Prokaryotic Cells in the Hydrothermal Vent Tube Worm Riftia pachyptila Jones: Possible Chemoautotrophic Symbionts

Luciferase genes cloned from the unculturable luminous bacteroid symbiont of the Caribbean flashlight fish, Kryptophanaron alfredi

Light organs of anomalopid (flashlight) fish contain luminous bacteroids that have never been cultured and, consequently, have been difficult to study. We have characterized the luciferase (lux) region of DNA extracted from light organs of the Caribbean flashlight fish Kryptophanaron alfredi by hybridization of cloned Vibrio harveyi lux genes to restriction-endonuclease-digested, light organ DNA. Comparison of the hybridization pattern of light organ DNA with that of DNA of a putative symbiotic isolate provides a method for identifying the authentic luminous symbiont regardless of its luminescence, and was used to reject one such isolate. Light organ DNA was further used to construct a cosmid clone bank and the luciferase genes were isolated. Unlike other bacterial luciferase genes, the genes were not expressed in Escherichia coli. When placed under the control of the E. coli trp promoter, the genes were transcribed but no luciferase was detected, suggesting a posttranscriptional block to expression.

Hydrothermal Vent Animals: Distribution and Biology

Hydrothermal vent communities characterized by large clams, mussels, and vestimentiferan worms thrive on chemosynthetic microbial production. There are similarities in the animal distributions at vent communities from 20 degrees S to 46 degrees N on the Mid-Ocean Ridge in the Pacific Ocean and at cold sulfide seeps in the Gulf of Mexico. Vent communities, consisting of at least 16 previously unknown families of invertebrates, are at least 200 million years old. Since the life-span of a vent is only tens of years, the species survive by rapid growth and widespread dispersal of larvae with the subsequent colonization of new vents.

Geomicrobiology of Deep-Sea Hydrothermal Vents

During the cycling of seawater through the earth's crust along the mid-ocean ridge system, geothermal energy is transferred into chemical energy in the form of reduced inorganic compounds. These compounds are derived from the reaction of seawater with crustal rocks at high temperatures and are emitted from warm (</=25 degrees C) and hot ( approximately 350 degrees C) submarine vents at depths of 2000 to 3000 meters. Chemolithotrophic bacteria use these reduced chemical species as sources of energy for the reduction of carbon dioxide (assimilation) to organic carbon. These bacteria form the base of the food chain, which permits copious populations of certain specifically adapted invertebrates to grow in the immediate vicinity of the vents. Such highly prolific, although narrowly localized, deep-sea communities are thus maintained primarily by terrestrial rather than by solar energy. Reduced sulfur compounds appear to represent the major electron donors for aerobic microbial metabolism, but methane-, hydrogen-, iron-, and manganese-oxidizing bacteria have also been found. Methanogenic, sulfur-respiring, and extremely thermophilic isolates carry out anaerobic chemosynthesis. Bacteria grow most abundantly in the shallow crust where upwelling hot, reducing hydrothermal fluid mixes with downwelling cold, oxygenated seawater. The predominant production of biomass, however, is the result of symbiotic associations between chemolithotrophic bacteria and certain invertebrates, which have also been found as fossils in Cretaceous sulfide ores of ophiolite deposits.

Enzymatic defenses against oxygen toxicity in the hydrothermal vent animals Riftia pachyptila and Calyptogena magnifica

Two deep-sea hydrothermal vent organisms, the tube worm Riftia pachyptila and the clam Calyptogena magnifica, contain superoxide dismutase, dianisidine peroxidase, and glutathione peroxidase. The tube worm trophosome exhibits an iron-containing superoxide dismutase, ordinarily associated with prokaryotes and not previously seen in an animal tissue, in accord with the presence of symbiotic bacteria in this tissue. The enzymes which provide a defense against oxygen toxicity are thus present in these animals.

Thermal Vent Clam (Calyptogena magnifica) Hemoglobin

A heterodont bivalve mollusk Calyptogena magnifica, from the East Pacific Rise and the Galápagos Rift hydrothermal vent areas, contains abundant hemoglobin in circulating erythrocytes. No other known heterodont clam contains a circulating intracellular hemoglobin. The hemoglobin is tetrameric and has a relatively high oxygen affinity, which varies only slightly between 2 degrees and 10 degrees C. The presence of hemoglobin in the clam may facilitate the transport of oxygen to be used in chemoautotrophic hydrogen sulfide metabolism.

Sulfide Binding by the Blood of the Hydrothermal Vent Tube Worm Riftia pachyptila

The blood of the deep-sea hydrothermal vent tube worm Riftia pachyptila Jones contains a sulfide-binding protein that appears to concentrate sulfide from the environment and may function for sulfide transport to the internal endosymbiotic bacteria contained within the coelomic organ, the trophosome.

Blood Components Prevent Sulfide Poisoning of Respiration of the Hydrothermal Vent Tube Worm Riftia pachyptila

Respiration of plume tissue of the hydrothermal vent tube worm Riftia pachyptila is insensitive to sulfide poisoning in contrast to tissues of animals that do not inhabit vents. Permeability barriers may not be responsible for this insensitivity since plume homogenates are also resistant to sulfide poisoning. Cytochrome c oxidase of plume, however, is strongly inhibited by sulfide at concentrations less than 10 microM. Factors present in blood, but not in cytosol, prevent sulfide from inhibiting cytochrome c oxidase. Avoidance of sulfide poisoning of respiration in Riftia pachyptila thus appears to involve a blood-borne factor having a higher sulfide affinity than that of cytochrome c oxidase, with the result that appreciable amounts of free sulfide are prevented from accumulating in the blood and entering the intracellular compartment.

Microbiology of thiobacilli and other sulphur-oxidizing autotrophs, mixotrophs and heterotrophs

Recent studies on the ecophysiology of the obligate chemolithotroph Thiobacillus neapolitanus have given better insight into its specialization for an autotrophic mode of life. This appears not only from its high constitutive levels of autotrophic enzymes, but also from its possession of carboxysomes, which seem to be specialized organelles for CO2 fixation and concentrating reducing power. At the same time, these organisms are metabolically versatile with respect to nitrogen assimilation pathways, and during starvation are able to utilize endogenous resources such as polyglucose for carbon and energy. Studies on the facultative chemolithotrophs such as Thiobacillus novellus and Thiobacillus A2 have shown that they can grow mixotrophically on mixtures of inorganic and organic substrates, i.e. they can utilize these compounds simultaneously provided that they are growth limiting. Thiobacillus A2 displays a remarkable flexibility not only with respect to the organic substrates that it can utilize but, for example, also in the choice of various pathways for glucose metabolism. Competition experiments carried out between specialized and versatile thiobacilli strongly indicate that the ecological advantage of the versatile thiobacilli may lie not so much in their short-term flexibility, but rather in their ability to grow mixotrophically. Studies on most heterotrophic chemolithotrophs are still in their infancy. Promising progress has been made in the study of the physiology of Beggiatoa species. Renewed interest in the sulphur-oxidizing bacteria stems from recent findings about their role in food chains, and their possible application in industry.

Ecology of the bacteria of the sulphur cycle with special reference to anoxic-oxic interface environments

H2S is produced as a main end-product of anaerobic mineralization in anoxic, sulphate-rich environments by a diverse population of sulphate-reducing bacteria. The sulphate reducers can carry out an almost complete oxidation of detrital organic matter to CO2. The H2S consequently becomes an important electron carrier from the anoxic to the oxic world. Thiobacilli and other colourless sulphur bacteria have the potential to oxidize the H2S at the oxic-anoxic interface in sediments or stratified waters, but their role is still poorly understood. A comparison of sulphide oxidation processes in the chemoclines of the Black Sea, the Solar Lake and in A beggiatoa mat indicated that depth scales and retention times of coexisting O2 and H2S regulate the bacterial involvement in the sulphide oxidation. The H2S specialists, Beggiatoa and Thiovulum, are optimally adapted to compete with the autocatalytic oxidation of H2S by O2. Microelectrode measurements show retention times of O2-H2S in the bacterial mats or veils of less than 1 s. In photic chemoclines of stratified waters or sulfureta, the phototrophic sulphur bacteria or cyanobacteria interact with the sulphide oxidation at the O2-H2S interface. Short cycles between H2S and intermediate oxidation products, So or S2O2 3-, are created. The bacteria of the sulfuretum are highly adapted to the diurnal rhythm of light, O2 and H2S.

Blood Function in the Hydrothermal Vent Vestimentiferan Tube Worm

Extracellular hemoglobin in the whole blood of Riftia pachyptila has a high oxygen affinity (P50 = 1.8 millimeters of mercury at 3 degrees C), a moderate decrease in oxygen affinity at higher temperatures (P50 = 2.7 millimeters of mercury at 14 degrees C), a small effect of carbon dioxide on oxygen affinity (Delta log P50/Delta pH =-0.12), and a high oxygen carrying capacity (up to 11 milliliters of oxygen per 100 milliliters of blood). These characteristics are compatible with the high oxygen demand of chemoautotrophic metabolism in the variable vent environment.

Riftia pachyptila Jones: Observations on the Vestimentiferan Worm from the Galapagos Rift

The obturacular plume, composed of numerous tentacles, is suggested as a site for the uptake of molecular "food" by Riftia pachyptila (Pogonophora) from the Galápagos Rift; symbiotic bacteria are another possible source of nutrition. Differing organizations of the linings of the five major body cavities of Riftia demonstrate the inadequacy of "coelom" as a descriptive term.