Molecular differentiation by PCR-RFLP technique shows that Antarctic fishes of the genus Notothenia, claimed to be two species, are phenotypic varieties with the same genetic pattern (Table 1)

The taxonomy of Antarctic fishes has been predominantly based on morphological characteristics rather than on genetic criteria. A typical example is the Notothenia group, which includes N. coriiceps Richardson, 1844, N. neglecta Nybelin, 1951 and N. rossii Richardson, 1844. The Polymerase Chain Reaction and Restriction Fragment Length Polymorphism (PCR-RFLP) technique was used to determine whether N. coriiceps Richardson, 1844 and N. neglecta Nybelin, 1951 are different or whether they are the same species with morphological, physiological and behavioural variability. N. rossii was used as control. Mitochondrial DNA (mtDNA) was isolated from muscle specimens of N. coriiceps Richardson, 1844, N. neglecta Nybelin, 1951 and N. rossii, which were collected in Admiralty Bay, King George Island. The DNA was used to amplify a fragment (690 base pairs) of the mitochondrial gene coding region of NADH dehydrogenase subunit 2. Further, the amplicon was digested with the following restriction enzymes: DdeI, HindIII and RsaI. The results showed a variation of the digestion pattern of the fragment amplified between N. rossii, and N. coriiceps Richardson, 1844 or N. neglecta Nybelin, 1951. However, no differences were found between N. coriiceps Richardson, 1844 and N. neglecta Nybelin, 1951, on the grounds of the same genetic pattern shown by the two fish.

Niche differentiation among mat-forming, sulfide-oxidizing bacteria in chemosynthetic ecosystems of the Nile Deep Sea Fan (Eastern Mediterranean Sea)

Sulfidic muds of cold seeps on the Nile Deep Sea Fan are populated by different types of mat-forming sulfide-oxidizing bacteria. The predominant sulfide oxidizers of three different mats were identified by microscopic and phylogenetic analyses as (i) Arcobacter species producing cotton-ball-like sulfur precipitates, (ii) large filamentous sulfur bacteria including Beggiatoa species, or (iii) single, spherical cells resembling Thiomargarita species. High resolution in situ microprofiles revealed different geochemical settings selecting for different mat types. Arcobacter mats occurred where oxygen and sulfide overlapped at the bottom water interface. Filamentous sulfide oxidizers were associated with non-overlapping, steep gradients of oxygen and sulfide. A dense population of Thiomargarita was favored by temporarily changing supplies of oxygen and sulfide. These results indicate that the decisive factors in selecting for different mat-forming bacteria within one deep-sea province are spatial or temporal variations in energy supply. Furthermore, the occurrence of Arcobacter spp.-related 16S rRNA genes in the sediments below all three types of mats, as well as on top of brine lakes of the Nile Deep Sea Fan, indicates that this group of sulfide oxidizers can switch between different life modes depending on the geobiochemical habitat setting.