No abstract

In the ocean's most extreme depths, pressures of 70 to 110 megapascals prevent the growth of all but the most hyperpiezophilic (pressure-loving) organisms. The physiological adaptations required for growth under these conditions are considered to be substantial. Efforts to determine specific adaptations permitting growth at extreme pressures have thus far focused on relatively few ␥-proteobacteria, in part due to the technical difficulties of obtaining piezophilic bacteria in pure culture. Here, we present the molecular phylogenies of several new piezophiles of widely differing geographic origins. Included are results from an analysis of the first deep-trench bacterial isolates recovered from the southern hemisphere (9.9-km depth) and of the first grampositive piezophilic strains. These new data allowed both phylogenetic and structural 16S rRNA comparisons among deep-ocean trench piezophiles and closely related strains not adapted to high pressure. Our results suggest that (i) the Circumpolar Deep Water acts as repository for hyperpiezophiles and drives their dissemination to deep trenches in the Pacific Ocean and (ii) the occurrence of elongated helices in the 16S rRNA genes increases with the extent of adaptation to growth at elevated pressure. These helix changes are believed to improve ribosome function under deep-sea conditions. Low temperature and high hydrostatic pressure structure deep-sea communities outside of hydrothermal vents. Tight selection by these and other environmental parameters is considered the cause of the conspicuous absence of many deep-sea taxonomic groups from the deepest ocean environments (8, 44).Both temperature and pressure exert their effects at many levels of bacterial physiology, from the structure of macromolecules to the rate of metabolic reactions. Adaptations to low temperature include alterations of membrane phospholipids, such as increased fatty acid unsaturation (43), enzymes characterized by high catalytic efficiency and reduced activation enthalpy (16,20,37,45), and high levels of cold shock proteins, RNA helicases (9), and posttranscriptional modification of tRNA molecules (15), all of which may reduce the formation of unfavorable nucleic acid secondary structures at low temperature. In contrast with enthalpy-based temperature effects, the underlying cause of pressure effects arises from the promotion of reduced system volumes, in accordance with Le Chatelier's principle (5). Despite these thermodynamic differences, low temperature and high pressure share a surprising number of influences on biological processes. For example, membrane fluidity, permeability, and phase are similarly altered by both parameters.As with psychrophiles, piezophiles ("high-pressure-loving" microbes) contain lipids with highly unsaturated fatty acids (6, 7). Indeed, the presence of unsaturated fatty acids is critical to growth ability at high pressure (3,4,19). Both low temperature and high pressure also alter protein quaternary structure (46) and nucleic acid secondary structure (50), and at ...

In the basal chordate amphioxus (Branchiostoma), somites extend the full length of the body. The anteriormost somites segment during the gastrula and neurula stages from dorsolateral grooves of the archenteron. The remaining ones pinch off, one at a time, from the tail bud. These posterior somites appear to be homologous to those of vertebrates, even though the latter pinch off from the anterior end of bands of presomitic mesoderm rather than directly from the tail bud. To gain insights into the evolution of mesodermal segmentation in chordates, we determined the expression of ten genes in nascent amphioxus somites. Five (Uncx4.1, NeuroD/atonal-related, IrxA, Pcdhdelta2-17/18, and Hey1) are expressed in stripes in the dorsolateral mesoderm at the gastrula stage and in the tail bud while three (Paraxis, Lcx, and Axin) are expressed in the posterior mesendoderm at the gastrula and neurula stages and in the tail bud at later stages. Expression of two genes (Pbx and OligA) suggests roles in the anterior somites that may be unrelated to initial segmentation. Together with previous data, our results indicate that, with the exception that Engrailed is only segmentally expressed in the anterior somites, the genetic mechanisms controlling formation of both the anterior and posterior somites are probably largely identical. Thus, the fundamental pathways for mesodermal segmentation involving Notch-Delta, Wnt/beta-catenin, and Fgf signaling were already in place in the common ancestor of amphioxus and vertebrates although budding of somites from bands of presomitic mesoderm exhibiting waves of expression of Notch, Wnt, and Fgf target genes was likely a vertebrate novelty. Given the conservation of segmentation gene expression between amphioxus and vertebrate somites, we propose that the clock mechanism may have been established in the basal chordate, while the wavefront evolved later in the vertebrate lineage.