments underlying the three DHABs investigated, their diversity and activity are likely to be more reduced in sediments underlying the brines.

Harmful and parasitic unicellular eukaryotes persist in a shallow lake under reconstruction (L. Karla, Greece)

The reconstructed Lake Karla, Greece, has been undergoing its water-filling period since November 2009. In this paper, we aimed at investigating whether the unicellular eukaryotes, including the toxic/parasitic ones, that have been found during mass fish kills in the lake (March–April 2010), persist during the first warm period of the lake (May, August, November 2010). Given that microscopic characterization of some of these eukaryotes is not adequate for their identification, we analysed the 18S rRNA gene diversity of plankton samples. All the found phylotypes belonged to the phyla of Mesomycetazoa, Chlorophyta, Fungi, Alveolata, Cercozoa, Cryptophyta and Stramenopiles. Some members of these groups seem to persist in Lake Karla as they have been found in early spring as well. These microscopic eukaryotes are either ichthyotoxic/parasitic (e.g. Pfiesteria sp./Pseudopfiesteria shumwayae, some Fungi, Mesomycetazoa, Lagenidium sp., Cercozoa) or indicative of hyper-eutrophic conditions (e.g. Oocystis sp., Scenedesmus spp.) and were rather abundant during the first spring–autumn period of the lake’s refilling process. These complex microscopic communities are expected to shape highly dynamic and variable food webs with the risk of repeated fish kills.

Establishment and Succession of an Epibiotic Community on Chromated Copper Arsenate-Treated Wood in Mediterranean Waters

Colonization and succession of an epibiotic animal community on chromated copper arsenate (CCA)-treated wood were studied for 18xa0months in the eastern Mediterranean (Saronikos Gulf, Aegean Sea). Pine wood panels, 200xa0×xa0100xa0×xa025xa0mm, impregnated with CCA at retentions of 0, 12, 24, and 48xa0kgxa0m−3 were used. The abundance or surface coverage of the most characteristic taxa (polychaetes, mollusca, crustacea bryozoa, sponges, ascidians) was measured in situ, while 12xa0months after submersion two panels of each retention were removed and examined in the laboratory. A total of 26 taxa were identified, among which polychaetes of the family Serpulidae dominated. The controls carried the largest number of species (17) but the lowest number of individuals. On panels with CCA retentions of 12 and 24xa0kgxa0m−3, 14 and 16 species were observed, respectively, while at 48xa0kgxa0m−3, only 9 species were found. Only the controls were affected by boring bivalves of the family Teredinidae and started to break up within 3xa0months of submersion. Statistically significant differences in barnacle and polychaete abundance were found between treated and untreated panels. There were no significant differences among panels treated at the three CCA loadings. Ordination by nonmetric multidimensional scaling showed a seasonal effect on the colonization of the treated panels, with the highest recruitment during the warmer months of the study.

Implementation of the Water Framework Directive: Lessons learned and future perspectives for an ecologically meaningful classification of the status of Greek lakes, Mediterranean region

The enactment of the Water Framework Directive (WFD) initiated scientific efforts to develop reliable methods for comparing prevailing lake conditions against reference (or non-impaired) states, using the state of a set biological elements. Drawing a distinction between impaired and natural conditions can be a challenging exercise, as it stipulates the robust delineation of reference conditions along with the establishment of threshold values for key environmental variables used as proxies for the degree of system impairment. Another important aspect is to ensure that water quality assessment is comparable among the different Member States. In this context, the present paper offers a constructive critique of the practices followed during the WFD implementation in Greece by pinpointing methodological weaknesses and knowledge gaps that undermine our ability to classify the ecological status of Greek lakes. One of the pillars of WDF is a valid lake typology that sets ecological standards transcending geographic regions and national boundaries. The national typology of Greek lakes has failed to take into account essential components (e.g. surface area, altitude, salinity). WFD compliance assessments based on descriptions of phytoplankton communities are oversimplified and as such should be revisited. Exclusion of most chroococcal species from the analysis of cyanobacteria biovolume in Greek lakes and most reservoirs in the Mediterranean Geographical Intercalibration Group (Greece, Spain, Portugal and Cyprus) is not consistent with the distribution of those taxa in lakes. Similarly, the total biovolume reference values and the indices used in their classification schemes reflect misunderstandings of WFD core principles. This hampers the comparability of ecological status across Europe and leads to quality standards that are too relaxed to provide an efficient target especially for the protection and management of Greek/transboundary lakes such as Lake Megali Prespa, one of the oldest lakes in Europe.