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Abstract: We report new in situ observations and laboratory studies of specimens of a small (diameter 2.4–7.5cm) strikingly hexagonal form originally described from sedimented steps in a wall of the axial valley of the Mid-Atlantic Ridge (water depth 3430–3575m) near 26°N, 45°W that appears to be identical to the iconic form Paleodictyon nodosum described as a trace fossil from Eocene flysch deposits at sites in Europe and Wales. Our findings follow: [•] The form is apparently agglutinated in sea floor sediment (a veneer of calcareous lutite over red metalliferous sediment) and consists of three equidistant rows of tiny holes (diameter ∼1mm) that intersect at an angle of 120° and continuously connect through vertical shafts (length 2–3mm) with an underlying horizontal network of tubes or tunnels identical with the fossil form. [•] The number of rows of holes and spacing of rows increase with overall diameter of the form indicative of organic growth. [•] The form is shaped like a shield surrounded by a lip and moat with surface relief (∼0.5cm) that is absent in the fossil form. The surface relief exposes the underlying red sediment and may have been produced either by excavation (constructional origin) or by infaunal growth (body form). [•] Protoplasm is absent in recovered specimens, as indicated by negative results of staining techniques, explained by either initial absence or loss. [•] Genetic sequencing of material from the form identified different foraminifera that had settled on the pattern of holes which acts as a baffle to trap organic matter. [•] Models in flume tanks show that the shield-like form deflects flow of ocean currents into a self-ventilating structure capable of aerating and of circulating organic particles through the tubes or tunnels. [•] Microbial counting techniques indicated background abundances within and outside the form. We come to two alternative interpretations of the findings resolvable with further studies: [•] The modern P. nodosum is a burrow consistent with interpretation of the ancient form as a trace fossil. [•] The modern P. nodosum is a compressed form of a hexactinellid sponge adapted to a sedimentary substrate, which means that the ancient form is a body fossil with possible affinity to the Ediacara fauna. [Copyright &y& Elsevier]

Abstract: Microbial communities of methanogenic, sulfate-reducing and methanotrophic prokaryotes from deep-sea environments were investigated by molecular phylogenetic analysis of the deduced amino acid sequences of the genes encoding for the methyl coenzyme M reductase (mcrA), dissimilatory sulfite reductase (dsrAB) and particulate methane monoxygenase (pmoA), respectively. Clone libraries of PCR amplified genes were constructed using DNA extracted from deep-sea vent chimneys (Rainbow and Logatchev hydrothermal vent fields, Mid-Atlantic Ridge, Atlantic Ocean; 9°N East Pacific Rise, Pacific Ocean) and from vertically subsampled sediment cores from cold-seep areas (Blake Ridge, western Atlantic Ocean; Florida Escarpment, Gulf of Mexico). Recombinant clones were screened by RFLP and representative dsrAB, mcrA and pmoA genes were sequenced. The dsrAB sequences grouped primarily within the orders Desulfobacterales, Syntrophobacterales and the Gram-positive order Clostridales. Cold-seep mcrA sequences were distributed among the ANME-2c, -2d and -2e groups, which were previously shown to be associated with the anaerobic oxidation of methane. This study also reports the first mcrA sequences from a high-temperature, black smoker chimney (Logatchev) to group within the ANME-2e subgroup. The majority of the remaining hydrothermal vent mcrA sequences were primarily related to thermophilic members of the anaerobic, methanogenic order Methanococcales. A shift in the dominant ANME-2 group with depth in the sediment for both Florida Escarpment and Blake Ridge mcrA libraries was detected. ANME-2d related clones were detected in the top zones of both cores, with the frequency of ANME-2e related clones increasing with depth. All pmoA sequences retrieved from the cold-seep sites were found to be related to Type I methanotrophic members of the γ-proteobacteria, and were primarily distributed among three major clusters of sequences. No Type II pmoA sequences related to methanotrophic members of the α-proteobacteria were detected, suggesting that the methanotrophic communities in these cold-seep areas are dominated by Type I γ-proteobacteria. [Copyright &y& Elsevier]