No abstract

We explore hypotheses that alternate foraging strategies, diet, or nutrient partitioning could help explain the success of scavenging Lysianassoids (Amphipoda) in hadal oligotrophic trenches (depths of 6-11 km) by examining the nutritional strategies of four lysianassoid species (Eurythenes gryllus, Scopelocheirus schellenbergi, Hirondellea dubia, and Uristes sp. nov.) collected with baited traps (6.3-10.8 km) from the oligotrophic Tonga and Kermadec Trenches (southwest Pacific Ocean). Diets and foraging strategies were examined by use of (1) the nascent DNA-based analysis of hindgut contents, which provides a 'snapshot' of recently ingested organisms, and (2) natural abundance isotopic signatures, which reflect the source of nutrition and relative trophic position. The scavenging guild exhibits remarkable trophic plasticity, and each amphipod species employs alternate foraging modes, including detrivory or predation, to supplement necrophagy. The nutritional strategies of some species appear to shift with age, depth, and even between trenches. Thus, there is no single ubiquitous hadal food web; rather it is influenced by depth and overlying surface productivity. Isotopic data suggest that coexisting species partition the dietary items, providing evidence of competition among members of the scavenging guild. The extreme foraging flexibility of scavenging amphipods may ultimately contribute to their success in severely foodlimited hadal ecosystems.

Determining the diets of marine invertebrates by gut content analysis is problematic. Many consumed organisms become unrecognizable once partly digested, while those with hard remains (e.g. diatom skeletons) may bias the analysis. Here, we adapt DNA-based methods similar to those used for microbial diversity surveys as a novel approach to study the diets of macrophagous (the deep-sea amphipods Scopelocheirus schellenbergi and Eurythenes gryllus) and microphagous (the bivalve Lucinoma aequizonata) feeders in the deep sea. Polymerase chain reaction (PCR) in conjunction with 'universal' primers amplified portions of the mitochondrial cytochrome c oxidase I (COI) gene for animals ingested by S. schellenbergi and E. gryllus and the 18S rRNA gene for lesser eukaryotes ingested by L. aequizonata. Amplified sequences were combined with sequences from GenBank to construct phylogenetic trees of ingested organisms. Our analyses indicate that S. schellenbergi, E. gryllus and L. aequizonata diets are considerably more diverse than previously thought, casting new light on the foraging strategies of these species. Finally, we discuss the strengths and weaknesses of this technique and its potential applicability to diet analyses of other invertebrates.

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 ...