| Abstract |
In order to describe the benthic microbial communities in different environments and the associated environmental factors that control their distribution sediment samples were collected from various regions of the Eastern Mediterranean Sea (e.g. Thermaikos Gulf, Cretan Sea, Levantine Sea, Thracian Sea, Augusta Bay). The bacterial community composition at selected stations was compared by parallel use of three fingerprinting methods: analysis of 16S rRNA gene fragment heterogeneity by denaturing gradient electrophoresis (DGGE), terminal restriction fragment length polymorphism (T-RFLP) and analysis of phospholipid-linked fatty acid composition (PLFA). DGGE banding patterns showed a significant separation of sediment communities from the Northern, more productive waters of the Thermaikos Gulf and the oligotrophic waters of the Cretan, S. Ionian and Levantine Sea. T-RFLP analysis clearly separated the communities of deep sediments from their shallow counterparts. We could also correlate the community composition to environmental state variables such as sediment carbon content and chlorophyll a. The regional variability of bacterial community composition and diversity was studied by comparative analysis of four large 16S rDNA clone libraries constructed from shallow and deep-sea sediment samples. The libraries were dominated by representatives of the Gamma-, Delta-, and Alphaproteobacteria, Acidobacteria/Holophaga, Planctomycetes, Actinobacteria, Cytophaga, Verrucomicrobia and the candidate division OP8. Most sequences clustered with uncultured 16S rDNA phylotypes from marine habitats, and many of the closest matches were clones from hydrocarbon seeps, benzene mineralizing consortia, sulfate-reducers, sulfur-oxidizers and ammonia oxidizers. All four communities were highly diverse and the estimated total sequence richness was found to be comparable to the estimates obtained from microbiota inhabiting terrestrial ecosystems. In a next step, the effect of mineralization processes on lipids distribution was investigated along a bathymetric gradient over the continental margin of northern Crete. PLFA homologue profiles denoted that aerobic gram negative and sulfur oxidizing bacteria dominated microbial communities while the anaerobic, gram positive and sulfate reducing bacteria occurred only in traces. The estimated descriptive ratios of n-alkanes (NA), the sum of short chain NA (C15-C20) and of long chain NA (C21-C36) to 17α(H),21β(H)-C30-hopane before as well as after 2-month incubation indicated the occurrence of hydrocarbon degradation processes whereas the increased ratios of saturated to unsaturated fatty acids indicated that bacterial communities starved at the end of the experiments. The impact of salinity and oxic/anoxic conditions on microbial communities in brine sediments from the Eastern Mediterranean (Bannock, LAtalante, Discovery and Urania basins) and in oxic sediments close to the brines was also quantified by PLFA analysis. n-Alkanes distributions revealed the occurrence of different pools of organic carbon originating either from terrestrial or marine source. The presence of a different microbial community between the oxic and anoxic stations was evident whereas salinity did not seem to play any significant role on microbial communities indicating that microbes inhabiting these sediments are probably halotolerant rather than halophilic. Finally, in order to evaluate the effects of the refractory organic material and the different contamination levels on microbial community composition sediment samples were collected from polluted and pristine regions of the Eastern Mediterranean Sea. Black carbon was found to constitute a significant part of the sedimentary carbon pool, whereas the occurrence of a different trophic state between the sampling stations was evident following a gradient from the most productive and contaminated region of Augusta Bay to the organic carbon-poor and pristine sediments of Thracian Sea. The parallel increase on microbial abundances and aliphatic hydrocarbons indicated that microbial communities have developed the ability to degrade either naturally occurring aliphatic hydrocarbons or hydrocarbons derived from oil contamination. The results of this study demonstrated that the amount of refractory organic carbon in sediments has important implications on benthic microbial community structure by affecting the bioavailability of the sedimentary C pool whereas total organic carbon appeared to be important only at the organic-poor sediments.
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Περιβαλλοντική μελέτη της βακτηριακής ποικιλότητας και των σχετιζομένων με αυτή μοριακών δεικτών σε ιζήματα της Ανατολικής Μεσογείου
